Chemical Reactions and Equations: Class 10 Science · Chapter 1

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

Published 12 Oct 2026 · Updated 12 Oct 2026

90 min
1 simulation
31 practice questions
26 MCQs

What you will learn in Chemical Reactions and Equations

Recognise a chemical reaction from what you observe: a new colour, a gas, a change of temperature or a new solid
Write word equations and chemical equations, and balance them by counting atoms so that mass is conserved
Complete a chemical equation with state symbols and the conditions written on the arrow
Classify reactions as combination, decomposition, displacement or double displacement, and as exothermic or endothermic
Identify the substance oxidised and the substance reduced by tracking which reactant takes in or gives up oxygen or hydrogen
Explain corrosion and rancidity as everyday effects of oxidation, and how we slow them down

Original companion text aligned to NCERT Science Class 10, Chapter 1, pp. 1–16. Not affiliated with or endorsed by NCERT.

Spotting a chemical reaction

Mix a spoon of curd into warm milk at night and by morning you have dahi. A bicycle left out through the monsoon grows an orange-brown crust on its chain. Dosa batter puffs up and turns sour, and a cut apple browns within minutes. Each time, what you started with has turned into something with a different taste, colour or texture.

In earlier classes you sorted changes into two kinds. In a physical change, such as ghee melting or sugar dissolving, no new substance forms. In a chemical change, at least one new substance appears, with properties of its own. Chemists call a chemical change a chemical reaction. So the practical question is: what can your eyes, nose and fingertips tell you? Three activities supply the evidence.

Activity 1.1NCERT Activity 1.1 · p. 1

A ribbon that burns with a dazzling light

You’ll need

a 3 cm strip of magnesium ribbon
sandpaper
a pair of tongs
a spirit lamp or burner
a china dish or a steel lid

Predict first

What do you expect to be left after the ribbon has burnt?

Nothing: the metal burns away into the airA white powderA shiny bead of melted metalBlack soot, as from a candle

Rub the ribbon with sandpaper until it is bright and silvery.
Grip one end with the tongs and hold the other end in the flame, at arm's length.
As soon as it catches, hold it over the dish so that the ash falls in. Look at the flame only from the corner of your eye.
Compare the powder in the dish with an unburnt piece of ribbon.

Table 1.1

Property	Ribbon	What is left
Colour
Shine
Bends or crumbles?

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Activity 1.3NCERT Activity 1.3 · p. 2

Fizz and warmth from zinc

You’ll need

4–5 granules of zinc
dilute hydrochloric acid or dilute sulphuric acid
a test tube in a stand
a dropper

Predict first

After the acid is added, how do you expect the bottom of the tube to feel?

Warmer than beforeColder than beforeJust the sameSlippery

Drop the zinc granules into the test tube.
Add dilute acid with the dropper until the granules are just covered.
Watch the surface of the zinc for a minute.
Wrap your fingers lightly round the bottom of the tube.

Table 1.2

What to check	What I noticed
Around the zinc
Temperature of the tube

Your entries are saved automatically.

Put the three activities side by side and you have a checklist. A reaction is probably taking place when you notice one or more of these:

a change of colour, like the yellow from two colourless liquids
a gas given off, like the hydrogen bubbles on zinc
a change of temperature, like the warm test tube
a change of state, like a new solid appearing in a liquid, or a metal crumbling to powder

5/5A new solid forms in the liquid.

Fig. 1.1Four clues that a chemical reaction may be happening.

Four test tubes stand in a row, each half full of a clear liquid. In the first step nothing has changed yet. In the colour step the liquid in the first tube turns yellow. In the gas step bubbles also rise through the second tube. In the warmth step a thermometer beside the third tube shows a higher reading. In the solid step a layer of new solid settles at the bottom of the fourth tube, so all four clues are visible together.

Treat these as clues, not proof. Water bubbles when it boils and ice changes state when it melts, yet nothing new forms: it is water all the way through. The real test is always whether a new substance has appeared. The simulation shows why a change of state on its own can fool you.

SimulationIce to steam: still water

A closed container of water particles with a thermometer and a temperature slider from minus 20 to 120 degrees Celsius. Below 0 °C the particles sit packed in a regular pattern as ice; between 0 and 100 °C they slide past one another as liquid water; above 100 °C they fly apart and fill the container as steam. A label names the state. The particles are the same water particles in every state; only their spacing and motion change.

Fig. 1.2Water particles from ice to steam. Press Play, then drag the temperature slider.

Try this

Start at −10 °C and slowly drag up to 110 °C. Does any particle ever turn into a different kind of particle?
Hold the slider just above 100 °C. The water is bubbling, but is anything new being made?
Drag back down to −10 °C. You get ice again. Could you get an unburnt magnesium ribbon back this easily?

1.1 Chemical Equations

A lab notebook full of sentences such as “magnesium burns in air, joining with oxygen to become magnesium oxide” would be slow to write and slow to read. Chemistry uses a shorthand instead, built up in two stages. Stage one keeps the names but drops the grammar, giving a word equation:

word equation

\begin{aligned} magnesium \\ + oxygen \\ \to magnesium oxide \end{aligned}

(1.1)

Everything to the left of the arrow gets used up: these are the reactants. Everything to the right is newly made: the products. A plus sign simply means “and”. The arrow shows which way the change runs, so read it as “react to give”. It is not an equals sign, because the two sides are different substances.

Definition

Reactant

A substance that is used up and changed in a chemical reaction; it is written to the left of the arrow.

ExampleMagnesium and oxygen, when a ribbon burns.

Definition

Product

A new substance formed in a chemical reaction; it is written to the right of the arrow.

ExampleMagnesium oxide, the white ash.

1.1.1 Writing a Chemical Equation

Names are still long, and they say nothing about how many atoms take part. So we replace each name with its chemical formula: $M g$ for magnesium, $O_{2}$ for oxygen gas (made of two-atom molecules) and $M g O$ for magnesium oxide. The result is a chemical equation.

skeletal equation

M g + O_{2} \overset{}{\to} M g O

(1.2)

Now count the atoms. The left side has one magnesium atom and two oxygen atoms; the right side has one of each. An oxygen atom seems to have vanished, and that cannot happen. An equation like this, with the right formulae but unequal atom counts, is called a skeletal equation. Think of it as a first draft that still has to be balanced.

Definition

Chemical equation

A short way of writing a reaction with chemical formulae: reactants on the left, an arrow, products on the right.

Example $M g + O_{2} \overset{}{\to} M g O$ (still to be balanced).

1.1.2 Balanced Chemical Equations

Why must the atom counts match? Picture the atoms as building blocks. A reaction pulls blocks apart and snaps them together in new ways, but it never makes a new block or destroys one. If you weigh a sealed flask before and after a reaction inside it, the reading does not change.

LawLaw of conservation of mass

In a chemical reaction the total mass stays the same: the products together weigh exactly as much as the reactants that formed them.

So in a correct equation each element must have the same number of atoms on both sides. Such an equation is a balanced equation. Some equations are balanced as soon as you write them. Zinc and sulphuric acid in Activity 1.3 give:

zinc and sulphuric acid

\begin{aligned} Z n + H_{2} S O_{4} \\ \overset{}{\to} Z n S O_{4} + H_{2} \end{aligned}

(1.3)

Tally it: zinc 1 and 1, hydrogen 2 and 2, sulphur 1 and 1, oxygen 4 and 4. Every element matches, so nothing needs to change. The magnesium equation is different. Fig. 1.3 fixes it with a particle picture.

3/3Two Mg: every atom balances.

Fig. 1.3Balancing

M g + O_{2} \overset{}{\to} M g O

by adding whole particles.

Atoms are drawn as labelled circles: Mg for magnesium and O for oxygen, with an oxygen molecule shown as two joined O circles. In the first step one Mg atom and one oxygen molecule give one MgO unit, and the tallies show 2 oxygen atoms on the left but only 1 on the right. In the second step a second MgO unit is added on the right, so oxygen now matches but magnesium is 1 on the left against 2 on the right. In the third step a second Mg atom is added on the left and both tallies match, with two Mg and two O on each side.

Definition

Balanced chemical equation

A chemical equation in which each element has the same number of atoms on the reactant side as on the product side.

Example $2 M g + O_{2} \overset{}{\to} 2 M g O$

Balancing has one golden rule: change only the numbers written in front of formulae, called coefficients. A coefficient says how many molecules (or formula units) you take. The small numbers inside a formula, the subscripts, belong to the substance itself. Change $H_{2} O$ to $H_{2} O_{2}$ and you have written hydrogen peroxide, an antiseptic and bleach, not water (Fig. 1.4).

3/3A changed subscript: a new substance.

Fig. 1.4A coefficient counts molecules; a subscript changes the molecule.

Molecules are drawn as joined circles, a large circle O for oxygen and small circles H for hydrogen. In the first step a single water molecule is shown. In the second step a coefficient of 2 gives two separate water molecules, which are still water. In the third step the subscript has been changed instead, giving one molecule with two oxygen and two hydrogen atoms in a chain: hydrogen peroxide, a different substance, marked with a cross as not allowed when balancing.

Balancing by hit-and-trial

Most equations you will meet this year can be balanced by a careful try-and-check routine, the hit-and-trial method:

Write the skeletal equation with correct formulae, then treat every formula as sealed: from now on you may only put numbers in front of it.
Tally the atoms of each element on each side.
Pick the biggest formula (the one with the most atoms) and, inside it, the element that appears most. Make that element equal first.
Balance the remaining elements one at a time. Leave elements that stand alone, such as $O_{2}$ , $H_{2}$ or $F e$ , for last.
Recount every atom. If all the coefficients share a factor, divide it out to get the smallest whole numbers.

Example 1.1

Balancing the gas in an LPG cylinder

Cooking gas (LPG) is a mixture of butane and propane, $C_{3} H_{8}$ . Propane burns in oxygen to give carbon dioxide and water. Balance the skeletal equation:

\begin{aligned} C_{3} H_{8} + O_{2} \\ \overset{}{\to} C O_{2} + H_{2} O \end{aligned}

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

Example 1.2

The odd–even trick: aluminium in air

A new aluminium kadhai soon gets a thin, dull coat of aluminium oxide, which protects the metal underneath. Balance the equation for its formation:

\begin{aligned} A l + O_{2} \\ \overset{}{\to} A l_{2} O_{3} \end{aligned}

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

Example 1.3

Treating a whole ion as one unit

Water works add aluminium sulphate, the main part of fitkari (alum), to muddy water. With an alkali such as sodium hydroxide it forms sticky aluminium hydroxide, which drags the mud down. Balance:

\begin{aligned} A l_{2} (S O_{4})_{3} \\ + N a O H \\ \overset{}{\to} A l (O H)_{3} \\ + N a_{2} S O_{4} \end{aligned}

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

Practise the routine on reactions of your own choosing with the balancer below. It counts the atoms for you, so you can concentrate on the strategy.

InteractiveEquation balancer

An interactive balancer. A menu picks one of nine skeletal equations, from calcium burning in oxygen to ammonia burning in oxygen, each with its reaction type. Every formula has a minus and a plus button that change its coefficient from 1 to 15. A table lists each element with its atom count on the left and on the right, marked with a tick when they match and a cross when they do not. A status line says whether the equation is balanced and whether the coefficients are the smallest whole numbers. A Hint button suggests which element to fix next, Reset sets every coefficient back to 1, and Show answer fills in the balanced coefficients.

Fig. 1.5Pick a reaction, then press − and + under each formula until every element balances.

Writing physical states

A balanced equation still hides something useful: is each substance a solid, a liquid, a gas or a solution? We show this with a state symbol in brackets after each formula.

Table 1.3State symbols

Symbol	Meaning	Example
(s)	solid	zinc granules, $Z n (s)$
(l)	liquid	water, $H_{2} O (l)$
(g)	gas, or vapour such as steam	hydrogen, $H_{2} (g)$
(aq)	aqueous: dissolved in water	dilute acid, $H_{2} S O_{4} (a q)$

With state symbols, the zinc reaction of (1.3) becomes:

with state symbols

\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}

Equations are often written without state symbols. Add them when they matter, for example when water reacts as steam or when a solid forms inside a solution, and whenever a question asks for them.

Conditions a reaction needs are written on the arrow: heat, light, a high pressure, or a catalyst, a substance that speeds up a reaction without being used up. Hydrogen peroxide from the first-aid box breaks down slowly on its own, but a pinch of manganese dioxide makes it fizz out oxygen fast:

catalyst on the arrow

\begin{aligned} 2 H_{2} O_{2} (a q) \\ \overset{M n O_{2}}{\to} 2 H_{2} O (l) \\ + O_{2} (g) \end{aligned}

Baking soda gives off carbon dioxide and steam when heated, one reason a cake rises in the oven:

heat on the arrow

\begin{aligned} 2 N a H C O_{3} (s) \\ \overset{heat}{\to} N a_{2} C O_{3} (s) \\ + H_{2} O (g) + C O_{2} (g) \end{aligned}

Example 1.4

From a sentence to a complete equation

Washing soda (sodium carbonate) solution reacts with dilute hydrochloric acid to give sodium chloride solution, water and carbon dioxide gas. Write the balanced equation with state symbols.

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

Quick check

1 of 4

Balance the magnesium equation:

M g

O_{2}

→

M g O

Show answer

Answer (1) 2, (2) 2

Two MgO units need two oxygen atoms, which one

O_{2}

supplies. They also need two magnesium atoms, so the answer is

2 M g + O_{2} \overset{}{\to} 2 M g O

Practice · 3 questions

Pause and practise 1: questions with answers

Short answerEasyNCERT in-text Questions p. 6, Q1
A magnesium ribbon from an old packet looks dull grey. Before burning it, a student rubs it with sandpaper until it shines. Why?
Show solution
Over time, magnesium reacts with oxygen (and carbon dioxide) in the air and gets a thin coat of magnesium oxide. This coat does not burn and keeps air away from the metal, so the ribbon is hard to light. Sanding exposes fresh metal, which catches fire easily.
2
Short answerMediumNCERT in-text Questions p. 6, Q2
Write the balanced equation for each reaction.
1. Sulphur dioxide + oxygen → sulphur trioxide
  Show solution
  $2 S O_{2}$ + $O_{2}$ → $2 S O_{3}$
  Sulphur 2 and 2, oxygen 4 + 2 = 6 and 6.
2. Iron(III) oxide + hydrogen → iron + water
  Show solution
  $F e_{2} O_{3}$ + $3 H_{2}$ → $2 F e$ + $3 H_{2} O$
  Iron 2 and 2, oxygen 3 and 3, hydrogen 6 and 6.
3. Magnesium + nitrogen → magnesium nitride ( $M g_{3} N_{2}$ )
  Show solution
  $3 M g$ + $N_{2}$ → $M g_{3} N_{2}$
  Three magnesium atoms are needed for the three in $M g_{3} N_{2}$ ; nitrogen is already 2 and 2.
3
Short answerMediumNCERT in-text Questions p. 6, Q3
Write each balanced equation with state symbols.
1. Lead nitrate solution is poured into sodium sulphate solution. White lead sulphate settles out, and sodium nitrate is left dissolved.
  Show solution
  $P b (N O_{3})_{2} (a q)$ + $N a_{2} S O_{4} (a q)$ → $P b S O_{4} (s)$ + $2 N a N O_{3} (a q)$
  Two nitrate groups on the left need two $N a N O_{3}$ , which also uses both sodium atoms.
2. Dilute nitric acid exactly neutralises potassium hydroxide solution. The products are water and dissolved potassium nitrate.
  Show solution
  $K O H (a q)$ + $H N O_{3} (a q)$ → $K N O_{3} (a q)$ + $H_{2} O (l)$
  Already balanced: K, N, H and O each match.

1.2 Types of Chemical Reactions

Think back to the magnesium ribbon. Every magnesium atom in the ribbon is still there in the white ash; it has simply let go of its old neighbours and taken oxygen atoms as new partners. That is all any reaction does. Some bonds break, others form, and the same atoms end up arranged as new substances. How atoms hold on to each other is the subject of Chapters 3 and 4.

Reactions fall into a few common patterns. Spotting the pattern helps you predict the products and write the equation. Fig. 1.6 shows four of them, with shapes standing in for atoms or groups of atoms.

1/5Four ways atoms can rearrange.

Fig. 1.6Four patterns of chemical reaction.

Four rows show reaction patterns using a circle A, a square B, a triangle C and a diamond D. Combination: A and B join into AB. Decomposition: AB splits into A and B. Displacement: A meets BC and takes B's place, giving AC and a free B. Double displacement: AB and CD swap partners to give AD and CB. In the first step all four rows are shown; each later step highlights one row and fades the other three.

1.2.1 Combination Reaction

Activity 1.4NCERT Activity 1.4 · p. 6

Quicklime meets water

You’ll need

two or three small lumps of quicklime (calcium oxide)
a glass beaker
water
a thermometer
a glass rod

Predict first

What will the thermometer show after you add water to quicklime?

The temperature drops sharplyThe temperature hardly changesThe thermometer cannot read inside a pasteThe temperature rises a lot

Put the lumps in the beaker and note the thermometer reading.
Pour in water slowly, a little at a time, stirring gently with the rod.
Hold the thermometer in the mixture and read it every half minute for three minutes.
Look at what the lumps have turned into.

Table 1.4

Time	Temperature (°C)
Before water
After 1 minute
After 3 minutes

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3/3Slaked lime forms and the mixture gets hot.

Fig. 1.7Quicklime and water combine and give out heat.NCERT Fig. 1.3 · p. 6

A beaker holds white lumps with a thermometer standing in it. In the first step the lumps are dry and the thermometer shows room temperature. In the second step water has been poured in, and small bubbles and the word hiss rise from the lumps. In the third step the beaker holds a milky paste of slaked lime, steam curls above it and the thermometer column has risen high.

quicklime → slaked lime

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

(1.13)

Count what went in and what came out: two substances in, only one out. Any reaction with that shape, several reactants and a single product, is a combination reaction. The reactants may be elements, like magnesium and oxygen, or compounds, like quicklime and water.

Definition

Combination reaction

A reaction in which two or more reactants join to form a single product.

ExampleCalcium oxide and water forming calcium hydroxide.

You have already met other combination reactions:

the magnesium ribbon in Activity 1.1, where two elements form one white product, $2 M g + O_{2} \overset{}{\to} 2 M g O$ , with plenty of heat and light
charcoal glowing in an angithi (a clay stove), $C + O_{2} \overset{}{\to} C O_{2}$
open bottles of ammonia and hydrochloric acid placed side by side: their gases meet in the air and form a white smoke of ammonium chloride, $N H_{3} + H C l \overset{}{\to} N H_{4} C l$
the upper stage of ISRO's heaviest rockets, where liquid hydrogen and liquid oxygen combine to form water, $2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O$ , releasing the energy that drives the rocket

Notice how often heat turns up. A reaction that hands heat out to its surroundings is an exothermic reaction (exo means out). Burning LPG (Example 1.1) is one, which is the whole point of a gas stove. The slow rotting of kitchen waste is another: dig into a compost pit on a winter morning and the inside is warm.

Definition

Exothermic reaction

A reaction that gives out heat to its surroundings as the products form.

ExampleBurning charcoal or LPG; quicklime and water.

Feel your forehead after a sprint. That warmth comes from a reaction running in every cell of your body. Glucose, made when you digest starchy food, is slowly “burnt” with the oxygen you breathe in. This controlled burning, respiration, releases the energy that keeps you alive and warm. Chapter 5 looks at it closely.

respiration of glucose

\begin{aligned} C_{6} H_{12} O_{6} (a q) \\ + 6 O_{2} (a q) \\ \overset{}{\to} 6 C O_{2} (a q) \\ + 6 H_{2} O (l) + energy \end{aligned}

(1.18)

1.2.2 Decomposition Reaction

Decomposition is combination run backwards: a single compound falls apart into two or more simpler substances. Pulling a compound apart means breaking bonds, and that costs energy, so these reactions usually need heat, light or electricity to keep going.

Activity 1.5NCERT Activity 1.5 · p. 8

Green crystals that turn brown

You’ll need

a spatula-tip of ferrous sulphate crystals (green vitriol)
a dry boiling tube
a test-tube holder
a burner

Predict first

What colour will the solid be after strong heating?

Reddish brownStill pale greenBright blueShiny silver

Note the colour of the crystals in the dry boiling tube.
Hold the tube with the holder, mouth pointing away from everyone, and heat it gently, then strongly.
Watch the cooler upper part of the tube and the colour of the solid.
Waft the air above the mouth towards your nose with your hand. Never sniff the tube directly.

Table 1.5

Stage	What I saw
Before heating
Gentle heating
Strong heating

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ferrous sulphate → ferric oxide

\begin{aligned} 2 F e S O_{4} (s) \\ \overset{heat}{\to} F e_{2} O_{3} (s) \\ + S O_{2} (g) + S O_{3} (g) \end{aligned}

(1.19)

3/3Lead nitrate: brown fumes of nitrogen dioxide.

Fig. 1.8Two thermal decompositions side by side.NCERT Figs. 1.4–1.5 · p. 8

Two boiling tubes are held at a slant over two burners. The left tube holds green ferrous sulphate crystals and the right tube a white lead nitrate powder. In the second step the left tube shows water droplets near its mouth and a reddish-brown solid at the bottom, with a label for the smell of burning sulphur. In the third step brown fumes of nitrogen dioxide fill the right tube and rise from its mouth, and a yellowish solid is left at its bottom.

A reaction in which one substance splits into simpler ones is a decomposition reaction. When heat drives it, we call it thermal decomposition. Industry depends on one in particular. Limestone heated in a kiln breaks down into quicklime and carbon dioxide; the lime kilns around Katni in Madhya Pradesh do exactly this, and the same reaction is the first step inside every cement kiln.

limestone → quicklime

\begin{aligned} C a C O_{3} (s) \\ \overset{heat}{\to} C a O (s) \\ + C O_{2} (g) \end{aligned}

(1.20)

Definition

Decomposition reaction

A reaction in which a single reactant breaks down into two or more simpler products.

ExampleLimestone heated to give quicklime and carbon dioxide.

lead nitrate

\begin{aligned} 2 P b (N O_{3})_{2} (s) \\ \overset{heat}{\to} 2 P b O (s) \\ + 4 N O_{2} (g) + O_{2} (g) \end{aligned}

(1.21)

Heat is not the only way to supply the energy. The next two activities use electricity and light.

Activity 1.7NCERT Activity 1.7 · p. 9

Splitting water with a battery

You’ll need

an electrolysis vessel with two electrodes coming up through its base
water with a few drops of dilute sulphuric acid
two test tubes
a 9 V battery, a switch and wires
wooden splints and matches

Predict first

Will the two tubes fill with gas at the same rate?

Yes, at exactly the same rateOnly one tube collects any gasOne fills about twice as fast as the otherNeither: water cannot be split

Fill the vessel with the acidified water so that both electrodes are covered.
Fill both test tubes to the brim, close each with a thumb, and turn them upside down over the electrodes, under the water.
Connect the electrodes to the battery through the switch, and switch on.
When one tube is nearly full of gas, switch off and lift each tube out, still upside down.
Your teacher brings a burning splint to the mouth of the fuller tube, and a glowing splint into the other.

Table 1.6

Electrode	Gas volume (mL)	Splint test
Joined to − terminal
Joined to + terminal

Your entries are saved automatically.

electrolysis of water

\begin{aligned} 2 H_{2} O (l) \overset{electricity}{\to} \\ 2 H_{2} (g) + O_{2} (g) \end{aligned}

3/3Hydrogen and oxygen collect, 2 : 1 by volume.

Fig. 1.9Electricity splits water into hydrogen and oxygen.NCERT Fig. 1.6 · p. 9

A lab apparatus with two upright glass tubes joined at the bottom, each with an electrode at its base, is wired to a battery: the left electrode to the negative terminal and the right one to the positive terminal. In the first step the switch is open and both tubes are full of water. In the second step the switch is closed, bubbles rise from both electrodes and gas starts to gather at the tops. In the third step the left tube holds twice as much gas as the right one; the gases are labelled hydrogen and oxygen, in a two to one ratio.

Activity 1.8NCERT Activity 1.8 · p. 9

Sunlight on silver chloride

You’ll need

about 1 g of white silver chloride
a china dish and a spatula
a coin
bright sunlight

Predict first

You put a coin on part of the white powder and leave the dish in the sun. What will you see when you lift the coin?

All the powder is still whiteThe whole layer has turned greyThe part under the coin is grey and the rest is whiteThe uncovered powder has gone grey; the part under the coin is still white

Spread the silver chloride in a thin layer in the dish with the spatula. (Silver salts leave dark stains on skin.)
Press a coin gently onto the middle of the layer.
Leave the dish in bright sunlight for 15–20 minutes.
Lift the coin and compare the two parts of the layer.

silver chloride in light

\begin{aligned} 2 A g C l (s) \overset{sunlight}{\to} \\ 2 A g (s) + C l_{2} (g) \end{aligned}

(1.22)

Silver bromide behaves the same way, and it was the light-sensitive chemical in black-and-white film:

silver bromide in light

\begin{aligned} 2 A g B r (s) \overset{sunlight}{\to} \\ 2 A g (s) + B r_{2} (g) \end{aligned}

(1.23)

3/3Exposed part grey (silver); covered part still white.

Fig. 1.10Light decomposes silver chloride only where it falls.NCERT Fig. 1.7 · p. 9

A dish of silver chloride is seen from above. In the first step the layer is white all over. In the second step a coin lies on the middle of the layer and the sun shines on the dish. In the third step the coin has been lifted: the exposed powder is grey because silver has formed, while a white disc remains where the coin blocked the light.

Look back at the last four activities. The burner, the battery and the sun were all doing work, pouring energy in to pull compounds apart. A reaction that soaks up energy like this is an endothermic reaction (endo means in). Some take in so much heat from their surroundings that the flask feels cold.

Definition

Endothermic reaction

A reaction that absorbs energy, usually as heat, from its surroundings.

ExampleLimestone breaking down in a hot kiln.

Try thisNCERT Carry out the following Activity · p. 10

A reaction that freezes water

You’ll need

a spatula of barium hydroxide crystals
half a spatula of ammonium chloride
a dry test tube and a glass rod

Predict first

What will you feel at the bottom of the tube after the solids are mixed?

It gets very coldIt gets hotNo change at allIt warms a little, then explodes

Your teacher mixes the two solids in the dry test tube with the glass rod.
Touch the bottom of the tube with your palm.
For a bigger effect, the mixture can be made in a small flask standing in a puddle of water on a wooden block. Wait two minutes, then try to lift the flask.

Quick check

1 of 4

Match each decomposition with the energy that drives it.

Tap an item on the left, then its match on the right.

Show answer

Answer

Limestone to quicklime : heat
Silver bromide in film : light
Water to hydrogen and oxygen : electricity

A kiln heats limestone, light darkens silver bromide, and a battery splits water.

Practice · 2 questions

Pause and practise 2: questions with answers

1
Short answerEasyNCERT in-text Questions p. 10, Q1
A painter buys white lumps marked “P” from a hardware shop and drops them into a drum of water. The water hisses and gets hot, and the milky liquid is later brushed onto walls.
1. Name P and give its formula.
  Show solution
  AnswerQuicklime, calcium oxide, $C a O$ .
  P is quicklime, calcium oxide, $C a O$ . It reacts vigorously with water, which explains the hissing and heat.
2. Write the equation for its reaction with water.
  Show solution
  $C a O (s)$ + $H_{2} O (l)$ → $C a (O H)_{2} (a q)$
  Slaked lime, calcium hydroxide, is the whitewash; heat is given out.
2
NumericalMediumNCERT in-text Questions p. 10, Q2
In the electrolysis of acidified water, 30 mL of gas collects over the electrode joined to the negative terminal.
1. How much gas, in mL, collects over the other electrode in the same time?
  mL
  Show answer
  Answer 15 mL
  Hydrogen collects at the negative electrode and oxygen at the positive one, in a 2 : 1 ratio, so $30 \div 2 = 15 m L$ of oxygen.
2. Name both gases and explain why one volume is double the other.
  Show solution
  The 30 mL is hydrogen and the 15 mL is oxygen. In $2 H_{2} O \overset{}{\to} 2 H_{2} + O_{2}$ , every two water molecules give two hydrogen molecules but only one oxygen molecule. Under the same conditions equal volumes of gases hold equal numbers of molecules, so hydrogen takes up twice the volume.

1.2.3 Displacement Reaction

Activity 1.9NCERT Activity 1.9 · p. 10

An iron nail in a blue solution

You’ll need

two iron nails and sandpaper
copper sulphate solution (neela thotha dissolved in water)
two small glass bowls
a pair of tongs

Predict first

After half an hour in copper sulphate solution, what will the nail look like?

Unchanged and greyCovered with blue crystalsDissolved away completelyCoated with a brownish layer

Sand both nails until they shine. Keep one dry, for comparison.
Pour copper sulphate solution into the two bowls to the same depth.
Lay the other nail in one bowl. Leave the second bowl as it is.
Look after 5 minutes and again after half an hour. Then lift the nail out with the tongs and compare it with the dry nail, and compare the colours of the two bowls.

Table 1.7

What to compare	After 5 min	After 30 min
Surface of the nail
Colour of the solution

Your entries are saved automatically.

iron displaces copper

\begin{aligned} F e (s) \\ + C u S O_{4} (a q) \\ \overset{}{\to} F e S O_{4} (a q) \\ + C u (s) \end{aligned}

(1.24)

3/3Close-up: iron goes into the solution, copper comes out.

Fig. 1.11Iron displaces copper from copper sulphate solution.NCERT Fig. 1.8 · p. 10

Two glass bowls of copper sulphate solution, labelled blue, sit side by side, and the right bowl has an iron nail lying in it. In the second step, after half an hour, the solution in the right bowl is labelled pale green and the nail has a brown coating labelled copper, while the left bowl is still blue. In the third step a magnified circle shows the surface of the nail: iron atoms leave as iron ions into the solution, and copper ions from the solution settle on the nail as copper atoms.

In this displacement reaction, a more reactive element pushes a less reactive one out of its compound. Iron is more reactive than copper, so it takes the sulphate and leaves copper as metal. Zinc does the same, and the blue solution turns colourless because zinc sulphate has no colour:

zinc displaces copper

\begin{aligned} Z n (s) \\ + C u S O_{4} (a q) \\ \overset{}{\to} Z n S O_{4} (a q) \\ + C u (s) \end{aligned}

(1.25)

Swap the roles and nothing happens. A copper wire left in iron sulphate solution stays bright, because copper is less reactive than iron and cannot push it out. In Chapter 3 you will rank metals in a reactivity series.

Definition

Displacement reaction

A reaction in which a more reactive element takes the place of a less reactive element in its compound.

ExampleIron displacing copper from copper sulphate solution.

1.2.4 Double Displacement Reaction

Activity 1.10NCERT Activity 1.10 · p. 11

Clear plus clear gives white

You’ll need

sodium sulphate solution
barium chloride solution
two test tubes
a measuring cylinder or dropper

Predict first

What will you see when the two clear solutions are mixed?

A white solid that clouds the liquidA blue solutionBubbles of gasNothing at all

Put 2 mL of barium chloride solution in a test tube and hold it against a dark background.
Add sodium sulphate solution a few drops at a time, shaking gently after each addition.
Stop when no more cloudiness appears, then let the tube stand for five minutes.

a white precipitate

\begin{aligned} N a_{2} S O_{4} (a q) \\ + B a C l_{2} (a q) \\ \overset{}{\to} B a S O_{4} (s) \\ + 2 N a C l (a q) \end{aligned}

(1.27)

3/3Barium sulphate settles as a precipitate.

Fig. 1.12In a double displacement, two compounds swap partners.NCERT Fig. 1.9 · p. 11

Ions are drawn as labelled shapes: sodium and barium as circles, sulphate and chloride as rounded squares. In the first step sodium is paired with sulphate and barium with chloride, as in the two starting solutions. In the second step crossing arrows show the partners swapping. In the third step barium sulphate sits as a white solid at the bottom of a beaker, labelled precipitate, while sodium and chloride stay dissolved in the liquid above it.

Picture each dissolved compound as pairs of dance partners: in one beaker sodium ions with sulphate ions, in the other barium ions with chloride ions. Pour the beakers together and the partners swap. Barium now meets sulphate, and that pair cannot stay dissolved, so it drops out as a solid. Sodium and chloride ions keep floating free in the water:

the ions that react

\begin{aligned} B a^{2 +} (a q) \\ + S O_{4}^{2 -} (a q) \\ \overset{}{\to} B a S O_{4} (s) \end{aligned}

Because the two compounds have exchanged partners, this is a double displacement reaction. Reactions like this, where an insoluble solid appears, are also called precipitation reactions, and many double displacements are of this kind. Add sodium hydroxide solution to blue copper sulphate solution, for instance, and a pale blue, jelly-like precipitate of copper hydroxide appears:

a blue precipitate

\begin{aligned} C u S O_{4} (a q) \\ + 2 N a O H (a q) \\ \overset{}{\to} C u (O H)_{2} (s) \\ + N a_{2} S O_{4} (a q) \end{aligned}

Definition

Double displacement reaction

A reaction in which two compounds exchange their ions to form two new compounds.

ExampleSodium sulphate and barium chloride giving barium sulphate and sodium chloride.

Definition

Precipitate

An insoluble solid that forms when solutions react; it clouds the liquid and then settles.

ExampleWhite barium sulphate; yellow lead iodide.

Quick check

1 of 3

When sodium sulphate and barium chloride solutions are mixed, which substance is the precipitate?

Sodium chlorideBarium chlorideSodium sulphateBarium sulphate

Show answer

Answer (D) Barium sulphate

Barium sulphate is insoluble and comes out as a white solid. Sodium chloride is a product too, but it stays dissolved.

1.2.5 Oxidation and Reduction

copper gains oxygen

\begin{aligned} 2 C u + O_{2} \\ \overset{heat}{\to} 2 C u O \end{aligned}

(1.28)

copper oxide loses oxygen

\begin{aligned} C u O + H_{2} \\ \overset{heat}{\to} C u + H_{2} O \end{aligned}

(1.29)

Follow the oxygen atom in (1.29). It starts attached to copper and ends attached to hydrogen. Chemists have a word for each end of that journey. Whatever gains oxygen has been oxidised (oxidation); whatever loses oxygen has been reduced (reduction). So hydrogen is oxidised to water, and copper oxide is reduced to copper.

An oxygen atom cannot leave one substance without landing on another, so the two processes always come as a pair. A reaction that contains both is a redox reaction, short for reduction–oxidation.

Definition

Oxidation

The gain of oxygen, or the loss of hydrogen, by a substance in a reaction.

ExampleCopper turning into copper oxide when heated in air.

Definition

Reduction

The loss of oxygen, or the gain of hydrogen, by a substance in a reaction. It always happens together with oxidation.

ExampleCopper oxide turning back into copper in hydrogen.

Iron is made by a redox reaction on a giant scale. In the blast furnaces of steel plants such as Bhilai and Jamshedpur, carbon monoxide pulls the oxygen out of iron ore. Fig. 1.13 tracks where the oxygen goes.

3/3 $C O$ gains oxygen: oxidised.

Fig. 1.13In a blast furnace, iron ore is reduced while carbon monoxide is oxidised.

The equation in which Fe2O3 plus 3CO gives 2Fe plus 3CO2 is written across the middle. In the first step it stands alone. In the second step an arrow below the equation runs from Fe2O3 to 2Fe, labelled loses oxygen and reduced. In the third step an arrow above it runs from 3CO to 3CO2, labelled gains oxygen and oxidised, so both halves of the redox reaction are marked.

Hydrogen works as a marker too, in the opposite sense: losing hydrogen is oxidation and gaining hydrogen is reduction. When hydrogen sulphide meets chlorine, the hydrogen sulphide loses its hydrogen and becomes sulphur, so it is oxidised. Chlorine gains that hydrogen and becomes hydrogen chloride, so it is reduced:

redox without oxygen

\begin{aligned} H_{2} S + C l_{2} \\ \overset{}{\to} 2 H C l + S \end{aligned}

Practice · 3 questions

Pause and practise 3: questions with answers

Short answerEasyNCERT in-text Questions p. 13, Q1
A student stirs blue copper sulphate solution with a zinc spatula for a few minutes. The blue colour slowly fades. Explain why, with an equation.
Show solution
Zinc is more reactive than copper, so it displaces copper from copper sulphate. Copper is deposited as a brown layer on the spatula, and the blue copper sulphate is replaced by colourless zinc sulphate, so the colour fades.
$Z n (s)$ + $C u S O_{4} (a q)$ → $Z n S O_{4} (a q)$ + $C u (s)$
Short answerMediumNCERT in-text Questions p. 13, Q2
Name two solutions that give a coloured precipitate when mixed, by double displacement. Write the equation, and name the precipitate and its colour.
Show solution
Copper sulphate with sodium hydroxide gives a pale blue precipitate of copper hydroxide:
$C u S O_{4} (a q)$ + $2 N a O H (a q)$ → $C u (O H)_{2} (s)$ + $N a_{2} S O_{4} (a q)$
Another answer: lead nitrate with potassium iodide gives a yellow precipitate of lead iodide.
3
Short answerMediumNCERT in-text Questions p. 13, Q3
Name the substance oxidised and the substance reduced in each reaction.
1. Tin from its ore: $S n O_{2} + 2 C \overset{}{\to} S n + 2 C O$
  Show solution
  $S n O_{2}$ loses oxygen, so it is reduced. Carbon gains oxygen to become CO, so it is oxidised.
2. Lead from its oxide: $P b O + C O \overset{}{\to} P b + C O_{2}$
  Show solution
  $P b O$ loses oxygen and is reduced. Carbon monoxide gains oxygen to become $C O_{2}$ and is oxidised.

1.3 Have You Observed the Effects of Oxidation Reactions in Everyday Life?

Oxidation is not only a laboratory idea. About a fifth of the air is oxygen, and it slowly oxidises the things around us. Two everyday results are corrosion and rancidity.

1.3.1 Corrosion

An iron gate painted last Diwali may show orange-brown flakes by the end of the monsoon. That flaky layer is rust, formed when iron reacts with oxygen and water. Other metals suffer too: a silver payal slowly turns black, and a copper or brass lota grows a dull green film. This slow eating-away of a metal by air, water, acids or other chemicals in its surroundings is called corrosion.

3/3Months later: rust, black tarnish and green coating.

Fig. 1.14Three metals, three kinds of corrosion.

Three objects stand side by side: a section of an iron gate, a silver anklet and a copper pot. In the first step all three are new and shiny. In the second step arrows labelled air and moisture reach all three. In the third step the gate has reddish-brown rust patches, the anklet has turned black and the pot has green patches, each labelled with the name of its coating.

Strictly, the black layer on silver is silver sulphide, formed with traces of sulphur compounds in the air, and the green film on copper is a basic copper carbonate built from air, water and carbon dioxide. Each is a slow reaction between a metal and its surroundings.

Rust is costly. A rusted bolt can bring down a signboard, a corroded water pipe leaks, and railway lines and sea-side structures need constant checking and repainting. So we put a barrier between the iron and the air: railings are painted, bicycle chains are oiled and many buckets are coated with zinc. Chapter 3 explains rusting and its prevention in more detail.

Definition

Corrosion

The slow damage of a metal by air, moisture, acids or other substances around it.

ExampleRusting of iron; black tarnish on silver; the green coat on copper.

1.3.2 Rancidity

Open a forgotten packet of namkeen, or smell oil that has been used for frying again and again. The stale, unpleasant smell and bitter taste are signs of rancidity: air has oxidised the fats and oils in the food. Light and warmth speed it up.

2/2Nitrogen inside: the chips stay fresh.

Fig. 1.15Nitrogen keeps oxygen away from the oil in chips.

Two sealed chip packets are drawn side by side. In the first step only the left packet is shown: it holds air, with oxygen molecules drawn as pairs of O circles among the chips, and wavy lines rising for a stale smell. In the second step the right packet appears, filled with nitrogen molecules drawn as pairs of N circles; its chips have no smell lines and are labelled fresh.

We slow rancidity by keeping oxygen away, or by slowing the oxidation:

Store food in airtight containers, in a cool, dark place or in the fridge.
Add antioxidants, substances that react with oxygen more readily than the fat does and so protect it. Vitamin E does this naturally in many oils; packet labels list others such as BHA or BHT.
Fill packets with nitrogen. Chip makers flush bags with nitrogen before sealing them, which is why a packet feels puffed up. Nitrogen does not react with the oil, so the chips stay crisp and fresh for much longer.

Definition

Rancidity

The oxidation of fats and oils in food by air, which spoils their smell and taste.

ExampleOld namkeen that smells stale.

Recap

Chemical Reactions and Equations: key points and summary

A reaction makes something new. Watch for a new colour, a gas, a temperature change or a solid appearing (Spotting a reaction).
A word equation names the reactants and products; a chemical equation uses formulae, with reactants on the left of the arrow and products on the right (Section 1.1).
A reaction only rearranges atoms, so every element must be counted equally on the two sides of the arrow: the equation must balance (Section 1.1.2).
Balance by changing coefficients only, never subscripts, and end with the smallest whole numbers (hit-and-trial).
State symbols (s), (l), (g) and (aq), and conditions written on the arrow, make an equation more informative (state symbols).
Combination: several reactants, one product, as when quicklime meets water (Section 1.2.1).
Decomposition: one compound splits into simpler substances, driven by heat, light or electricity (Section 1.2.2).
Heat out (exothermic): burning, respiration, quicklime with water. Energy in (endothermic): most decompositions.
Displacement: a more reactive metal pushes a less reactive one out of its compound, as iron does to copper (Section 1.2.3).
Double displacement: two compounds swap partner ions; when the swap makes an insoluble solid, it is a precipitation reaction too (Section 1.2.4).
A substance that takes in oxygen or gives up hydrogen is oxidised; one that gives up oxygen or takes in hydrogen is reduced. The two changes always come as a pair, in redox reactions (Section 1.2.5).
Corrosion of metals and rancidity of fats and oils are everyday effects of oxidation (Section 1.3).

Group activityNCERT Group Activity · p. 16

Hot or cold? Four reactions, one thermometer

You’ll need

four glasses marked A, B, C and D
a thermometer
fruit salt (such as ENO) and water
baking soda and vinegar
a pad of steel wool and vinegar
dilute hydrochloric acid and dilute sodium hydroxide solution (school lab)

Predict first

Which mixtures do you expect to get warmer?

A: fruit salt in waterB: baking soda in vinegarC: steel wool soaked in vinegarD: acid mixed with alkali

Select all that apply

Pour 50 mL of water into A and 50 mL of vinegar into B. Measure and record each temperature.
Add a spoon of fruit salt to A and a spoon of baking soda to B. Stir, wait a minute and record the temperatures again.
Soak the steel wool in vinegar for a minute and squeeze it out. Wrap it round the thermometer bulb inside glass C and cover the glass with paper. Record the temperature now and after five minutes.
In D, your teacher mixes 20 mL of dilute hydrochloric acid with 20 mL of dilute sodium hydroxide solution. Record the temperature before and after mixing.

Table 1.8

Glass	Start (°C)	End (°C)	Exo or endo?
A
B
C
D

Your entries are saved automatically.

A and B cool by a few degrees, so these reactions are endothermic; the fizz is carbon dioxide made by the reaction. C warms slowly: the vinegar strips the steel wool's protective film and the iron starts to rust, an exothermic oxidation. D warms quickly, because an acid reacting with an alkali releases heat. Comparing start and end temperatures is the simplest way to sort exothermic from endothermic reactions.

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 Questions p. 6, Q1	Pause and practise 1 · Q1
NCERT in-text Questions p. 6, Q2	Pause and practise 1 · Q2
NCERT in-text Questions p. 6, Q3	Pause and practise 1 · Q3
NCERT in-text Questions p. 10, Q1	Pause and practise 2 · Q1
NCERT in-text Questions p. 10, Q2	Pause and practise 2 · Q2
NCERT in-text Questions p. 13, Q1	Pause and practise 3 · Q1
NCERT in-text Questions p. 13, Q2	Pause and practise 3 · Q2
NCERT in-text Questions p. 13, Q3	Pause and practise 3 · Q3
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
NCERT Exercises Q16	Exercises · Q16
NCERT Exercises Q17	Exercises · Q17
NCERT Exercises Q18	Exercises · Q18
NCERT Exercises Q19	Exercises · Q19
NCERT Exercises Q20	Exercises · Q20

Quick check: MCQs on Chemical Reactions and Equations

Spotting a chemical reaction3 questions
Writing physical states4 questions
1.2.1 Combination Reaction3 questions
1.2.2 Decomposition Reaction4 questions
1.2.3 Displacement Reaction3 questions
1.2.4 Double Displacement Reaction3 questions
1.2.5 Oxidation and Reduction3 questions
1.3.2 Rancidity3 questions

Glossary · 15

Key terms (English–Hindi)

balanced equationसंतुलित समीकरणsantulit samikaran: An equation with the same number of atoms of each element on both sides. An unbalanced draft is a skeletal equation.First seen in the chapter →

chemical equationरासायनिक समीकरणrasayanik samikaran: A reaction written with chemical formulae: reactants on the left, an arrow, products on the right.First seen in the chapter →
combination reactionसंयोजन अभिक्रियाsanyojan abhikriya: Two or more reactants join to form a single product.First seen in the chapter →
corrosionसंक्षारणsankshaaran: The slow eating-away of a metal by air, moisture, acids or other substances around it.First seen in the chapter →

decomposition reactionवियोजन अभिक्रियाviyojan abhikriya: One reactant breaks down into two or more simpler products.First seen in the chapter →
displacement reactionविस्थापन अभिक्रियाvisthapan abhikriya: A more reactive element takes the place of a less reactive one in its compound.First seen in the chapter →
double displacement reactionद्विविस्थापन अभिक्रियाdvivisthapan abhikriya: Two compounds exchange ions to form two new compounds.First seen in the chapter →

endothermic reactionऊष्माशोषी अभिक्रियाushmashoshi abhikriya: A reaction that takes in energy, usually as heat.First seen in the chapter →
exothermic reactionऊष्माक्षेपी अभिक्रियाushmakshepi abhikriya: A reaction that gives out heat.First seen in the chapter →

oxidationउपचयनupachayan: Gain of oxygen or loss of hydrogen by a substance.First seen in the chapter →

precipitateअवक्षेपavakshep: An insoluble solid that forms in a solution during a reaction.First seen in the chapter →
productउत्पादutpad: A new substance formed in a reaction.First seen in the chapter →

rancidityविकृतगंधिताvikritgandhita: The spoiling of fats and oils in food by oxidation, which changes their smell and taste.First seen in the chapter →
reactantअभिकारकabhikarak: A substance that is used up and changed in a reaction.First seen in the chapter →
reductionअपचयनapachayan: Loss of oxygen or gain of hydrogen by a substance. It always pairs with oxidation (a redox reaction).First seen in the chapter →

Chemical Reactions and Equations: Class 10 Science · Chapter 1

What you will learn in Chemical Reactions and Equations

Spotting a chemical reaction

1.1 Chemical Equations

1.1.1 Writing a Chemical Equation

1.1.2 Balanced Chemical Equations

Balancing by hit-and-trial

Writing physical states

Pause and practise 1: questions with answers

1.2 Types of Chemical Reactions

1.2.1 Combination Reaction

1.2.2 Decomposition Reaction

Pause and practise 2: questions with answers

1.2.3 Displacement Reaction

1.2.4 Double Displacement Reaction

1.2.5 Oxidation and Reduction

Pause and practise 3: questions with answers

1.3 Have You Observed the Effects of Oxidation Reactions in Everyday Life?

1.3.1 Corrosion

1.3.2 Rancidity

Chemical Reactions and Equations: key points and summary

Practice questions with answers (NCERT “Exercises”)

Find your NCERT exercise question

Quick check: MCQs on Chemical Reactions and Equations

Key terms (English–Hindi)

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