Acids, Bases and Salts: Class 10 Science · Chapter 2

Science · Companion notes, simulations and practice · Official NCERT chapter PDF ↗

Published 12 Oct 2026 · Updated 12 Oct 2026

100 min
39 practice questions
25 MCQs

What you will learn in Acids, Bases and Salts

Use natural, synthetic and olfactory indicators to tell acids from bases
Write word and balanced equations for the reactions of acids and bases with metals, carbonates, oxides and each other
Explain that acids give hydronium ions and bases give hydroxide ions only in water, and why their solutions conduct electricity
Use the pH scale to compare how acidic or basic solutions are, and distinguish strong from weak acids
Describe how pH matters in the body, the soil, digestion, tooth decay and stings
Predict whether a salt solution is acidic, basic or neutral, and group salts into families
Describe how sodium hydroxide, bleaching powder, baking soda, washing soda and Plaster of Paris are made and used
Explain water of crystallisation using copper sulphate, washing soda and gypsum

Original companion text aligned to NCERT Science Class 10, Chapter 2, pp. 17–36. Not affiliated with or endorsed by NCERT.

Picture the end of a big family feast: puris, chhole, pickles and a second helping of gulab jamun. An hour later someone complains of a burning feeling in the chest, the discomfort we usually call acidity. What would you offer?

Before you begin

Which of these would actually ease the burning feeling?

A glass of lemon sodaA spoon of milk of magnesiaA piece of sour mango pickle

Show answer

Answer (B) A spoon of milk of magnesia

Milk of magnesia is a mild base, so it cancels some of the extra stomach acid. Lemon soda and mango pickle taste sour because they already contain acids: they would add to the problem, not cure it.

You probably chose by using one idea: acids and bases cancel each other's effects. You met both in earlier classes. Acids (अम्ल) taste sour, while bases (क्षारक) taste bitter and feel soapy. In a laboratory, though, we never taste anything. We use an indicator (सूचक) instead: a substance whose colour, or sometimes smell, is different in an acid and in a base.

Litmus is the best-known indicator: an acid turns blue litmus red, and a base turns red litmus blue. Turmeric (haldi) is another. It stays yellow in acids but turns reddish-brown in a base. Traditional kumkum is made this way: yellow turmeric is mixed with lime (chuna), which is a base, and the mixture turns deep red. Laboratories add man-made (synthetic) indicators to the list, for example methyl orange and phenolphthalein.

In this chapter you will see how acids and bases react with metals, carbonates, oxides and each other, and what all acids have in common. You will also meet the pH scale, which measures how acidic a solution is, and follow the salt from the sea as it becomes caustic soda, bleaching powder, baking soda and washing soda.

2.1 Understanding the Chemical Properties of Acids and Bases

To compare acids and bases fairly, we test the same set of laboratory chemicals with several indicators, then react them with metals, carbonates, oxides and one another. Watch for patterns: every acid will turn out to behave in much the same way.

2.1.1 Acids and Bases in the Laboratory

A school laboratory keeps a few acids and bases in labelled bottles. The usual acids are hydrochloric acid ( $H C l$ ), sulphuric acid ( $H_{2} S O_{4}$ ), nitric acid ( $H N O_{3}$ ) and acetic acid ( $C H_{3} C O O H$ ), the acid in vinegar. The usual bases are sodium hydroxide ( $N a O H$ ), potassium hydroxide ( $K O H$ ), calcium hydroxide ( $C a (O H)_{2}$ ), magnesium hydroxide ( $M g (O H)_{2}$ ) and ammonium hydroxide ( $N H_{4} O H$ ). Each indicator in Table 2.1 has its own pair of colours.

Table 2.1How common indicators respond

	In an acid	In a neutral liquid	In a base
Blue litmus paper	turns red	stays blue	stays blue
Red litmus paper	stays red	stays red	turns blue
Methyl orange	red	yellow	yellow
Phenolphthalein	colourless	colourless	pink
Turmeric	yellow	yellow	reddish-brown

Notice that each indicator separates only two groups. Phenolphthalein is colourless in an acid and in water, so it picks out bases. Methyl orange is yellow in water and in a base, so it picks out acids.Methyl orange turns fully red only below about pH 3. In a very weak acid it shows orange, or even yellow. Litmus separates acids from bases, provided you use both colours of paper.

4/4Phenolphthalein is pink in the base, colourless in the other two.

Fig. 2.1One tile, three liquids, four indicators: each colour change sorts the liquids differently.NCERT Table 2.1 · p. 18

A white spotting tile with three columns of hollows, labelled acid, water and base, and four rows labelled blue litmus, red litmus, methyl orange and phenolphthalein. In the first step every hollow is empty. In the second step the two litmus rows fill in: blue litmus turns red only in the acid, and red litmus turns blue only in the base. In the third step the methyl orange row shows red in the acid and yellow in water and in the base. In the last step the phenolphthalein row shows colourless in the acid and water and pink in the base. Every colour is written as a word in its hollow.

Activity 2.1NCERT Activity 2.1 · p. 18

Colour-test the chemical shelf

You’ll need

dilute hydrochloric acid, dilute sulphuric acid and vinegar
sodium hydroxide solution, lime water and dilute ammonia solution
distilled water
red and blue litmus paper, methyl orange and phenolphthalein
a white tile or seven watch-glasses, and droppers

Predict first

What do you expect phenolphthalein to do in lime water?

turn pinkstay colourlessturn red

Put one drop of each liquid in its own hollow of a white tile, or on its own watch-glass, and label it.
Touch a fresh strip of blue litmus to each drop, then a strip of red litmus. Note any change.
On a fresh set of drops add one drop of methyl orange; on another fresh set, one drop of phenolphthalein.
Record each colour and decide for every liquid: acid, base or neither?

Table 2.2Indicator colours

Liquid	Blue litmus	Red litmus	Methyl orange	Phenolphthalein	Acid, base or neutral?
Dilute hydrochloric acid
Dilute sulphuric acid
Vinegar (acetic acid)
Sodium hydroxide solution
Lime water
Ammonia solution
Distilled water

Your entries are saved automatically.

Some substances signal with smell instead of colour. These are olfactory indicators (olfactory means to do with the sense of smell). Onion, vanilla essence and clove oil keep their smell in an acid but lose it in a base. Such indicators allow a person who cannot see colours, or cannot see at all, to test a liquid.

Activity 2.2NCERT Activity 2.2 · p. 18

Test with your nose

You’ll need

half an onion
two strips of clean cotton cloth or filter paper
vanilla essence and clove oil
dilute hydrochloric acid and dilute sodium hydroxide solution
four test tubes

Predict first

A drop of sodium hydroxide falls on the onion-scented strip. What happens to the onion smell?

It fades awayIt gets strongerIt does not change

Rub both strips well with the cut face of the onion. Seal them in a small plastic bag for an hour so that they soak up the smell.
Smell both strips. Put a few drops of dilute hydrochloric acid on one strip and dilute sodium hydroxide on the other. Smell them again.
Pour dilute acid into two test tubes and dilute sodium hydroxide into two more. Add a drop of vanilla essence to one acid tube and one base tube, and a drop of clove oil to the other two.
Shake each tube and smell it by wafting the air above it towards your nose.

Table 2.3Smell test

Indicator	Smell in acid	Smell in base
Onion
Vanilla essence
Clove oil

Your entries are saved automatically.

Quick check

1 of 3

A colourless liquid turns phenolphthalein pink. What does this tell you about the liquid?

It is acidicIt is neutralIt is basic

Show answer

Answer (C) It is basic

Phenolphthalein turns pink only in a base. In an acid and in a neutral liquid it stays colourless.

Practice · 1 question

Indicators (in-text): questions with answers

Short answerMediumNCERT in-text Question, p. 18
Three unlabelled test tubes contain an acid, a base and distilled water. You have only blue litmus paper. How will you identify each liquid?
Show solution
AnswerBlue litmus turns red only in the acid. Dip that reddened strip into the other two: it turns blue again in the base and stays red in the water.
Dip a blue strip into each tube. Only the acid turns it red, so that tube is the acid.
You now hold a red strip. Dip it into the two remaining tubes. In the base it turns blue again; in distilled water it stays red. So the base and the water are identified too.

2.1.2 How do Acids and Bases React with Metals?

Drop a few granules of zinc into dilute acid and the metal soon starts to fizz. The bubbles are a gas, and the next activity identifies it.

Activity 2.3NCERT Activity 2.3 · p. 19

What gas does zinc give with an acid?

You’ll need

zinc granules
dilute sulphuric acid, dilute hydrochloric acid and acetic acid
two test tubes and a stand
a wooden splint and matches

Predict first

The gas is collected in an upside-down tube, and a burning splint is brought to its mouth. What do you expect?

The flame grows brighterThe flame goes out quietlyA short 'pop' is heard

Put three or four zinc granules in a test tube held in a stand. Pour in dilute sulphuric acid to a depth of about 3 cm.
Watch the surface of the zinc. Hold a second, empty test tube upside down over the mouth of the first for about half a minute, so that the gas collects in it.
Keeping the collecting tube upside down, move it away and bring a burning splint to its mouth. Listen.
Repeat with dilute hydrochloric acid and then with acetic acid. Compare how fast the bubbles form.

3/3A burning splint at the mouth: the gas burns with a pop. It is hydrogen.

Fig. 2.2Collecting hydrogen from zinc and dilute acid, and testing it with a flame.NCERT Fig. 2.1 · p. 19

On the left, a test tube in a stand holds zinc granules in dilute sulphuric acid. In the first step bubbles rise from the zinc. In the second step an empty test tube is held upside down over the mouth of the first, and the gas, labelled hydrogen, collects in it because it is lighter than air. In the third step the upside-down tube has been moved aside and a burning splint is held at its mouth; a burst mark with the word pop shows the test result.

In this reaction the metal pushes hydrogen out of the acid and takes its place. The other product, zinc sulphate, is a salt (लवण): a compound made when the hydrogen of an acid is replaced by a metal.

\begin{aligned} Z n (s) + H_{2} S O_{4} (a q) \\ \overset{}{\to} Z n S O_{4} (a q) \\ + H_{2} (g) \end{aligned}

(2.1)

With hydrochloric acid the reaction $Z n + 2 H C l \overset{}{\to} Z n C l_{2} + H_{2}$ gives zinc chloride instead. The gas is the same; only the salt depends on the acid. In general:

\begin{aligned} acid + metal \\ ⟶ salt + hydrogen \end{aligned}

(2.2)

sodium zincate

\begin{aligned} 2 N a O H (a q) + Z n (s) \\ \overset{}{\to} N a_{2} Z n O_{2} \\ + H_{2} (g) \end{aligned}

(2.3)

Sodium zincate is an unusual salt: the metal ends up inside its negative ion, $Z n O_{2}^{2 -}$ , together with oxygen. Only a few metals, such as zinc and aluminium, react with bases like this. Iron and copper, for example, do not react with sodium hydroxide at all.

2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

Squeeze lemon juice onto a pinch of baking soda and it froths at once. Baking soda is sodium hydrogencarbonate; washing soda is sodium carbonate. Both belong to large families of compounds that react with every acid in the same way.

Activity 2.5NCERT Activity 2.5 · p. 20

Fizz, then the lime-water test

You’ll need

a spatula-tip each of washing soda (sodium carbonate) and baking soda (sodium hydrogencarbonate)
dilute hydrochloric acid
freshly made, clear lime water
two boiling tubes, corks with bent delivery tubes, two test tubes

Predict first

The gas from the fizzing tubes is bubbled through clear lime water. What will you see?

The lime water turns milkyThe lime water turns blueNothing changes

Label two boiling tubes 1 and 2. Put washing soda in tube 1 and baking soda in tube 2.
Pour a little dilute hydrochloric acid into each, and quickly fit the cork with its delivery tube.
Let the gas from each tube bubble through lime water in a test tube. Note what happens.
Keep bubbling the gas through one lime-water tube for a few minutes more, and watch carefully.

2/3The lime water turns milky: insoluble calcium carbonate forms.

Fig. 2.3The lime-water test for carbon dioxide, and what happens when the gas keeps flowing.NCERT Fig. 2.2 · p. 20

On the left a corked boiling tube holds baking soda and dilute hydrochloric acid; a bent delivery tube leads the gas into a test tube of lime water on the right. In the first step the powder fizzes and the lime water is clear. In the second step the lime water is milky, labelled calcium carbonate. In the third step, after much more gas, the lime water is clear again, labelled calcium hydrogencarbonate, which dissolves.

The reactions in the two tubes are:

\begin{aligned} N a_{2} C O_{3} + 2 H C l \\ \overset{}{\to} 2 N a C l + H_{2} O \\ + C O_{2} \end{aligned}

(2.4)

\begin{aligned} N a H C O_{3} + H C l \\ \overset{}{\to} N a C l + H_{2} O \\ + C O_{2} \end{aligned}

(2.5)

Carbon dioxide reacts with the calcium hydroxide in lime water to form calcium carbonate. Calcium carbonate does not dissolve, so it appears as a fine white precipitate that makes the liquid look milky:

\begin{aligned} C a (O H)_{2} + C O_{2} \\ \overset{}{\to} C a C O_{3} + H_{2} O \end{aligned}

(2.6)

With more carbon dioxide, the precipitate dissolves again, because it changes into calcium hydrogencarbonate, which is soluble in water:

\begin{aligned} C a C O_{3} + H_{2} O \\ + C O_{2} \\ \overset{}{\to} C a (H C O_{3})_{2} \end{aligned}

(2.7)

Calcium carbonate is everywhere around you: it makes up limestone, chalk and marble, and also egg shells and sea shells. That is why acids slowly eat into marble statues and buildings. Every metal carbonate and hydrogencarbonate gives the same three kinds of product with an acid:

\begin{aligned} carbonate + acid \\ ⟶ salt + C O_{2} \\ + water \end{aligned}

(2.8)

2.1.4 How do Acids and Bases React with each other?

An acid fizzes with a metal and with a carbonate. When an acid meets a base, however, nothing seems to happen: no bubbles and no colour. An indicator lets us see the reaction.

Activity 2.6NCERT Activity 2.6 · p. 21

Count the drops

You’ll need

dilute sodium hydroxide solution
dilute hydrochloric acid in a dropper bottle
phenolphthalein solution
a test tube

Predict first

You add acid, drop by drop, to sodium hydroxide coloured pink by phenolphthalein. What will happen to the pink colour?

It turns blueIt grows darkerAt some drop it disappears

Pour about 2 mL of dilute sodium hydroxide into a test tube, let two drops of phenolphthalein fall into it, and note the colour.
Add dilute hydrochloric acid one drop at a time, shaking after each drop. Count the drops until the colour disappears.
Now add one or two drops of sodium hydroxide. Does the colour come back?

3/4No $O H^{-}$ left: only salt and water, so it is colourless.

Fig. 2.4Each drop of acid removes hydroxide ions; when none are left, the pink goes.

On the left is a test tube with a dropper above it; on the right, a panel shows the ions in the liquid. In the first step the liquid is pink and the panel shows many sodium ions and hydroxide ions. In the second step some acid has been added: hydroxide ions are fewer, water molecules have appeared, and the liquid is still pink. In the third step the liquid is colourless and the panel holds only sodium ions, chloride ions and water. In the last step one drop of base has been added: a few hydroxide ions are back and the liquid is pink again.

The reaction in the tube is:

\begin{aligned} N a O H + H C l \\ \overset{}{\to} N a C l + H_{2} O \end{aligned}

(2.9)

Definition

Neutralisation reaction

A reaction in which an acid and a base cancel each other's properties, producing a salt and water.

ExampleHydrochloric acid and sodium hydroxide give sodium chloride (common salt) and water.

\begin{aligned} base + acid \\ ⟶ salt + water \end{aligned}

(2.10)

Example 2.1

More base, more acid

In a test, 25 mL of a sodium hydroxide solution is exactly neutralised by 15 mL of a hydrochloric acid solution. What volume of the same acid will neutralise 40 mL of the same sodium hydroxide solution?

Try it yourself first. Then reveal the solution one step at a time.

2.1.5 Reaction of Metallic Oxides with Acids

\begin{aligned} C u O (s) + 2 H C l (a q) \\ \overset{}{\to} C u C l_{2} (a q) \\ + H_{2} O (l) \end{aligned}

(2.11)

This is just like neutralisation: a metal oxide plus an acid gives a salt and water. So metal oxides behave as bases, and we call them basic oxides. Magnesium oxide, the white ash left when magnesium ribbon burns (Chapter 1), dissolves in acids in the same way.

\begin{aligned} metal oxide + acid \\ ⟶ salt + water \end{aligned}

(2.12)

2.1.6 Reaction of a Non-metallic Oxide with Base

If metal oxides act as bases, do non-metal oxides act as acids? You have already run the test without knowing it. In the lime-water test of Activity 2.5, carbon dioxide met a base, calcium hydroxide, and the products were calcium carbonate and water: a salt and water, exactly what an acid would have given. So carbon dioxide, the oxide of the non-metal carbon, behaves as an acid, and non-metal oxides are called acidic oxides. Sulphur dioxide and nitrogen dioxide from vehicles and factories dissolve in rain to form acids, which is how acid rain forms.

5/5Base + non-metal oxide → salt + water. A salt forms every time.

Fig. 2.5Five reactions, one common product: every one of them makes a salt.

A chart with five rows, each an arrow from a pair of reactants to the products. Step one shows acid plus metal giving salt plus hydrogen. Step two adds acid plus carbonate giving salt plus carbon dioxide plus water. Step three adds acid plus base giving salt plus water. Step four adds acid plus metal oxide giving salt plus water. Step five adds base plus non-metal oxide giving salt plus water, and every salt in the product column is highlighted.

Example 2.2

Complete and balance

Complete and balance each equation, and name the type of reaction.

$M g O + H C l \overset{}{\to}$
$K O H + H_{2} S O_{4} \overset{}{\to}$
$N a O H + C O_{2} \overset{}{\to}$

Try it yourself first. Then reveal the solution one step at a time.

Practice · 3 questions

Reactions of acids and bases (in-text): questions with answers

Short answerEasyNCERT in-text Questions, p. 22, Q1
Your grandmother stores lemon pickle only in glass or ceramic jars, never in brass or copper pots. Explain why.
Show solution
AnswerThe pickle is acidic. Over time the acid attacks copper and brass and forms salts of copper and zinc that are harmful to eat.
Lemon pickle contains citric acid. Brass is an alloy of copper and zinc. Over days and weeks, especially with air present, the acid reacts with these metals and forms copper and zinc salts.
These salts dissolve in the pickle, spoil its taste and can make people ill. Glass and ceramic do not react with food acids, so they are safe containers.
2
Short answerEasyNCERT in-text Questions, p. 22, Q2
A strip of magnesium ribbon is dropped into dilute sulphuric acid.
1. Name the gas given off and write the balanced equation.
  Show solution
  The gas is hydrogen.
  $\begin{aligned} M g + H_{2} S O_{4} \\ \overset{}{\to} M g S O_{4} + H_{2} \end{aligned}$
2. How will you test for this gas?
  Show solution
  Collect the gas in an upside-down test tube and bring a burning splint to its mouth. The gas burns with a pop, which shows it is hydrogen.
Short answerMediumNCERT in-text Questions, p. 22, Q3
A white solid X fizzes when dilute nitric acid is added. The gas puts out a burning candle and turns lime water milky. One product is calcium nitrate. Identify X and write the balanced equation.
Show solution
AnswerX is calcium carbonate.
The gas puts out a flame and turns lime water milky, so it is carbon dioxide. A gas like this comes from a carbonate and an acid. The salt is calcium nitrate, so the metal is calcium: X is calcium carbonate, $C a C O_{3}$ .
$\begin{aligned} C a C O_{3} \\ + 2 H N O_{3} \\ \overset{}{\to} C a (N O_{3})_{2} \\ + H_{2} O + C O_{2} \end{aligned}$

2.2 What do All Acids and All Bases have in Common?

Here is a puzzle. Glucose ( $C_{6} H_{12} O_{6}$ ), alcohol ( $C_{2} H_{5} O H$ ) and hydrochloric acid ( $H C l$ ) all contain hydrogen atoms; glucose has twelve in every molecule. Yet only hydrochloric acid tastes sour, turns litmus red and fizzes with zinc. So having hydrogen cannot be the whole story. A battery and a bulb reveal the difference.

Activity 2.8NCERT Activity 2.8 · p. 22

Which solutions carry a current?

You’ll need

a 6 V battery
a torch bulb in a holder, a switch and connecting wires
two steel drawing pins pushed through a piece of stiff card
a small glass or beaker
glucose solution, alcohol, dilute hydrochloric acid, dilute sulphuric acid, sodium hydroxide solution and lime water

Predict first

Which liquid will make the bulb glow?

glucose solutionalcoholdilute hydrochloric acid

Push the two drawing pins through the card about 2 cm apart. Rest the card on the glass so that the points dip into the liquid.
Join the pins, the bulb, the switch and the battery in one loop.
Pour in each liquid in turn, switch on and watch the bulb. Rinse the glass and pins with water between liquids.

Table 2.4Which liquids conduct?

Liquid	Does the bulb glow?
Glucose solution
Alcohol
Dilute hydrochloric acid
Dilute sulphuric acid
Sodium hydroxide solution
Lime water

Your entries are saved automatically.

2/3Dilute HCl: $H^{+}$ and $C l^{-}$ ions move. The bulb glows.

Fig. 2.6Only liquids that contain ions complete the circuit.NCERT Fig. 2.3 · p. 22

A battery, a switch and a bulb are joined in a loop that is broken by two pins dipping into a glass of liquid. In the first step the glass holds glucose solution drawn as whole molecules and the bulb is dark. In the second step it holds dilute hydrochloric acid drawn as hydrogen ions and chloride ions drifting towards opposite pins, and the bulb glows. In the third step it holds sodium hydroxide solution, drawn as sodium ions and hydroxide ions, and the bulb glows again.

In water, every acid releases hydrogen ions, $H^{+}$ , together with a negative ion that differs from acid to acid:

Table 2.5The ions acids release in water

Acid	Positive ion	Negative ion
$H C l$	$H^{+}$	$C l^{-}$ (chloride)
$H N O_{3}$	$H^{+}$	$N O_{3}^{-}$ (nitrate)
$H_{2} S O_{4}$	$H^{+}$ (two)	$S O_{4}^{2 -}$ (sulphate)
$C H_{3} C O O H$	$H^{+}$	$C H_{3} C O O^{-}$ (acetate)

The positive ion is always the same. So it is the hydrogen ions an acid produces in solution, written $H^{+} (a q)$ , that make it acidic. Glucose and alcohol keep their hydrogen atoms firmly bonded inside their molecules. They release no $H^{+}$ ions, so they show no acidic properties. Alkalis, as you will see next, share a common ion of their own.

2.2.1 What Happens to an Acid or a Base in a Water Solution?

Does an acid need water to form ions? Hydrogen chloride is a gas at room temperature, and hydrochloric acid is simply this gas dissolved in water. So we can compare the dry gas with its solution.

Activity 2.9NCERT Activity 2.9 · p. 23

Dry gas, moist paper

You’ll need

a spatula of solid sodium chloride
concentrated sulphuric acid (teacher only)
a dry boiling tube with a cork and delivery tube
dry and moist strips of blue litmus paper

Predict first

Which strip of blue litmus will turn red in the hydrogen chloride gas?

only the dry striponly the moist stripboth strips

Your teacher puts the sodium chloride in a dry boiling tube and adds a little concentrated sulphuric acid. Hydrogen chloride gas comes out of the delivery tube.
Hold a dry strip of blue litmus in the gas, then a moist strip.
On a humid day the gas is first passed through a drying tube filled with anhydrous calcium chloride, so that it stays dry.

2/3Water takes the hydrogen: $H_{3} O^{+}$ and $C l^{-}$ ions form.

Fig. 2.7Water pulls hydrogen ions off hydrogen chloride; without water there are no ions.NCERT Fig. 2.4 · p. 23

Molecules are drawn as joined circles. In the first step hydrogen chloride molecules float near a dry strip of blue litmus, which stays blue. In the second step water molecules are added: each hydrogen chloride molecule hands its hydrogen to a water molecule, forming a hydronium ion and a chloride ion. In the third step the strip, now moist and touched by hydronium ions, has turned red.

Water pulls the hydrogen away from the chlorine. The hydrogen leaves as a bare $H^{+}$ ion, which cannot stay on its own and at once attaches to a water molecule:

\begin{aligned} H C l + H_{2} O \\ \overset{}{\to} H_{3} O^{+} + C l^{-} \end{aligned}

(2.13)

H^{+} + H_{2} O \overset{}{\to} H_{3} O^{+}

(2.14)

Definition

Hydronium ion

The ion $H_{3} O^{+}$ , formed when a hydrogen ion attaches to a water molecule. It is the form in which $H^{+}$ exists in water, so $H^{+} (a q)$ and $H_{3} O^{+}$ mean the same thing.

Bases, when they dissolve, release hydroxide ions, $O H^{-}$ :

\begin{aligned} N a O H \\ \overset{}{\to} N a^{+} + O H^{-} \end{aligned}

(2.15)

Potassium hydroxide releases $K^{+}$ and $O H^{-}$ ions in the same way, and each unit of magnesium hydroxide that dissolves releases one $M g^{2 +}$ ion and two $O H^{-}$ ions.

Definition

Alkali

A base that dissolves in water, giving hydroxide ions ( $O H^{-}$ ).

ExampleSodium hydroxide and potassium hydroxide are alkalis.

Now neutralisation looks very simple. Write any acid as $H X$ and any base as $M O H$ :

\begin{aligned} H X + M O H \\ \overset{}{\to} M X + H_{2} O \end{aligned}

(2.16)

The $M^{+}$ and $X^{-}$ ions remain in the solution as the salt. The real change is that hydrogen ions and hydroxide ions join to form water:

\begin{aligned} H^{+} (a q) + O H^{-} (a q) \\ \overset{}{\to} H_{2} O (l) \end{aligned}

(2.17)

Activity 2.10NCERT Activity 2.10 · p. 24

Does mixing with water give out heat?

You’ll need

two small beakers, each about a quarter full of water
a thermometer
concentrated sulphuric acid (teacher only)
a few sodium hydroxide pellets

Predict first

What will the thermometer show when the acid is added?

a fall in temperatureno changea rise in temperature

Note the temperature of the water.
Your teacher slowly adds a few drops of concentrated sulphuric acid and swirls the beaker.
Note the temperature again and feel the outside of the beaker.
Repeat with fresh water and a few pellets of sodium hydroxide.

Table 2.6Temperature change on mixing

Added to water	Before (°C)	After (°C)
Concentrated sulphuric acid
Sodium hydroxide pellets

Your entries are saved automatically.

3/3The same ions in more water: fewer in each millilitre.

Fig. 2.8Dilution spreads the same ions through more water, so each millilitre holds fewer.NCERT Fig. 2.5 · p. 24

In the first step a small flask of concentrated acid is drawn with many ions packed into a small volume. In the second step the acid is poured in a thin stream down a glass rod into a larger beaker of water, with a curved arrow for stirring and a few heat marks. In the third step the same number of ions is spread through the much larger volume, with a box marking one millilitre that now holds far fewer ions.

Definition

Dilution

Adding water to an acid or a base. The same $H_{3} O^{+}$ or $O H^{-}$ ions spread through a bigger volume, so each millilitre holds fewer of them.

Quick check

1 of 4

Why does dilute hydrochloric acid conduct electricity, while glucose solution does not?

Its molecules carry the currentIt forms ions in waterGlucose contains no hydrogen

Show answer

Answer (B) It forms ions in water

A current through a liquid needs moving ions. HCl splits into

H_{3} O^{+}

and

C l^{-}

in water. Glucose does contain hydrogen, but it does not form ions.

Practice · 6 questions

Ions in water (in-text): questions with answers

Short answerEasyNCERT in-text Questions, p. 25, Q1
Ethanol ( $C_{2} H_{5} O H$ ) and nitric acid ( $H N O_{3}$ ) both contain hydrogen. Why does only nitric acid solution behave as an acid?
Show solution
AnswerNitric acid releases $H^{+} (a q)$ ions in water; ethanol does not.
Acidic properties come from hydrogen ions in solution. Nitric acid splits in water into $H^{+} (a q)$ (as $H_{3} O^{+}$ ) and $N O_{3}^{-}$ ions.
In ethanol the hydrogen atoms stay bonded inside the molecules, so its solution has no extra hydrogen ions and shows no acidic properties.
Short answerEasyNCERT in-text Questions, p. 25, Q2
A student writes: 'An acid solution conducts electricity because its molecules move between the wires.' Correct the statement.
Show solution
AnswerIt conducts because the acid forms ions in water, and the moving ions carry the current.
Neutral molecules cannot carry a current. In water an acid splits into ions, for example $H_{3} O^{+}$ and $C l^{-}$ from HCl. These charged ions move towards the wires, and their movement is the current.
Short answerMediumNCERT in-text Questions, p. 25, Q3
A strip of dry blue litmus is held in a jar of dry hydrogen chloride gas and stays blue. What simple change would make it turn red, and why?
Show solution
AnswerMoisten the litmus strip. With water present, HCl forms $H_{3} O^{+}$ ions, and these turn litmus red.
Dry hydrogen chloride exists as molecules, not ions, so it cannot act as an acid. If the strip is moistened, the gas dissolves in the water on it, and the hydronium ions formed turn the litmus red:
$\begin{aligned} H C l + H_{2} O \\ \overset{}{\to} H_{3} O^{+} + C l^{-} \end{aligned}$
Short answerMediumNCERT in-text Questions, p. 25, Q4
A lab assistant must make dilute sulphuric acid from the concentrated acid. In what order should she mix the acid and the water? What could go wrong the other way round?
Show solution
AnswerAdd the acid slowly to the water with stirring. Adding water to the acid can make it boil and spit acid, and the glass may crack.
Mixing is strongly exothermic. When acid is added slowly to a large amount of water, the water absorbs the heat and the mixture only warms up.
If water is poured onto concentrated acid, the small amount of water boils at once. The mixture can splash out and cause burns, and the container may crack from the sudden heating.
Quick answerEasyNCERT in-text Questions, p. 25, Q5
A solution of hydrochloric acid is mixed with an equal volume of water. The concentration of $H_{3} O^{+}$ ions …
doublesbecomes about halfstays the same
Show answer
Answer (B) becomes about half
The same number of hydronium ions is spread through twice the volume, so their concentration is roughly halved.
Quick answerEasyNCERT in-text Questions, p. 25, Q6
More pellets of potassium hydroxide are dissolved in a potassium hydroxide solution. The concentration of $O H^{-}$ ions …
increasesdecreasesstays the same
Show answer
Answer (A) increases
Each extra unit of KOH releases one more $O H^{-}$ ion into about the same volume, so the concentration increases.

2.3 How Strong are Acid or Base Solutions?

Litmus answers only one question: acid or base? It cannot tell a squeeze of lemon from battery acid. Chemists rate acidity on the pH scale, which usually runs from 0 to 14 and follows three rules:

pH 7 is neutral, like pure water.
Below 7 is acidic. The lower the pH, the more hydronium ions the solution holds.
Above 7 is basic. The higher the pH, the more hydroxide ions the solution holds.

Notice the direction: the scale runs against the hydrogen ions, so more $H_{3} O^{+}$ gives a lower pH. Each step of one unit is a tenfold change. A solution of pH 3 has ten times as many hydrogen ions as one of pH 4, and a hundred times as many as one of pH 5. (The letter p comes from Potenz, German for power.)

To read the pH we use a universal indicator, a blend of several indicators that changes colour step by step: red, orange and yellow for acids, green near neutral, and blue to violet for bases. Strips of paper soaked in it are the quickest way to measure pH in the laboratory.

Definition

pH scale

A scale, usually from 0 to 14, that shows how acidic or basic a solution is. pH 7 is neutral, below 7 is acidic and above 7 is basic; a lower pH means more hydrogen ions.

4/4Baking soda (8.3), milk of magnesia (10.5) and lime water (12.4) are basic.

Fig. 2.9Everyday substances on the pH scale. The colours are only a rough guide to universal indicator.NCERT Fig. 2.6 · p. 25

A horizontal bar numbered 0 to 14 is shaded in bands: red and orange at the acidic end, yellow, green around 7, then blue and violet at the basic end. In the first step arrows below the bar point left for more hydrogen ions and right for more hydroxide ions. In the second step lemon juice is marked near pH 2 and tomato juice near pH 4. In the third step pure water is marked at pH 7, labelled neutral. In the last step baking soda solution is marked near pH 8.3, milk of magnesia near 10.5 and lime water near 12.4.

InteractivepH explorer

A pH scale from 0 to 14, shaded in rough universal-indicator colours, with a marker that moves along it. A slider sets the pH in steps of 0.1, and buttons jump to substances such as dilute hydrochloric acid, stomach acid, lemon juice, vinegar, tomato juice, black coffee, clean rain, milk, pure water, blood, sea water, baking soda solution, soap solution, milk of magnesia, washing soda solution, lime water and dilute sodium hydroxide. Readouts give the pH, whether the solution is strongly or weakly acidic, neutral, or weakly or strongly basic, and how many times more or fewer hydrogen ions it has than pure water. Five swatches name the colour that blue litmus, red litmus, methyl orange, phenolphthalein and universal indicator would show at that pH, and a note adds context, such as when tooth enamel would be attacked, when rain would count as acid rain, or when a liquid is corrosive.

Fig. 2.10Pick an everyday substance or drag the slider. See its pH, how many hydrogen ions it has compared with pure water, and what five indicators would show.

Example 2.3

Comparing two solutions

Lemon juice has a pH of about 2 and black coffee about 5. (a) Which has more hydrogen ions, and how many times more? (b) What colour will methyl orange show in each? (Methyl orange is red below pH 3.1 and yellow above pH 4.4.)

Try it yourself first. Then reveal the solution one step at a time.

Activity 2.11NCERT Activity 2.11 · p. 26

Measure the pH of things around you

You’ll need

universal indicator paper and its colour chart
small samples of the liquids in the table
a clean dropper or glass rod for each sample

Put a drop of each sample on a fresh piece of pH paper.
Within a few seconds, match the colour with the chart and note the pH.
Decide whether each sample is acidic, neutral or basic.

Table 2.7pH of everyday liquids

Sample	Colour of pH paper	pH (approx.)	Acidic, neutral or basic?
Lemon juice
Buttermilk (chaas)
Soda water
Tea without milk
Tap water
Toothpaste in water
Soap solution
Baking soda solution

Your entries are saved automatically.

Does a low pH always mean a strong acid? Not quite. Take hydrochloric acid and acetic acid at the same concentration. Almost every HCl molecule splits into ions, but only a small fraction of acetic acid molecules do, so the acetic acid solution has far fewer hydrogen ions. Hydrochloric acid is therefore a strong acid, and acetic acid is a weak acid. Hydrochloric, sulphuric and nitric acids are strong; acetic, citric, lactic and carbonic acids are weak. In the same way, sodium hydroxide and potassium hydroxide are strong bases, while ammonium hydroxide is a weak base.

2/2HCl splits completely; acetic acid splits only slightly.

Fig. 2.11Same concentration, very different numbers of hydrogen ions.

Two beakers side by side, labelled hydrochloric acid and acetic acid. In the first step each beaker holds eight acid molecules drawn as joined pairs, with the label same amount. In the second step all eight molecules in the hydrochloric acid beaker have split into hydrogen ions and chloride ions, while in the acetic acid beaker only one molecule has split and seven remain whole. The labels read strong acid, many ions, and weak acid, few ions.

Definition

Strong acid

An acid that splits almost completely into ions in water, such as hydrochloric acid. A weak acid, such as acetic acid, splits only partly, so it gives fewer hydrogen ions at the same concentration.

Strong is not the same as concentrated. A few drops of hydrochloric acid in a bucket of water make a dilute solution of a strong acid. It is still strong, because the little acid present has split completely into ions.

2.3.1 Importance of pH in Everyday Life

pH and living things

Living things are fussy about pH. Your blood is held very close to pH 7.4, and the body's chemistry works only within a narrow band, roughly 7.0 to 7.8. Even a small shift outside it makes a person seriously ill.

Rain is naturally a little acidic, about pH 5.6, because it dissolves some carbon dioxide from the air. When oxides of sulphur and nitrogen from smoke make it more acidic than that, we call it acid rain. In lakes fed by acid rain, fish eggs fail to hatch and many water animals die out, because their bodies, like ours, need a steady pH.

pH of the soil

Plants need the right pH too. Most crops grow best in soil close to neutral, about pH 6 to 7.5. Tea bushes prefer acidic soil, one reason why they thrive on the hill slopes of Assam and Darjeeling. Farmers test their soil before treating it. A soil that is too acidic is treated with a base: quicklime (calcium oxide), slaked lime (calcium hydroxide) or powdered chalk (calcium carbonate).

Activity 2.12NCERT Activity 2.12 · p. 27

Test the soil near you

You’ll need

soil from two or three places (a garden bed, a field, a roadside)
test tubes and water
filter paper and a funnel
universal indicator paper

Put a spoonful of one soil sample in a test tube with about three times as much water, and shake well.
Let it settle, then filter the muddy water into a clean tube.
Test the clear filtrate with universal indicator paper and note the pH.
Repeat for the other samples. Note which plants grow where each sample came from.

Table 2.8Soil pH

Sample	Where it came from	pH (approx.)	Plants growing there
1
2
3

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pH in our digestive system

Read the label on an antacid and you will usually find a base: magnesium hydroxide (sold as milk of magnesia) or sodium hydrogencarbonate. Why would a medicine be a base? Because the trouble it treats is an acid. Your stomach releases hydrochloric acid, at about pH 1.5 to 3.5, to help break down food and to kill germs. A layer of mucus shields the stomach wall from it. A heavy or spicy meal can trigger too much acid, which irritates and burns. A mild base neutralises the excess, which is why milk of magnesia was the right answer to the opening question:

\begin{aligned} M g (O H)_{2} + 2 H C l \\ \overset{}{\to} M g C l_{2} + 2 H_{2} O \end{aligned}

(2.18)

pH and tooth decay

Follow your mouth's pH after a sweet in Fig. 2.12. Bacteria living in the sticky film on your teeth ferment the sugar into acids within minutes, and the pH dips. The outer layer of each tooth, the enamel, is calcium hydroxyapatite, a calcium phosphate mineral and the hardest material your body makes. Water alone cannot dissolve it, but acid can: below about pH 5.5 calcium starts to leach out of it.

3/3Saliva slowly neutralises the acid; the pH is back to normal in about 45 minutes.

Fig. 2.12Every sweet snack starts a dip in mouth pH. The shaded part is when enamel is under attack.

A graph of pH in the mouth, from 4 to 8, against time, from before eating to 60 minutes after a snack. A dashed line at pH 5.5 is labelled enamel dissolves below. In the first step the curve is flat at about 6.8 before eating. In the second step a sweet snack is eaten at time zero and the curve falls to about pH 5 within ten minutes; the part below 5.5 is shaded. In the third step the curve climbs back above 5.5 after about 25 minutes and reaches 6.8 by about 45 minutes, as saliva neutralises the acid.

Each sweet snack starts a new dip, so nibbling all day is worse for teeth than one treat. Rinsing or brushing after eating takes away the bacteria's food, and most toothpastes are slightly basic, so they neutralise acid as well.

Self-defence with chemicals

Some animals and plants defend themselves with chemistry. The red weaver ants that build leaf nests in mango trees bite and squirt methanoic acid (formic acid) into the wound. Nettle plants keep the same acid in hollow hairs that snap off in your skin. A honeybee's sting also leaves acidic venom behind. Because the culprit is an acid, a mild base gives relief: a paste of baking soda dabbed on the spot.

Table 2.9Some acids found in natureNCERT Table 2.3 · p. 28

Where it is found	Acid
Lemon, orange	citric acid
Tamarind, grapes	tartaric acid
Curd, buttermilk	lactic acid
Vinegar	acetic acid
Tomato, spinach	oxalic acid
Apple	malic acid
Amla (Indian gooseberry)	ascorbic acid (vitamin C)
Ant bite, nettle sting	methanoic acid

Practice · 4 questions

The pH scale (in-text): questions with answers

1
Short answerMediumNCERT in-text Questions, p. 28, Q1
Solution X has a pH of 9, and solution Y has a pH of 5.
1. Which solution has more hydrogen ions?
  XY
  Show answer
  Answer (B) Y
  Y has the lower pH, so it has more $H^{+}$ ions.
2. Which is acidic and which is basic?
  Show solution
  Y (pH 5, below 7) is acidic. X (pH 9, above 7) is basic.
3. How many times more hydrogen ions does it have?
  Show answer
  Answer 10000
  The difference is 9 − 5 = 4 units, so $10^{4} = 10 000$ times.
Quick answerEasyNCERT in-text Questions, p. 28, Q2
As the concentration of $H^{+} (a q)$ ions in a solution increases, the solution becomes …
more basic, with a higher pHmore acidic, with a lower pHmore acidic, with a higher pH
Show answer
Answer (B) more acidic, with a lower pH
More hydrogen ions make a solution more acidic, and pH runs opposite to hydrogen-ion concentration, so the pH falls.
Short answerMediumNCERT in-text Questions, p. 28, Q3
Pure water, and even lime water, contain some $H^{+} (a q)$ ions. Why, then, is lime water called basic?
Show solution
AnswerIn lime water hydroxide ions far outnumber hydrogen ions, so it behaves as a base.
Every water solution contains some hydrogen ions and some hydroxide ions. In pure water the two are equal, so it is neutral (pH 7).
In lime water, calcium hydroxide adds a large number of $O H^{-}$ ions, so they far outnumber the $H^{+}$ ions. The solution is therefore basic, with a pH of about 12.
Short answerMediumNCERT in-text Questions, p. 28, Q4
A soil test on a field near a tea garden shows pH 4.2. The farmer wants to grow wheat, which prefers soil near neutral. What could he add, and why?
Show solution
AnswerQuicklime, slaked lime or powdered chalk: these bases neutralise the excess acid and raise the pH.
pH 4.2 is strongly acidic for wheat. Adding a base neutralises some of the acid in the soil and raises its pH towards neutral.
Suitable bases are quicklime (calcium oxide), slaked lime (calcium hydroxide) or powdered chalk (calcium carbonate).

2.4 More about Salts

Salts have turned up throughout this chapter: zinc sulphate from zinc and acid, sodium chloride from neutralisation, calcium carbonate in lime water. Now let us look at salts themselves: how they are named and grouped, how acidic or basic their solutions are, and how a few important ones are made and used.

2.4.1 Family of Salts

A salt has two parts: a positive ion from the base (a metal ion, or the ammonium ion $N H_{4}^{+}$ ) and a negative ion from the acid. The negative ion gives the second half of the name: chlorides come from hydrochloric acid, sulphates from sulphuric acid, nitrates from nitric acid, carbonates from carbonic acid and acetates from acetic acid. Chemists sort salts into families, rather like surnames: every sodium salt shares the name sodium, and every chloride shares the name chloride. So each salt belongs to two families at once; potassium chloride is both a potassium salt and a chloride.

Activity 2.13NCERT Activity 2.13 · p. 28

Sort salts into families

For each salt, write its formula and name the acid and the base it can be made from. Then sort the salts into families.

Table 2.10Salts and their parents

Salt	Formula	Acid	Base
Potassium nitrate
Calcium chloride
Zinc sulphate
Ammonium nitrate
Magnesium chloride
Sodium sulphate
Potassium chloride
Calcium carbonate

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2.4.2 pH of Salts

Are all salts neutral? Common salt is, but many others are not.

Activity 2.14NCERT Activity 2.14 · p. 29

Are salt solutions neutral?

You’ll need

a pinch each of potassium chloride, sodium sulphate, ammonium chloride, copper sulphate, sodium carbonate and sodium hydrogencarbonate
distilled water and six test tubes
red and blue litmus paper, universal indicator paper

Predict first

Will every one of these salt solutions have pH 7?

No: some are acidic and some basicYes: salts are always neutralNo: every salt solution is acidic

Dissolve a pinch of each salt in about 5 mL of distilled water. (Tap water may not itself be neutral.)
Test each solution with both colours of litmus paper and with universal indicator paper.
Note the pH and name the acid and the base each salt comes from.

Table 2.11pH of salt solutions

Salt	pH	Parent acid	Parent base	Acidic, basic or neutral?
Potassium chloride
Sodium sulphate
Ammonium chloride
Copper sulphate
Sodium carbonate
Sodium hydrogencarbonate

Your entries are saved automatically.

Table 2.12What decides the pH of a salt solution

Made from	Examples	Solution	pH
strong acid + strong base	$K C l$ , $N a_{2} S O_{4}$ , $N a C l$	neutral	7
strong acid + weak base	$N H_{4} C l$ , $C u S O_{4}$	acidic	below 7
weak acid + strong base	$N a_{2} C O_{3}$ , $N a H C O_{3}$	basic	above 7

The rule is easy to remember: a salt's solution leans towards its stronger parent. When both parents are strong, they balance each other and the salt is neutral. Copper sulphate comes from strong sulphuric acid and weak copper hydroxide, so its solution is acidic. Sodium carbonate comes from strong sodium hydroxide and weak carbonic acid, so its solution is basic.

Quick check

1 of 2

Which of these salts give an acidic solution in water?

N H_{4} C l

K N O_{3}

C u S O_{4}

N a_{2} C O_{3}

Select all that apply

Show answer

Answer

(A) $N H_{4} C l$
(C) $C u S O_{4}$

N H_{4} C l

and

C u S O_{4}

come from a strong acid and a weak base, so they are acidic.

K N O_{3}

is neutral (both parents strong) and

N a_{2} C O_{3}

is basic (weak acid, strong base).

2.4.3 Chemicals from Common Salt

Sodium chloride, the neutral salt of hydrochloric acid and sodium hydroxide, is the salt on your dining table. Where does it come from? Most of India's salt comes from the sea. Along the coasts of Gujarat and Tamil Nadu, sea water is let into shallow pans, the sun evaporates the water, and the crystals are raked into white heaps.

Some salt is dug out of the ground instead. Millions of years ago shallow seas dried up and left thick layers of salt that were later buried. These are mined today as rock salt, whose large crystals are often tinted pink or brown by impurities; the pink sendha namak used during fasts is rock salt. India has rock-salt mines at Mandi in Himachal Pradesh.

Common salt is also a raw material for the chemical industry. Fig. 2.13 shows how it leads to sodium hydroxide, chlorine, hydrogen, bleaching powder, baking soda and washing soda.

5/5Heat baking soda, then crystallise with water: washing soda.

Fig. 2.13One cheap raw material, six useful chemicals.

Common salt, sodium chloride, sits at the top. In the second step an arrow labelled electrolysis of brine leads to three boxes: sodium hydroxide, chlorine and hydrogen. In the third step an arrow from chlorine, labelled plus dry slaked lime, leads to bleaching powder. In the fourth step a second branch from common salt, labelled plus ammonia, carbon dioxide and water, leads to baking soda, sodium hydrogencarbonate. In the last step an arrow labelled heat leads from baking soda to sodium carbonate, and another labelled plus ten water leads to washing soda.

Sodium hydroxide

Factories make sodium hydroxide by electrolysing brine, a strong solution of common salt in water. Three products come out together, and two of them give the method its name, the chlor-alkali process: chlorine and an alkali, sodium hydroxide.

\begin{aligned} 2 N a C l + 2 H_{2} O \\ \overset{e l e c t r i c i t y}{\to} 2 N a O H \\ + C l_{2} + H_{2} \end{aligned}

(2.19)

In Fig. 2.14, watch the two electrodes. The positive one (the anode) releases chlorine, the negative one (the cathode) releases hydrogen, and the liquid around the cathode turns into sodium hydroxide solution. Industry has a use for every one of them.

3/3Phenolphthalein turns pink near the cathode: NaOH has formed there.

Fig. 2.14The chlor-alkali process on a small scale.NCERT Fig. 2.8 · p. 30

A U-shaped tube filled with brine has a carbon electrode in each arm, joined to a battery: the left electrode is positive, the anode, and the right is negative, the cathode. In the first step the switch is open and the brine holds sodium ions and chloride ions. In the second step the current flows: bubbles of chlorine gas rise at the anode and bubbles of hydrogen gas at the cathode. In the third step a drop of phenolphthalein turns the liquid around the cathode pink, labelled sodium hydroxide forms here.

Compare

Uses of the three products

Hydrogen

fuel, including rocket fuel
hardening vegetable oils into vanaspati
making ammonia for fertilisers

Chlorine

disinfecting drinking water and swimming pools
making PVC pipes
bleaches, disinfectants and pesticides

Sodium hydroxide

soaps and detergents
paper making
artificial fibres such as rayon
removing grease from metals

Bleaching powder

Chlorine from the chlor-alkali process is passed over dry slaked lime to make bleaching powder:

\begin{aligned} 2 C a (O H)_{2} + 2 C l_{2} \\ \overset{}{\to} C a (C l O)_{2} \\ + C a C l_{2} + 2 H_{2} O \end{aligned}

(2.20)

Its active ingredient is calcium hypochlorite, so we write it as $C a (C l O)_{2}$ for short, although the powder sold in shops is a mixture of several calcium compounds. (Older books write it as $C a O C l_{2}$ .) It smells of chlorine.

Bleaching powder disinfects drinking water: a little is added to wells, village tanks and water supplies, especially after floods. Textile mills use it to whiten cotton and linen, paper mills to bleach wood pulp, and laundries to whiten clothes. Chemical industries also use it as an oxidising agent.

Baking soda

Baking soda is sodium hydrogencarbonate, $N a H C O_{3}$ , also called sodium bicarbonate. In Indian kitchens it puffs up bhaturas and dhoklas, and a pinch in the water helps chickpeas soften faster when boiled. It is made from sodium chloride, water, carbon dioxide and ammonia:

\begin{aligned} N a C l + H_{2} O \\ + C O_{2} + N H_{3} \\ \overset{}{\to} N H_{4} C l \\ + N a H C O_{3} \end{aligned}

(2.21)

Its solution is only gently basic, about pH 8.3, and it does not burn the skin, so small amounts can be eaten and used to neutralise acids safely. When it is heated, for example in a hot pan, it decomposes:

\begin{aligned} 2 N a H C O_{3} \\ \overset{h e a t}{\to} N a_{2} C O_{3} \\ + H_{2} O + C O_{2} \end{aligned}

(2.22)

Uses of baking soda:

Baking powder. Baking soda is mixed with a weak edible acid such as tartaric acid. When the mixture is wetted or heated, the two react and release carbon dioxide, whose bubbles puff up cakes, bread and dhokla and leave them light and airy (equation 2.23). The fruit salt sold for acidity is a similar mixture of baking soda and citric acid.
Antacid. A small dose in water mops up excess stomach acid.
Soda–acid fire extinguishers. Inside the extinguisher it meets an acid and produces a jet of carbon dioxide (see Group activity II).

baking powder

\begin{aligned} N a H C O_{3} + H^{+} \\ \overset{}{\to} N a^{+} + C O_{2} \\ + H_{2} O \end{aligned}

(2.23)

Washing soda

Washing soda is sodium carbonate that has crystallised from water, with ten water molecules attached to each unit. The sodium carbonate itself can come from heating baking soda (equation 2.22). Its solution is far more strongly basic than baking soda's, at about pH 11.5.

washing soda

\begin{aligned} N a_{2} C O_{3} + 10 H_{2} O \\ \overset{}{\to} N a_{2} C O_{3} \cdot 10 H_{2} O \end{aligned}

(2.24)

Ten water molecules in a dry-looking crystal? Section 2.4.4 explains how that can be.

Washing soda softens hard water: it removes permanent hardness by turning the dissolved calcium and magnesium salts into insoluble carbonates, so soap lathers again. It is also a household cleaner for greasy pots, tiles and drains. Industry uses sodium carbonate to make glass, soap and paper, and to make other sodium compounds such as borax.

Example 2.4

Identify the compounds

A white powder X is stirred into dhokla batter to make it rise. Heated in a dry tube, X gives a white solid Y, water, and a gas Z that turns lime water milky. Y, dissolved in water and crystallised, gives a salt used to soften hard water. Identify X, Y, Z and the salt, and write the equations.

Try it yourself first. Then reveal the solution one step at a time.

Quick check

1 of 4

In the chlor-alkali process, which gas is given off at the anode?

hydrogenchlorineoxygen

Show answer

Answer (B) chlorine

Chloride ions give up electrons at the positive electrode (the anode), forming chlorine. Hydrogen forms at the cathode.

2.4.4 Are the Crystals of Salts really Dry?

Copper sulphate crystals look and feel completely dry. Yet heat them and water comes out.

Activity 2.15NCERT Activity 2.15 · p. 32

Water from dry crystals

You’ll need

a few crystals of blue copper sulphate
a dry boiling tube and a test-tube holder
a burner
a dropper with water

Predict first

What colour do you expect the crystals to become after heating?

whitedark bluegreen

Hold the dry tube with a holder, tilted and pointing away from people, and heat a few crystals gently.
Watch the colour of the crystals, and look for droplets near the cooler mouth of the tube.
Let the tube cool. Then add two or three drops of water to the powder and touch the bottom of the tube.

2/3The crystals turn white; water condenses near the mouth.

Fig. 2.15Heating drives the water of crystallisation out; adding water puts it back.NCERT Fig. 2.9 · p. 32

A boiling tube held at a slant contains crystals at its bottom. In the first step the crystals are blue and labelled CuSO4 5H2O. In the second step a flame heats the tube: the crystals have become a white powder, labelled anhydrous, and droplets of water appear near the cooler mouth. In the third step a dropper adds water to the cooled powder, which turns blue again, with heat marks showing that the tube grows warm.

Definition

Water of crystallisation

Water molecules built into a salt's crystals in a fixed ratio: a set number for each formula unit of the salt.

ExampleCopper sulphate crystals hold five: $C u S O_{4} \cdot 5 H_{2} O$ .

So the dot in $C u S O_{4} \cdot 5 H_{2} O$ means that five water molecules belong to each formula unit of the crystal. Washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ , has ten. Such crystals are not wet: the water is built into the crystal and does not wet your hand. Heating drives it out:

blue to white

\begin{aligned} C u S O_{4} \cdot 5 H_{2} O \\ \overset{h e a t}{\to} C u S O_{4} + 5 H_{2} O \end{aligned}

(2.25)

The white mineral gypsum, $C a S O_{4} \cdot 2 H_{2} O$ , holds two water molecules for every unit of calcium sulphate. India mines most of its gypsum in Rajasthan, and it is the raw material for a very useful powder.

Plaster of Paris

Heat gypsum to about 373 K and it loses part of its water, becoming calcium sulphate hemihydrate, better known as Plaster of Paris:

\begin{aligned} C a S O_{4} \cdot 2 H_{2} O \\ \overset{373 K}{\to} C a S O_{4} \cdot \frac{1}{2} H_{2} O \\ + \frac{3}{2} H_{2} O \end{aligned}

(2.26)

Plaster of Paris is a white powder. Stir it into water and within minutes it turns back into gypsum and sets rock-hard:

\begin{aligned} C a S O_{4} \cdot \frac{1}{2} H_{2} O \\ + \frac{3}{2} H_{2} O \\ \overset{}{\to} C a S O_{4} \cdot 2 H_{2} O \end{aligned}

(2.27)

No crystal holds half a molecule of anything. Count in pairs instead: for every two calcium sulphate units there is one water molecule, $(C a S O_{4})_{2} \cdot H_{2} O$ , which averages out to a half per unit.

Fig. 2.16Gypsum and Plaster of Paris turn into each other by losing or gaining water.

Two boxes face each other in a loop. The left box is gypsum, calcium sulphate dihydrate, and the right box is Plaster of Paris, the hemihydrate. The upper arrow, from gypsum to Plaster of Paris, is labelled heat to 373 K and loses one and a half water. The lower arrow, from Plaster of Paris back to gypsum, is labelled add water and sets hard.

Doctors use Plaster of Paris to make casts that hold broken bones in the right position. It is also used for toys, decorative mouldings and statues, for false ceilings, and to smooth walls before painting. Many festival idols are now cast in it. Set plaster does not dissolve easily, so immersing such idols pollutes rivers and lakes, and clay idols are encouraged instead. The name comes from the large gypsum deposits around Paris, where the plaster was made for centuries.

Quick check

1 of 3

One formula unit of washing soda has molecules of water of crystallisation.

Show answer

Answer 10

Washing soda is

N a_{2} C O_{3} \cdot 10 H_{2} O

Practice · 5 questions

Salts (in-text): questions with answers

Quick answerEasyNCERT in-text Questions, p. 33, Q1
Your textbook writes bleaching powder as $C a (C l O)_{2}$ . Its chemical name is calcium .
Show answer
Answer hypochlorite
$C a (C l O)_{2}$ is calcium hypochlorite.
Quick answerEasyNCERT in-text Questions, p. 33, Q2
Chlorine is passed over which dry substance to make bleaching powder?
quicklime, $C a O$ limestone, $C a C O_{3}$ slaked lime, $C a (O H)_{2}$ gypsum, $C a S O_{4} \cdot 2 H_{2} O$
Show answer
Answer (C) slaked lime, $C a (O H)_{2}$
Dry slaked lime, calcium hydroxide, reacts with chlorine to give bleaching powder.
Quick answerEasyNCERT in-text Questions, p. 33, Q3
A laundry's water forms scum with soap because it holds dissolved calcium and magnesium salts. Which sodium compound should the laundry add to fix this?
baking sodacommon saltcaustic sodawashing soda
Show answer
Answer (D) washing soda
Washing soda, $N a_{2} C O_{3} \cdot 10 H_{2} O$ , turns the calcium and magnesium salts that cause hardness into insoluble carbonates.
Short answerMediumNCERT in-text Questions, p. 33, Q4
Baking soda is heated in a dry test tube, and the gas given off is bubbled through lime water. What will you observe? Write the equation for the change in the test tube.
Show solution
AnswerThe lime water turns milky, because carbon dioxide is given off.
Baking soda decomposes on heating into sodium carbonate, water and carbon dioxide. The carbon dioxide turns lime water milky.
$\begin{aligned} 2 N a H C O_{3} \\ \overset{h e a t}{\to} N a_{2} C O_{3} \\ + H_{2} O + C O_{2} \end{aligned}$
Short answerEasyNCERT in-text Questions, p. 33, Q5
A sculptor mixes Plaster of Paris with water to cast a statue. Write the equation for what happens as it sets.
Show solution
AnswerIt takes up water and turns back into gypsum.
Plaster of Paris combines with water to form gypsum, which is a hard solid.
$\begin{aligned} C a S O_{4} \cdot \frac{1}{2} H_{2} O \\ + \frac{3}{2} H_{2} O \\ \overset{}{\to} C a S O_{4} \cdot 2 H_{2} O \end{aligned}$

Recap

Acids, Bases and Salts: key points and summary

Indicators such as litmus, turmeric, methyl orange and phenolphthalein change colour in acids and bases; olfactory indicators change smell (Section 2.1.1).
Acids react with metals to give a salt and hydrogen. Zinc and aluminium also give hydrogen with sodium hydroxide, forming a salt whose negative ion contains the metal and oxygen (Section 2.1.2).
With an acid, a carbonate or hydrogencarbonate fizzes and forms a salt, water and carbon dioxide, the gas that makes lime water milky (Section 2.1.3).
Acid + base → salt + water is neutralisation. Metal oxides are basic and non-metal oxides are acidic (Section 2.1.4, Section 2.1.6).
Acids and alkalis conduct electricity in water because they form ions (Section 2.2).
Acids form $H^{+} (a q)$ (that is, $H_{3} O^{+}$ ) ions only in water; bases give $O H^{-}$ ions, and bases that dissolve in water are alkalis (Section 2.2.1).
Mixing a concentrated acid or base with water gives out much heat, so acid is always added slowly to water (Activity 2.10).
The pH scale (0 to 14) measures hydrogen-ion concentration: 7 is neutral, below 7 acidic and above 7 basic, and each unit is a tenfold change (Section 2.3).
Strong acids and bases split almost completely into ions; weak ones split only partly (strong and weak acids).
Living things work within narrow pH ranges: blood, soil, digestion and teeth all depend on pH (Section 2.3.1).
A salt solution is neutral, acidic or basic depending on the strengths of its parent acid and base (Section 2.4.2).
Common salt is the raw material for sodium hydroxide, chlorine, hydrogen, bleaching powder, baking soda and washing soda (Section 2.4.3).
Many salt crystals hold a fixed number of water molecules per formula unit, their water of crystallisation, as in $C u S O_{4} \cdot 5 H_{2} O$ ; Plaster of Paris sets by taking water back (Section 2.4.4).

Group activity INCERT Group Activity (I) · p. 35

Make a red-cabbage indicator

You’ll need

two or three red (purple) cabbage leaves
hot water (ask an adult), a bowl and a strainer
five small glasses
lemon juice, vinegar, soda water, baking soda solution and washing soda solution

Tear the leaves into small pieces, cover them with hot water and leave them for 15 minutes.
Strain off the purple liquid: this is your indicator.
Pour a little of each test liquid into its own glass, add a spoonful of indicator to each, and note the colours.

Table 2.13Red-cabbage indicator

Liquid	Colour with cabbage indicator	Acidic or basic?
Lemon juice
Vinegar
Soda water
Baking soda solution
Washing soda solution

Your entries are saved automatically.

Group activity IINCERT Group Activity (II) · p. 36

Build a soda–acid fire extinguisher

You’ll need

a plastic squeeze bottle with a nozzle cap
a strong solution of baking soda (about half a cup)
a small glass vial and some thread
vinegar
a candle on a metal tray

Pour the baking soda solution into the bottle.
Tie a thread round the neck of the vial, fill it with vinegar and lower it carefully into the bottle so that it hangs upright above the solution. Close the cap with the thread trapped under it.
Outdoors, light a candle on a tray. Tip the bottle so that the vinegar mixes with the solution, point the nozzle at the flame and watch.

Find your NCERT exercise question

Our practice questions are written for ncert.ai and follow the NCERT exercises. Find the one that matches the question in your textbook; its answer is with it.

In your NCERT textbook	Practice question here
NCERT in-text Question, p. 18	Indicators (in-text) · Q1
NCERT in-text Questions, p. 22, Q1	Reactions of acids and bases (in-text) · Q1
NCERT in-text Questions, p. 22, Q2	Reactions of acids and bases (in-text) · Q2
NCERT in-text Questions, p. 22, Q3	Reactions of acids and bases (in-text) · Q3
NCERT in-text Questions, p. 25, Q1	Ions in water (in-text) · Q1
NCERT in-text Questions, p. 25, Q2	Ions in water (in-text) · Q2
NCERT in-text Questions, p. 25, Q3	Ions in water (in-text) · Q3
NCERT in-text Questions, p. 25, Q4	Ions in water (in-text) · Q4
NCERT in-text Questions, p. 25, Q5	Ions in water (in-text) · Q5
NCERT in-text Questions, p. 25, Q6	Ions in water (in-text) · Q6
NCERT in-text Questions, p. 28, Q1	The pH scale (in-text) · Q1
NCERT in-text Questions, p. 28, Q2	The pH scale (in-text) · Q2
NCERT in-text Questions, p. 28, Q3	The pH scale (in-text) · Q3
NCERT in-text Questions, p. 28, Q4	The pH scale (in-text) · Q4
NCERT in-text Questions, p. 33, Q1	Salts (in-text) · Q1
NCERT in-text Questions, p. 33, Q2	Salts (in-text) · Q2
NCERT in-text Questions, p. 33, Q3	Salts (in-text) · Q3
NCERT in-text Questions, p. 33, Q4	Salts (in-text) · Q4
NCERT in-text Questions, p. 33, Q5	Salts (in-text) · Q5
NCERT Exercises Q1	Exercises · Q1
NCERT Exercises Q2	Exercises · Q2
NCERT Exercises Q3	Exercises · Q3
NCERT Exercises Q4	Exercises · Q4
NCERT Exercises Q5	Exercises · Q5
NCERT Exercises Q6	Exercises · Q6
NCERT Exercises Q7	Exercises · Q7
NCERT Exercises Q8	Exercises · Q8
NCERT Exercises Q9	Exercises · Q9
NCERT Exercises Q10	Exercises · Q10
NCERT Exercises Q11	Exercises · Q11
NCERT Exercises Q12	Exercises · Q12
NCERT Exercises Q13	Exercises · Q13
NCERT Exercises Q14	Exercises · Q14
NCERT Exercises Q15	Exercises · Q15

Quick check: MCQs on Acids, Bases and Salts

Before you begin1 question
2.1.1 Acids and Bases in the Laboratory3 questions
2.1.6 Reaction of a Non-metallic Oxide with Base4 questions
2.2.1 What Happens to an Acid or a Base in a Water Solution?4 questions
Self-defence with chemicals4 questions
2.4.2 pH of Salts2 questions
Washing soda4 questions
Plaster of Paris3 questions

Glossary · 15

Key terms (English–Hindi)

acidअम्लamla: A substance that gives hydrogen ions ( $H^{+}$ ) in water. Acids taste sour, turn blue litmus red and have a pH below 7.First seen in the chapter →
alkaliक्षारkshar: A base that dissolves in water, giving hydroxide ions, such as sodium hydroxide.
antacid: A mild base, such as milk of magnesia, taken to neutralise excess acid in the stomach.First seen in the chapter →

baseक्षारकksharak: A substance that reacts with an acid to give a salt and water. Bases turn red litmus blue; those that dissolve give $O H^{-}$ ions.First seen in the chapter →

chlor-alkali process: Electrolysis of brine (sodium chloride solution) to make chlorine, hydrogen and sodium hydroxide.First seen in the chapter →

dilutionतनुकरणtanukaran: Adding water to an acid or base, which lowers the concentration of its ions.

hydronium ionहाइड्रोनियम आयनhaaidroniyam aayan: The ion $H_{3} O^{+}$ : a hydrogen ion joined to a water molecule, the form $H^{+}$ takes in water.

indicatorसूचकsoochak: A substance whose colour (or smell) is different in acids and in bases, such as litmus, methyl orange or phenolphthalein.First seen in the chapter →

neutralisationउदासीनीकरणudaaseenikaran: The reaction in which an acid and a base cancel each other, forming a salt and water.

olfactory indicatorगंधीय सूचकgandhiya soochak: A substance whose smell changes in an acid or a base, such as onion, vanilla or clove oil.First seen in the chapter →

pH scale: A scale from 0 to 14 for acidity: 7 is neutral, below 7 acidic, above 7 basic. A lower pH means more hydrogen ions.First seen in the chapter →

saltलवणlavan: A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion, for example in neutralisation.First seen in the chapter →
strong acidप्रबल अम्लprabal amla: An acid that splits almost completely into ions in water, such as HCl. A weak acid, such as acetic acid, splits only partly.First seen in the chapter →

universal indicatorसार्वत्रिक सूचकsaarvatrik soochak: A mixture of indicators that shows a different colour at each pH.First seen in the chapter →

water of crystallisationक्रिस्टलन जलkristalan jal: Water locked into a salt's crystals in a fixed ratio to the salt, as in $C u S O_{4} \cdot 5 H_{2} O$ .

Acids, Bases and Salts: Class 10 Science · Chapter 2

What you will learn in Acids, Bases and Salts

2.1 Understanding the Chemical Properties of Acids and Bases

2.1.1 Acids and Bases in the Laboratory

Indicators (in-text): questions with answers

2.1.2 How do Acids and Bases React with Metals?

2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

2.1.4 How do Acids and Bases React with each other?

2.1.5 Reaction of Metallic Oxides with Acids

2.1.6 Reaction of a Non-metallic Oxide with Base

Reactions of acids and bases (in-text): questions with answers

2.2 What do All Acids and All Bases have in Common?

2.2.1 What Happens to an Acid or a Base in a Water Solution?

Ions in water (in-text): questions with answers

2.3 How Strong are Acid or Base Solutions?

2.3.1 Importance of pH in Everyday Life

pH and living things

pH of the soil

pH in our digestive system

pH and tooth decay

Self-defence with chemicals

The pH scale (in-text): questions with answers

2.4 More about Salts

2.4.1 Family of Salts

2.4.2 pH of Salts

2.4.3 Chemicals from Common Salt

Sodium hydroxide

Bleaching powder

Baking soda

Washing soda

2.4.4 Are the Crystals of Salts really Dry?

Plaster of Paris

Salts (in-text): questions with answers

Acids, Bases and Salts: key points and summary

Practice questions with answers (NCERT “Exercises”)

Find your NCERT exercise question

Quick check: MCQs on Acids, Bases and Salts

Key terms (English–Hindi)

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