Carbon and its Compounds: Class 10 Science · Chapter 4

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

Published 12 Oct 2026 · Updated 12 Oct 2026

105 min
32 practice questions
24 MCQs

What you will learn in Carbon and its Compounds

Explain why carbon forms covalent bonds and draw electron-dot structures of molecules with single, double and triple bonds
Relate the structures of diamond, graphite and fullerenes to their properties
Use catenation and tetravalency to explain the variety of carbon compounds, and tell saturated and unsaturated chains, branches, rings and isomers apart
Identify functional groups and homologous series, and name simple carbon compounds by IUPAC rules
Describe combustion, oxidation, addition and substitution reactions with balanced equations
Describe the properties and reactions of ethanol and ethanoic acid, including esterification and saponification
Explain how soaps and detergents clean through micelles and why soap forms scum in hard water

Original companion text aligned to NCERT Science Class 10, Chapter 4, pp. 58–78. Not affiliated with or endorsed by NCERT.

Make a quick list of what is in your kitchen: LPG in the cylinder, rice and dal, a plastic bucket, a cotton dishcloth. They look and behave nothing alike, yet nearly all of them are built around one element, carbon (कार्बन).

Carbon is not common. It is only about 0.02 % of the Earth's crust, mostly locked up in coal, petroleum and carbonate rocks such as limestone. Carbon dioxide is roughly 0.04 % of the air. Yet every living cell, from a tulsi leaf to your own skin, is made of carbon compounds. This chapter asks what makes carbon so special, and then looks at carbon compounds you meet every day: fuels, alcohol, vinegar, soaps and detergents.

Activity 4.1NCERT Activity 4.1 · p. 58

What is your morning made of?

You’ll need

a notebook and pen
two or three classmates

Predict first

Sort everything you used this morning into three groups. Which group do you expect to be the biggest?

Things made of metalThings made of glass or clayEverything else: food, cloth, paper, plastic, wood

Write down twelve things you used or ate between waking up and reaching school: toothpaste, tea, a steel tumbler, your uniform, a notebook and so on.
Put your lists together with two or three classmates.
Sort every item into the table. If an item is made of two materials, like a pressure cooker with a plastic handle, tick both columns.

Table 4.1Sort your items

Item	Metal	Glass or clay	Something else

Your entries are saved automatically.

4.1 Bonding in Carbon – The Covalent Bond

In Chapter 3 you met ionic compounds such as common salt: hard solids that melt only when very hot and conduct electricity when molten or dissolved. Carbon compounds behave very differently. Most of them are gases, liquids or soft solids that melt easily, and almost none conduct electricity. Compare the numbers in Table 4.2.

Table 4.2Carbon compounds melt and boil far below an ionic compound

Compound	Where you meet it	Melts at (°C)	Boils at (°C)
Butane, $C_{4} H_{10}$	LPG cylinder	−138	−0.5
Propanone, $C H_{3} C O C H_{3}$	nail-polish remover	−95	56
Ethanol, $C_{2} H_{5} O H$	hand sanitiser	−114	78
Naphthalene, $C_{10} H_{8}$	mothballs	80	218
Sodium chloride, $N a C l$ (ionic)	table salt	801	1465

What do these numbers tell us? To melt or boil a substance, heat must pull its particles away from one another. Salt needs over 800 °C because its ions are locked together by strong attractions. LPG needs almost no heat at all, so its molecules can only be clinging to each other loosely. And a substance that carries no current has no free ions. Whatever holds the atoms of a carbon compound together, it is not the electron transfer that makes ions.

Carbon has atomic number 6, so its electrons are arranged as 2 in the K shell and 4 in the L shell. With four valence electrons it is exactly halfway to an octet, and both obvious ways of completing it are blocked:

Gain four? The ion $C^{4 -}$ would carry ten electrons but have only six protons to attract them. The four newcomers would be held so loosely that the ion could not hold together.
Lose four? Each electron removed makes the next one harder to pull away. Stripping off all four to leave a tiny $C^{4 +}$ ion with only two electrons would cost an enormous amount of energy.

2/3As $C^{4 -}$ , six protons would have to hold ten electrons.

Fig. 4.1Carbon is stuck halfway: gaining or losing four electrons are both too costly.NCERT p. 59

A carbon atom drawn as a nucleus marked 6+ with two electrons in the inner shell and four in the outer shell. Step 1 labels it carbon, atomic number 6, shells 2, 4, with four valence electrons. Step 2 adds four extra electrons as crosses on the outer shell to make C4−, and notes that six protons must hold ten electrons, a grip that is too weak. Step 3 shows the four outer electrons moving away to leave C4+, and notes that pulling them off costs a huge amount of energy.

Carbon takes a third way: it shares. Picture two neighbours who each drop one coin into a joint money box; both can count the box as their own. Two atoms can likewise each put one electron into a pair that sits between them, and both count that pair in their outer shell. Hydrogen, oxygen, nitrogen and chlorine bond like this too, and their small molecules show the idea most clearly. Step through Fig. 4.2 as you read.

Hydrogen, $H_{2}$ . A hydrogen atom has one electron and needs one more to match the full K shell of helium. Two hydrogen atoms each put their electron into one shared pair, and both are satisfied. One shared pair is a single covalent bond, drawn as a line: $H - H$ .

Oxygen, $O_{2}$ . Oxygen (2, 6) is two electrons short of an octet. Each atom puts two electrons into the space between them, making two shared pairs: a double bond, $O = O$ . Each oxygen also keeps two pairs to itself, called lone pairs.

Nitrogen, $N_{2}$ . Nitrogen (2, 5) needs three more electrons, so each atom puts three into the middle. Three shared pairs make a triple bond, $N \equiv N$ . This bond is very strong, one reason why the nitrogen of the air is so unreactive.

Methane, $C H_{4}$ . Methane is the main gas in CNG and in biogas from a gobar gas plant. Its carbon shares one pair with each of four hydrogen atoms, so carbon is surrounded by eight electrons and each hydrogen by two. Because carbon forms four bonds in its compounds, we say it is tetravalent.

4/4 $C H_{4}$ : carbon shares a pair with each of four H atoms.

Fig. 4.2Shared pairs (shaded) become bond lines: one, two or three pairs make single, double or triple bonds.NCERT Figs 4.1–4.5 · p. 60

Each step shows one molecule as an electron-dot structure on the left and as a bond-line formula on the right. Electrons from one atom are dots and from the other are crosses; shared pairs sit in shaded bands between the atoms. Step 1: H2, one shared pair, the single bond H–H. Step 2: O2, two shared pairs and two lone pairs on each oxygen, a double bond. Step 3: N2, three shared pairs and one lone pair on each nitrogen, a triple bond. Step 4: CH4, carbon sharing one pair with each of four hydrogen atoms, drawn as four single bonds.

Definition

Covalent bond

A bond formed when two atoms share a pair of electrons, so that each atom reaches a stable, full outer shell. Atoms may share one, two or three pairs.

Example $H - H$ (one pair), $O = O$ (two pairs), $N \equiv N$ (three pairs).

Why are covalent compounds so easy to melt? Think of a net bag full of footballs. Each ball is stitched together strongly, but the balls only rest loosely against one another, so tipping them out is easy. In the same way, the covalent bonds inside a molecule are strong, but the forces between molecules are weak. Little heat is needed to pull the molecules apart, so melting and boiling points are low. And since sharing creates no ions, these compounds are generally poor conductors of electricity.

Try this

Your turn: four more molecules

Draw electron-dot structures for chlorine ( $C l_{2}$ ; Cl is 2, 8, 7), water ( $H_{2} O$ ), ammonia ( $N H_{3}$ ; N is 2, 5) and carbon dioxide ( $C O_{2}$ ). Show only the valence electrons. Decide whether each bond is single, double or triple, then open the answer.

Fig. 4.3Answers: chlorine, water and ammonia have single bonds; carbon dioxide has two double bonds.NCERT p. 60

Four electron-dot structures in a grid, with shared pairs in shaded bands. Chlorine, Cl2: one shared pair and three lone pairs on each chlorine. Water, H2O: oxygen shares one pair with each hydrogen and keeps two lone pairs. Ammonia, NH3: nitrogen shares one pair with each of three hydrogens and keeps one lone pair. Carbon dioxide, CO2: carbon shares two pairs with each oxygen, and each oxygen keeps two lone pairs.

Chlorine and water have only single bonds. Ammonia has three single bonds and a lone pair on nitrogen. Carbon dioxide has two double bonds, $O = C = O$ , so carbon again makes four bonds.

Example 4.1

Electron dots for methanal

Methanal, $H C H O$ , is the gas dissolved in formalin, used to preserve specimens in a biology lab. Its carbon is bonded to two hydrogen atoms and one oxygen atom. Draw its electron-dot structure and say what kind of bond joins C and O.

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

Allotropes of carbon

Pure carbon itself comes in several forms. A diamond in a ring, the graphite "lead" in your pencil and the soot on a lamp chimney are all nothing but carbon atoms. Different forms of the same element, with their atoms arranged differently, are called allotropes.

Definition

Allotropes

Different forms of the same element in which the atoms are bonded in different arrangements.

ExampleDiamond, graphite and fullerenes are allotropes of carbon.

Diamond's atoms are linked in every direction. Each carbon holds four neighbours placed at the corners of a tetrahedron, those neighbours hold four more, and so on right through the crystal: one giant, rigid three-dimensional network. That makes diamond the hardest natural substance. It does not conduct electricity, because every valence electron is tied up in a bond.

Graphite is built in flat sheets. Within a sheet every carbon is joined to just three neighbours, forming a honeycomb of hexagons. To keep carbon's valency at four, one of those three links is pictured as a double bond. The sheets are stacked loosely, like the pages of a book, and slide over one another easily. So graphite is soft and slippery: it leaves a mark on paper and works as a lubricant. Electrons can travel along the sheets, so graphite, unusually for a non-metal, conducts electricity and is used for electrodes.

2/3Each C bonded to three in flat sheets that slide.

Fig. 4.4Same atoms, different arrangements: diamond, graphite and

C_{60}

.NCERT More to Know · p. 61

Step 1, diamond: a carbon atom bonded to four neighbours, each of which bonds to more atoms, forming a rigid three-dimensional network; labels say each carbon is bonded to four, it is the hardest natural solid and does not conduct. Step 2, graphite: one flat layer of hexagonal rings and a side view of three stacked layers with the middle one shifted to show sliding; labels say each carbon is bonded to three in sheets and graphite conducts electricity. Step 3, fullerene: a hollow ball of 60 carbon atoms made of pentagons and hexagons, like a football.

Compare

Diamond and graphite

Diamond

Each C bonded to 4 others
Rigid 3-D network
Hardest natural substance
Does not conduct electricity
Jewellery, cutting and drilling tools

Graphite

Each C bonded to 3 others in a plane
Layers of hexagons that slide
Soft and slippery
Good conductor of electricity
Pencils, lubricants, electrodes

Since both are pure carbon, diamond and graphite have the same chemical properties: each burns in plenty of oxygen to give only carbon dioxide. Their physical properties differ because their atoms are arranged differently.

Quick check

1 of 4

Why does carbon form covalent bonds instead of ions?

It has too few protons to attract any electronsCarbon atoms are too large to form ionsIt already has a full outer shellGaining or losing four electrons is too hard, so it shares

Show answer

Answer (D) Gaining or losing four electrons is too hard, so it shares

With four valence electrons, carbon would have to gain or lose four. A

C^{4 -}

ion holds its extra electrons too weakly, and making

C^{4 +}

costs too much energy, so carbon shares. Carbon is in fact a small atom.

Practice · 2 questions

Practice 4.1 (NCERT in-text questions, p. 61): questions with answers

Short answerMediumNCERT in-text Q1, p. 61
Carbon disulphide, $C S_{2}$ , is a liquid used in making viscose rayon. Sulphur (2, 8, 6) has six valence electrons, like oxygen. Draw the electron-dot structure of $C S_{2}$ and name the type of each bond.
Show solution
Answer $S = C = S$ : two double bonds; each S keeps two lone pairs.
Carbon has 4 valence electrons and each S has 6. Carbon shares two pairs with each sulphur, so it has 4 shared pairs (8 electrons). Each sulphur has 2 shared pairs and 2 lone pairs, also 8.
Check: $4 + 6 + 6 = 16$ electrons, or 8 pairs: 4 bonding and 4 lone ✓. The structure is $S = C = S$ , just like $O = C = O$ .
Short answerMediumNCERT in-text Q2, p. 61
Sulphur also forms rings. Draw the electron-dot structure of a ring of six sulphur atoms, $S_{6}$ . How many shared pairs and lone pairs does the molecule have?
Show solution
Answer6 shared pairs (single bonds) and 12 lone pairs.
Sulphur needs two more electrons, so it forms two single bonds, one with each neighbour in the ring. Six atoms in a ring give six S–S bonds.
Each S uses 2 of its 6 valence electrons in bonds and keeps 4 as two lone pairs: $6 \times 2 = 12$ lone pairs. Check: 36 electrons = 18 pairs = 6 + 12 ✓.

4.2 Versatile Nature of Carbon

Make a list of every substance chemists have ever described and most of the entries, many millions of them, contain carbon. The other hundred-odd elements together account for far fewer. Your body, your food and your clothes are carbon compounds. What lets one element build so much? Two of its properties work together.

(i) Catenation. Carbon atoms bond strongly to one another. Since each carbon can link to as many as four others, carbon builds chains hundreds or thousands of atoms long, chains with side branches, and closed rings. This self-linking is called catenation. The links between carbons may be single, double or triple bonds, a difference that sorts compounds into two kinds in Section 4.2.1.

Carbon's nearest rival is silicon, the element below it in the periodic table. Silicon–hydrogen compounds stop at chains of about seven or eight silicon atoms, and even these break down or catch fire easily. Carbon–carbon bonds are strong and stable, which is why the chains in plastics and starch can be so long.

Definition

Catenation

The ability of atoms of an element to bond to one another, forming long chains, branches or rings. Carbon shows it more than any other element.

(ii) Tetravalency. Every carbon atom has four bonds to offer. It can spend them on other carbons, or on hydrogen, oxygen, nitrogen, sulphur, chlorine and more, and each new partner changes how the compound behaves. These bonds also last. A carbon atom is small, so the shared electrons sit close to its nucleus and are gripped firmly. In larger atoms the shared pairs sit farther out and the bonds are weaker.

Definition

Tetravalency

Having a valency of four. A carbon atom forms four covalent bonds.

4.2.1 Saturated and Unsaturated Carbon Compounds

How do we work out the structure of a carbon compound from its formula? Take ethane, $C_{2} H_{6}$ . First join the two carbon atoms with a single bond. Each carbon now has three valencies left over. Fill each one with a hydrogen atom, and all six hydrogens are used up. Step through Fig. 4.5 to watch this, with the matching electron-dot structure below it.

3/4With only four H, each carbon has a valency to spare, so the carbons share a second pair: $C_{2} H_{4}$ .

Fig. 4.5Join the carbons first, then fill the free valencies; any valency left over becomes another C–C bond.NCERT Figs 4.6–4.7 · p. 63

The top half shows bond-line structures and the bottom half electron-dot structures, with carbon electrons as dots and hydrogen electrons as crosses. Step 1: two carbon atoms joined by a single bond, with dashed stubs for the three free valencies on each carbon and three unpaired electrons on each. Step 2: ethane, C2H6, each carbon bonded to three hydrogens, seven shared pairs, saturated. Step 3: ethene, C2H4, two hydrogens per carbon, so the carbons share two pairs, a double bond; six shared pairs. Step 4: ethyne, C2H2, one hydrogen per carbon and a triple bond; five shared pairs.

Try propane, $C_{3} H_{8}$ , the same way. The chain $C - C - C$ leaves 3 + 2 + 3 = 8 free valencies, exactly enough for eight hydrogens: $C H_{3} - C H_{2} - C H_{3}$ . With every bond single, propane is saturated, and like other saturated compounds it reacts only reluctantly.

Ethene, $C_{2} H_{4}$ , has two hydrogens fewer than ethane. Once its four hydrogens are in place, each carbon is still one bond short, and the two carbons fix this by sharing a second pair with each other: $C H_{2} = C H_{2}$ . Ethyne, $C_{2} H_{2}$ , is short of two bonds per carbon, so its carbons share three pairs: $H C \equiv C H$ . Ethene and ethyne are unsaturated. Their double and triple bonds make them more reactive than saturated compounds, as you will see in Section 4.3.3.

Definition

Saturated compound

A carbon compound in which the carbon atoms are joined only by single bonds.

ExampleEthane, $C H_{3} - C H_{3}$ ; propane, $C H_{3} - C H_{2} - C H_{3}$ .

Definition

Unsaturated compound

A carbon compound with at least one double or triple bond between carbon atoms.

ExampleEthene, $C H_{2} = C H_{2}$ ; ethyne, $H C \equiv C H$ .

4.2.2 Chains, Branches and Rings

Nothing stops a carbon chain at three atoms. Table 4.3 carries the family of saturated carbon–hydrogen compounds on to six carbons. Watch the boiling point climb as the chain grows.

Table 4.3The first six alkanes

Name	Formula	Condensed formula	Boils at (°C)	At 25 °C
Methane	$C H_{4}$	$C H_{4}$	−162	gas
Ethane	$C_{2} H_{6}$	$C H_{3} C H_{3}$	−89	gas
Propane	$C_{3} H_{8}$	$C H_{3} C H_{2} C H_{3}$	−42	gas
Butane	$C_{4} H_{10}$	$C H_{3} (C H_{2})_{2} C H_{3}$	−0.5	gas
Pentane	$C_{5} H_{12}$	$C H_{3} (C H_{2})_{3} C H_{3}$	36	liquid
Hexane	$C_{6} H_{14}$	$C H_{3} (C H_{2})_{4} C H_{3}$	69	liquid

Look again at butane. Four carbon atoms can be joined in two ways. They can form one unbroken chain, or a chain of three with the fourth carbon on the middle one. Filling in the hydrogens gives two different molecules with the same formula, $C_{4} H_{10}$ . The first is butane; the second is called 2-methylpropane (or isobutane). Molecules like these, sharing one molecular formula but built differently, are structural isomers.

Definition

Structural isomers

Compounds that have the same molecular formula but different arrangements of their atoms.

ExampleButane and 2-methylpropane, both $C_{4} H_{10}$ .

Carbon chains can also close into rings. Six carbons in a ring, each carrying two hydrogens, make cyclohexane, $C_{6} H_{12}$ , a saturated ring. Benzene, $C_{6} H_{6}$ , is also a ring of six carbons, but each carbon carries only one hydrogen. So the ring has three double bonds in alternate places and is unsaturated. So "chain or ring" and "saturated or unsaturated" are two separate questions you can ask about any carbon compound.

2/42-methylpropane: the same $C_{4} H_{10}$ with a branch, an isomer of butane.

Fig. 4.6A straight chain, a branched chain and two rings. Butane and 2-methylpropane are isomers.NCERT Figs 4.8–4.10 · p. 65

Carbon atoms are shown as the letter C joined by bonds; hydrogen atoms are left out of the rings. Step 1: butane, a zigzag chain of four carbons, formula C4H10, condensed CH3–CH2–CH2–CH3. Step 2: 2-methylpropane, three carbons in a row with the fourth joined to the middle one, also C4H10, marked same formula. Step 3: cyclohexane, six carbons in a hexagon with single bonds, C6H12, a saturated ring. Step 4: benzene, a six-carbon hexagon with three alternating double bonds, C6H6, an unsaturated ring.

Compounds made of carbon and hydrogen only are called hydrocarbons. They fall into three families:

Compare

Three families of hydrocarbons

Alkanes

Only single C–C bonds (saturated)
Names end in -ane
Methane, $C H_{4}$

Alkenes

At least one C=C double bond
Names end in -ene
Ethene, $C_{2} H_{4}$

Alkynes

At least one C≡C triple bond
Names end in -yne
Ethyne, $C_{2} H_{2}$

4.2.3 Will you be my Friend?

Carbon's friends are not only hydrogen atoms. Swap one hydrogen on a chain for a chlorine atom, or for an oxygen carrying a hydrogen, and you get a new compound with new habits. The one rule is that every carbon must still end up with four bonds. Atoms such as chlorine, bromine, oxygen, nitrogen and sulphur, built into a carbon compound in place of hydrogen, are called heteroatoms, meaning "other atoms".

Methanol and butanol molecules differ in size, yet both fizz with sodium and both can be oxidised to acids. What they share is the –OH group. An atom or small cluster of atoms that sets a compound's chemistry, whatever chain it hangs on, is called a functional group. In Table 4.4 the dash on each group is the bond that joins it to the chain where a hydrogen used to be.

Definition

Functional group

An atom or group of atoms attached to a carbon chain that decides the chemical properties of the compound.

Example–OH in alcohols, –COOH in carboxylic acids.

Table 4.4Common functional groups

Heteroatom	Class of compound	Functional group	Example
Cl or Br	Haloalkane	–Cl, –Br	chloroethane, $C H_{3} C H_{2} C l$
Oxygen	Alcohol	–OH	ethanol, $C H_{3} C H_{2} O H$
Oxygen	Aldehyde	–CHO	ethanal, $C H_{3} C H O$
Oxygen	Ketone	C=O inside the chain	propanone, $C H_{3} C O C H_{3}$
Oxygen	Carboxylic acid	–COOH	ethanoic acid, $C H_{3} C O O H$

6/6The end carbon becomes –COOH: butanoic acid.

Fig. 4.7Change the group and you change the family and the name. Every carbon keeps four bonds.NCERT Tables 4.3–4.4 · p. 66

A butane chain with every atom drawn. In each step the group at the right-hand end, or on the second carbon, changes and is highlighted, and the name and formula below change too. Step 1: butane, C4H10, an alkane. Step 2: an end hydrogen replaced by chlorine, 1-chlorobutane, C4H9Cl. Step 3: an –OH group, butan-1-ol, C4H10O, an alcohol. Step 4: the end carbon carries a double-bonded oxygen and a hydrogen, butanal, C4H8O, an aldehyde. Step 5: the double-bonded oxygen sits on the second carbon instead, butanone, C4H8O, a ketone. Step 6: the end carbon carries a double-bonded oxygen and an –OH, butanoic acid, C4H8O2, a carboxylic acid.

4.2.4 Homologous Series

Line up the chloroalkanes $C H_{3} C l$ , $C_{2} H_{5} C l$ , $C_{3} H_{7} C l$ and $C_{4} H_{9} C l$ . Each carries the same –Cl group, on a chain one carbon longer than the one before, and they react in almost the same ways because the –Cl group decides that. A family like this is a homologous series (समजातीय श्रेणी).

Definition

Homologous series

A family of carbon compounds with the same functional group, in which each member differs from the next by a $C H_{2}$ unit.

ExampleAlkanes $C H_{4}$ , $C_{2} H_{6}$ , $C_{3} H_{8}$ …; alcohols $C H_{3} O H$ , $C_{2} H_{5} O H$ …

Each step along a series adds one $C H_{2}$ unit. With C = 12 u and H = 1 u, that is $12 + 2 = 14$ u, so neighbouring members always differ in molecular mass by 14 u. The alkanes of Table 4.3 form such a series: $C H_{4}$ , $C_{2} H_{6}$ , $C_{3} H_{8}$ , $C_{4} H_{10}$ and so on.

Count the atoms and a pattern appears. An alkane with $n$ carbon atoms has $2 n + 2$ hydrogen atoms. A double bond takes away two hydrogens and a triple bond four, which gives the general formulas in Table 4.5. Alkenes and alkynes start at $n = 2$ , since a C=C or C≡C bond needs two carbons.

Table 4.5General formulas of the hydrocarbon series

Series	General formula	1st member	2nd	3rd
Alkanes	$C_{n} H_{2 n + 2}$	$C H_{4}$	$C_{2} H_{6}$	$C_{3} H_{8}$
Alkenes	$C_{n} H_{2 n}$	$C_{2} H_{4}$	$C_{3} H_{6}$	$C_{4} H_{8}$
Alkynes	$C_{n} H_{2 n - 2}$	$C_{2} H_{2}$	$C_{3} H_{4}$	$C_{4} H_{6}$

Look at the alcohols in Fig. 4.8: each extra $C H_{2}$ makes the molecule heavier and its boiling point higher. Physical properties creep along a series like this, one step per member. Solubility creeps too: the three smallest alcohols mix with water in any amount, while butanol dissolves only partly. Chemical behaviour does not creep. It is set by the functional group, so every member reacts in much the same way.

3/3Boiling points rise steadily: 65, 78, 97 and 118 °C.

Fig. 4.8Each member adds

C H_{2}

(14 u); the boiling point rises step by step while the chemistry stays the same.NCERT p. 67

Four columns show methanol CH3OH 32 u, ethanol C2H5OH 46 u, propan-1-ol C3H7OH 60 u and butan-1-ol C4H9OH 74 u. Step 1 shows only the formulas and masses. Step 2 adds curved arrows between neighbours, each labelled +CH2 and +14 u. Step 3 adds bars for the boiling points, 65, 78, 97 and 118 °C, rising steadily from left to right.

Example 4.2

Finding a member of a series

Write the formula of the fifth member of the alkene series. Find its molecular mass, and how much heavier it is than the fourth member (C = 12 u, H = 1 u).

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

Activity 4.2NCERT Activity 4.2 · p. 67

The acid family, one step at a time

You’ll need

a notebook
atomic masses: C = 12 u, H = 1 u, O = 16 u

Predict first

By how many mass units do you expect

C H_{3} C O O H

and

C_{2} H_{5} C O O H

to differ?

Write the formulas of the first four carboxylic acids: $H C O O H$ , $C H_{3} C O O H$ , $C_{2} H_{5} C O O H$ and $C_{3} H_{7} C O O H$ .
For each neighbouring pair, find the difference in formula and in molecular mass, and fill in the table.
Do all three pairs show the same difference? Is this family a homologous series?
Now write the first four members (up to four carbons) of two other series in Table 4.4, for example the aldehydes and the chloroalkanes.

Table 4.6Differences between neighbours

Pair	Difference in formula	Difference in mass (u)
$H C O O H$ and $C H_{3} C O O H$
$C H_{3} C O O H$ and $C_{2} H_{5} C O O H$
$C_{2} H_{5} C O O H$ and $C_{3} H_{7} C O O H$

Your entries are saved automatically.

4.2.5 Nomenclature of Carbon Compounds

With millions of compounds, chemists need names that describe the structure. The system used worldwide is set by IUPAC, the International Union of Pure and Applied Chemistry. It builds a name from two parts: a stem that counts the carbon atoms, and a prefix or suffix that names the functional group.

Count the carbon atoms in the chain, including any carbon inside –CHO or –COOH. The count gives the stem: 1 meth-, 2 eth-, 3 prop-, 4 but-, 5 pent-, 6 hex-. Five carbons with only single bonds make pentane.
Name the group. Chlorine and bromine are written as a prefix (chloro-, bromo-); most other groups as a suffix (Table 4.7).
Drop the final e before a vowel. If the suffix begins with a vowel, the -e of the alkane name goes. So pentane → pentan + -ol = pentanol; butane → butan + -one = butanone. A prefix changes nothing: chlorobutane.
Show unsaturation in the ending. A C=C bond changes -ane to -ene, and a C≡C bond to -yne: butane, butene, butyne.

Table 4.7Prefixes and suffixes

Class	Prefix or suffix	Example
Haloalkane	prefix chloro-, bromo-	chloromethane, $C H_{3} C l$
Alcohol	suffix -ol	methanol, $C H_{3} O H$
Aldehyde	suffix -al	pentanal, $C_{4} H_{9} C H O$
Ketone	suffix -one	butanone, $C H_{3} C O C_{2} H_{5}$
Carboxylic acid	suffix -oic acid	butanoic acid, $C_{3} H_{7} C O O H$
Alkene	suffix -ene	propene, $C_{3} H_{6}$
Alkyne	suffix -yne	ethyne, $C_{2} H_{2}$

Example 4.3

Naming from a condensed formula

Name these compounds: (a) $C H_{3} C H_{2} C H_{2} C H O$ (b) $C H_{3} C O C H_{2} C H_{3}$ (c) $C H_{3} C H_{2} C \equiv C H$ .

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

InteractiveCarbon compound builder

An interactive builder. The learner picks one of nine families (alkane, alkene, alkyne, chloroalkane, bromoalkane, alcohol, aldehyde, ketone, carboxylic acid) and a chain of one to six carbon atoms; choices that cannot exist, such as a one-carbon alkene, are greyed out. The widget draws the full structural formula with every atom and bond, gives the IUPAC name, with the position number where one is needed, and any common name, the formula and the molecular mass, and confirms that every carbon has four bonds. A strip lists the members of the chosen homologous series with their masses, 14 u apart. A quiz switch hides the name and offers three names to choose from.

Fig. 4.9Pick a family and a chain length to see the full structure, the name and the formula. Switch on the quiz to name it first.

Quick check

1 of 4

A five-carbon chain with an –OH group on an end carbon is named .

Show answer

Answer pentanol

pent- + -ane → pentan + -ol = pentanol (in full, pentan-1-ol).

Practice · 5 questions

Practice 4.2 (NCERT in-text questions, pp. 68–69): questions with answers

1
Short answerHardNCERT in-text Q1, p. 68
Write condensed formulas for all the structural isomers of hexane, $C_{6} H_{14}$ . How many are there?
Show solution
AnswerFive.
There are five isomers:
$C H_{3} (C H_{2})_{4} C H_{3}$ , hexane
$(C H_{3})_{2} C H (C H_{2})_{2} C H_{3}$ , 2-methylpentane
$(C_{2} H_{5})_{2} C H C H_{3}$ , 3-methylpentane
$(C H_{3})_{2} C H C H (C H_{3})_{2}$ , 2,3-dimethylbutane
$(C H_{3})_{3} C C H_{2} C H_{3}$ , 2,2-dimethylbutane
Short answerEasyNCERT in-text Q2, p. 68
A classmate says carbon forms millions of compounds "because it is a non-metal". Correct her: name the two properties of carbon that really explain it, and say what each means.
Show solution
AnswerCatenation and tetravalency.
Catenation: carbon atoms bond to one another in long chains, branches and rings. Tetravalency: each carbon forms four bonds, to carbon or to atoms such as H, O, N, S and Cl. Together they allow an enormous variety of structures. Being a non-metal has nothing to do with it; most non-metals show neither property strongly.
Short answerMediumNCERT in-text Q3, p. 68
Write the molecular formula of cyclobutane, a ring of four carbon atoms, and describe its electron-dot structure.
Show solution
Answer $C_{4} H_{8}$ ; 12 shared pairs and no lone pairs.
Each carbon uses two valencies on its two carbon neighbours and has two left for hydrogen: $4 \times 2 = 8$ H, so the formula is $C_{4} H_{8}$ .
In the dot structure each carbon shares one pair with each neighbouring carbon and one with each of its two hydrogens. That makes 4 C–C pairs + 8 C–H pairs = 12 shared pairs, and every carbon has an octet.
4
DiagramMediumNCERT in-text Q4, p. 69
Write the condensed structural formula of each compound.
1. propanoic acid
  Show solution
  Three carbons, the last in –COOH: $C H_{3} C H_{2} C O O H$ .
2. chlorobutane. Are isomers possible?
  Show solution
  $C H_{3} C H_{2} C H_{2} C H_{2} C l$ (1-chlorobutane). Yes: the Cl can sit on the second carbon, $C H_{3} C H_{2} C H C l C H_{3}$ , and the chain itself can branch, so there are four isomers in all.
3. pentanal
  Show solution
  $C H_{3} (C H_{2})_{3} C H O$ : five carbons, including the –CHO carbon.
4. pentanone. Why are two answers possible?
  Show solution
  The C=O can be on carbon 2, $C H_{3} C O C_{3} H_{7}$ (pentan-2-one), or on carbon 3, $C_{2} H_{5} C O C_{2} H_{5}$ (pentan-3-one). It cannot be at the end, which would make an aldehyde.
5
Short answerEasyNCERT in-text Q5, p. 69
Name these compounds.
1. $C H_{3} C H_{2} C H_{2} C l$
  Show solution
  Chloropropane (1-chloropropane): three carbons, Cl at the end.
2. $C H_{3} C H O$
  Show solution
  Ethanal: two carbons, an aldehyde.
3. $C H_{3} C H_{2} C H_{2} C \equiv C H$
  Show solution
  Pentyne (pent-1-yne): five carbons, a triple bond.
4. $H C O O H$
  Show solution
  Methanoic acid: one carbon, in the –COOH group.

4.3 Chemical Properties of Carbon Compounds

Light a gas stove, start a scooter or fire a thermal power station, and you are using the most important reaction of carbon compounds: burning. We start there, and then meet three gentler kinds of reaction.

4.3.1 Combustion

Diamond, graphite and charcoal all meet the same end in a hot enough fire: they become carbon dioxide. Carbon compounds burn too, and any hydrogen they contain leaves as water. Each of these is an oxidation (Chapter 1) that gives out a great deal of heat and light.

Carbon burns

C + O_{2} \overset{}{\to} C O_{2}

(4.1)

Methane (CNG) burns

\begin{aligned} C H_{4} + 2 O_{2} \\ \overset{}{\to} C O_{2} + 2 H_{2} O \end{aligned}

(4.2)

Ethanol burns

\begin{aligned} C_{2} H_{5} O H + 3 O_{2} \\ \overset{}{\to} 2 C O_{2} + 3 H_{2} O \end{aligned}

(4.3)

Check that the last two equations are balanced by counting C, H and O atoms on each side.

Example 4.4

Balancing the burning of LPG

Cooking gas is mostly butane, $C_{4} H_{10}$ . Write a balanced equation for its complete combustion.

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

Activity 4.3NCERT Activity 4.3 · p. 69

Which flames leave soot?

You’ll need

a camphor tablet (kapoor)
a few flakes of naphthalene (from a mothball)
a few drops of ethanol in a china dish
a small piece of candle wax
a steel plate or lid, tongs and matches

Predict first

Which of these do you expect to burn with the cleanest flame?

camphornaphthaleneethanol

With your teacher, burn a small amount of each substance in turn on a spatula or in the dish.
Note the colour of each flame and whether smoke rises from it.
Hold the steel plate with tongs a little above each flame for a few seconds and look for a black deposit.

Table 4.8What each flame did

Substance	Flame colour	Smoky?	Soot on plate?
Camphor
Naphthalene
Ethanol
Candle wax

Your entries are saved automatically.

2/2Too little air: incomplete combustion, a yellow flame and soot.

Fig. 4.10Enough air gives a blue, clean flame; too little gives a yellow, sooty one.NCERT Activity 4.4 · p. 69

A Bunsen burner below a steel plate. Step 1: the air hole is open, with an arrow showing air going in; the flame is a small blue cone and the plate stays clean; the labels say the gas burns completely. Step 2: the air hole is covered and marked with a cross; the flame is tall, yellow and wavy, soot particles rise from it and a black patch forms under the plate; the labels say the gas burns incompletely.

Two things decide how clean a flame is. One is the fuel. A saturated hydrocarbon such as methane has plenty of hydrogen for each carbon and usually burns blue, while unsaturated, carbon-rich fuels such as ethyne or naphthalene tend to burn yellow and smoky. The other is the air: starve even methane of oxygen and part of its carbon escapes as soot. A kitchen burner is designed with this in mind, drawing in air through holes near its base and mixing it with the gas before it burns. When those holes clog with spilt food, the flame turns yellow, soot blackens the base of the patila, and you pay for gas that is not cooking your food.

Fossil fuels are not pure carbon compounds. Coal and petroleum carry small amounts of sulphur and nitrogen, which leave a chimney or an exhaust pipe as sulphur dioxide and nitrogen oxides. These gases irritate our lungs and dissolve in rain to make it acidic.

4.3.2 Oxidation

Activity 4.5NCERT Activity 4.5 · p. 70

Watch the purple disappear

You’ll need

about 2 mL of ethanol in a test tube
a beaker of hot water (a water bath)
5 % alkaline potassium permanganate solution
a dropper

Predict first

You add purple potassium permanganate drop by drop to warm ethanol. What happens to the first few drops?

The purple colour disappearsThe purple colour staysThe mixture turns green and boils

Stand the test tube of ethanol in the hot water for a minute. Never warm ethanol over a flame.
Add the permanganate solution one drop at a time, shaking gently after each drop.
Watch what happens to the colour of the first few drops, then keep adding until the colour no longer goes away.

2/3Each drop is used up oxidising the ethanol.

Fig. 4.11While ethanol remains, each drop is used up; once all of it is oxidised, the purple stays.NCERT Activity 4.5 · p. 70

A test tube of ethanol stands in a beaker of warm water, with a dropper of purple potassium permanganate above it. Step 1: the ethanol is colourless and a purple drop hangs from the dropper. Step 2: the drop falls in and its colour vanishes, because it is used up oxidising the ethanol to ethanoic acid. Step 3: after many drops the liquid stays purple, because no ethanol is left to oxidise.

Burning is oxidation taken all the way: every carbon ends up in carbon dioxide. A milder oxidation can stop part of the way. Warm ethanol with alkaline potassium permanganate, $K M n O_{4}$ , or acidified potassium dichromate, $K_{2} C r_{2} O_{7}$ . The end carbon that carries the –OH becomes a –COOH group, and the alcohol turns into the carboxylic acid with the same number of carbons:

Ethanol to ethanoic acid

\begin{aligned} C H_{3} C H_{2} O H \\ \to_{h e a t}^{K M n O_{4}} C H_{3} C O O H \end{aligned}

(4.4)

Substances like these, which supply oxygen to another substance, are called oxidising agents. A slower version of the same change gives us vinegar: bacteria oxidise the ethanol in fermented sugarcane juice or fruit juice to ethanoic acid.

Definition

Oxidising agent

A substance that supplies oxygen to another substance, that is, oxidises it.

ExampleAlkaline $K M n O_{4}$ and acidified $K_{2} C r_{2} O_{7}$ oxidise ethanol to ethanoic acid.

4.3.3 Addition Reaction

A double or triple bond is a weak spot where new atoms can add on. Pass hydrogen and ethene over finely divided nickel or palladium, and each carbon picks up one more hydrogen atom, giving saturated ethane. The metal is a catalyst: it lets the reaction run quickly at a modest temperature and is still there, unchanged, when the reaction is over. Because two molecules join to form one, this is an addition reaction.

Ethene to ethane

\begin{aligned} C H_{2} = C H_{2} + H_{2} \\ \to_{h e a t}^{N i} C H_{3} - C H_{3} \end{aligned}

(4.5)

Definition

Addition reaction

A reaction in which atoms add across a double or triple bond, turning an unsaturated compound into a saturated one.

ExampleHydrogenation of an oil with a nickel catalyst.

Industry uses this reaction to hydrogenate vegetable oils. Oils such as groundnut, mustard or sunflower oil have long carbon chains with several double bonds, and they are liquid. Bubbling hydrogen through hot oil containing finely powdered nickel saturates many of these bonds. The straighter, saturated chains pack together more easily, and the oil sets into a soft solid fat, vanaspati ghee.

3/3One H adds to each carbon and the double bond becomes single.

Fig. 4.12On a nickel catalyst,

H_{2}

adds across the C=C bond and the oil becomes a saturated fat.NCERT p. 71

Part of an oil molecule: four carbon atoms in a row with their hydrogens, the middle two joined by a double bond. Step 1: a hydrogen molecule, H–H, lies below the chain; the labels read vegetable oil, C=C, unsaturated. Step 2: a nickel surface appears under the hydrogen and the H–H bond breaks. Step 3: one hydrogen atom joins each of the two middle carbons, the double bond becomes single, and the label reads vanaspati ghee, saturated chain.

Labels on cooking-oil bottles often boast of "PUFA" and "MUFA", the poly- and mono-unsaturated fatty acids of vegetable oils. Butter, ghee and vanaspati, by contrast, are mostly saturated. Eating a lot of saturated fat is linked with heart disease, which is why doctors recommend cooking mainly with unsaturated oils.

4.3.4 Substitution Reaction

An alkane has no double bond to open, so there is nowhere for new atoms to add, and most reagents simply leave it alone. Chlorine in sunlight is an exception. Light a mixture of methane and chlorine and they react fast: a chlorine atom pushes out a hydrogen atom and takes its seat, and the hydrogen leaves as hydrogen chloride. Because one atom swaps in for another, this is called a substitution reaction.

Methane and chlorine

\begin{aligned} C H_{4} + C l_{2} \\ \overset{s u n l i g h t}{\to} C H_{3} C l + H C l \end{aligned}

(4.6)

With more chlorine the swapping continues, giving $C H_{2} C l_{2}$ , $C H C l_{3}$ (chloroform) and $C C l_{4}$ , with one $H C l$ released at each step. Longer alkanes have many hydrogens that can be replaced, so they usually give a mixture of products.

2/5One H replaced: chloromethane, $C H_{3} C l$ .

Fig. 4.13In sunlight, Cl takes the place of H one atom at a time, and each swap releases HCl.NCERT p. 71

A methane molecule drawn as a carbon atom bonded to four hydrogen atoms, with a sun in the corner. Step 1: methane, CH4. Step 2: the right-hand hydrogen is replaced by a chlorine atom, giving chloromethane, CH3Cl, and one HCl. Step 3: the top hydrogen is replaced too, giving dichloromethane, CH2Cl2. Step 4: the left hydrogen is replaced, giving trichloromethane (chloroform), CHCl3. Step 5: all four are chlorine, tetrachloromethane, CCl4, and four HCl have formed.

Definition

Substitution reaction

A reaction in which an atom or group in a molecule is replaced by a different atom or group.

Example $C H_{4} + C l_{2} \overset{}{\to} C H_{3} C l + H C l$ in sunlight.

Practice · 2 questions

Practice 4.3 (NCERT in-text questions, p. 71): questions with answers

Short answerMediumNCERT in-text Q1, p. 71
In the lab, propan-1-ol is warmed with acidified potassium dichromate and turns into propanoic acid. Why is this change called an oxidation?
Show solution
AnswerThe compound gains oxygen (and loses hydrogen).
Propan-1-ol is $C_{3} H_{8} O$ and propanoic acid is $C_{3} H_{6} O_{2}$ . The product has one more oxygen atom and two fewer hydrogen atoms. Gaining oxygen (or losing hydrogen) is oxidation, and the dichromate is the oxidising agent that supplies the oxygen.
Short answerMediumNCERT in-text Q2, p. 71
A glass-blower's torch burns LPG mixed with pure oxygen instead of air. Why?
Show solution
AnswerPure oxygen burns the fuel completely and far hotter.
Air is about 78 % nitrogen, which takes no part in burning but soaks up heat, and it supplies oxygen only slowly. With pure oxygen the fuel burns completely and quickly, so the flame is clean and much hotter, hot enough to soften glass. Gas welders mix ethyne with oxygen for the same reason.

4.4 Some Important Carbon Compounds – Ethanol and Ethanoic Acid

Two carbon compounds deserve a closer look because we make and use them in huge amounts: ethanol and ethanoic acid. They differ in only one group, yet they behave very differently.

4.4.1 Properties of Ethanol

Read the label on a hand sanitiser: its main ingredient is usually ethanol, $C_{2} H_{5} O H$ , about 70 % of it. Ethanol, often just called alcohol, is a colourless liquid that boils at 78 °C (Table 4.2), so it evaporates quickly from your hands. It mixes with water in any ratio, and it dissolves many substances that water cannot: tincture of iodine, for instance, is iodine dissolved in ethanol.

Ethanol is also the intoxicating substance in alcoholic drinks, and it is a poison. Even a little of it dulls judgement and makes a person drunk. Drinking the pure liquid (absolute alcohol) can kill, and years of drinking damage the liver and other organs.

(i) Reaction with sodium. Drop sodium into ethanol and it fizzes, much as reactive metals do in water or acids. The gas is hydrogen, and the sodium ends up in sodium ethoxide, an ionic compound.

Sodium ethoxide forms

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

(4.7)

(ii) Removing water: making ethene. Concentrated sulphuric acid has a strong thirst for water. Warm ethanol with an excess of it to 443 K (170 °C), and the acid strips an H from one carbon and an OH from the other, leaving ethene. A substance that takes water out of a compound like this is called a dehydrating agent.

Dehydration of ethanol

\begin{aligned} C H_{3} C H_{2} O H \\ \to_{443 K}^{H_{2} S O_{4}} C H_{2} = C H_{2} \\ + H_{2} O \end{aligned}

(4.8)

4.4.2 Properties of Ethanoic Acid

The sour taste of sirka in a jar of pickled onions comes from ethanoic acid, $C H_{3} C O O H$ , also called acetic acid. Kitchen vinegar is just 5–8 % ethanoic acid in water. The pure acid is quite different: it is corrosive, and it freezes at 290 K (17 °C), so in a cold lab it sets into icy crystals. That is why it is sold as glacial acetic acid. Ethanoic acid belongs to the carboxylic acids, the family with the –COOH group.

In water, only a small fraction of ethanoic acid molecules release $H^{+}$ ions, while every molecule of hydrochloric acid does. So, strength for strength, ethanoic acid gives far fewer $H^{+}$ ions: it is a weak acid, and hydrochloric acid is a strong one.

Activity 4.7NCERT Activity 4.7 · p. 73

Same strength, different acidity

You’ll need

dilute hydrochloric acid and dilute ethanoic acid of the same strength
white vinegar from the kitchen
blue litmus paper
universal indicator paper and its colour chart

Predict first

Both acids are equally dilute. How will their pH values compare?

Both will have the same pHEthanoic acid will have the lower pHHydrochloric acid will have the lower pH

Put a drop of each sample on blue litmus paper.
Put a drop of each on universal indicator paper and read the pH from the chart.

Table 4.9Comparing the acids

Sample	Blue litmus	Indicator colour	pH
Dilute hydrochloric acid
Dilute ethanoic acid
Vinegar

Your entries are saved automatically.

(i) Esterification. The pineapple, banana or mango flavour in a sweet is often made in a factory, not an orchard: it is an ester. An ester forms when a carboxylic acid and an alcohol join, helped along by a little concentrated sulphuric acid, and water is the other product. Esters smell sweet or fruity, so they are used in perfumes and flavourings.

Activity 4.8NCERT Activity 4.8 · p. 73

Make a fruity smell

You’ll need

2 mL absolute ethanol
2 mL glacial ethanoic acid
4 or 5 drops of concentrated sulphuric acid
a test tube, a beaker of hot water and a beaker of cold water

Predict first

What kind of smell do you expect from the product?

sharp, like vinegarfruity and sweetno smell at all

Mix the ethanol and the glacial ethanoic acid in the test tube; your teacher adds the drops of concentrated sulphuric acid.
Stand the tube in the hot-water bath for about ten minutes.
Pour the mixture into the cold water. Do not sniff it directly: wave the air above it towards your nose.

3/3The two remaining pieces bond: ethyl ethanoate, an ester.

Fig. 4.14The acid loses –OH and the alcohol loses H; these form water, and the rest join up as an ester.NCERT Activity 4.8 · p. 73

Step 1: ethanoic acid, CH3–C(=O)–O–H, and ethanol, H–O–CH2CH3, side by side; the O–H of the acid and the H of the alcohol are circled. Step 2: these atoms move away together and form a water molecule, H–O–H. Step 3: the oxygen of the alcohol joins the carbon of the acid, giving the ester ethyl ethanoate, CH3–C(=O)–O–CH2CH3, which has a fruity smell.

Esterification

\begin{aligned} C H_{3} C O O H \\ + C_{2} H_{5} O H \\ \overset{a c i d}{\to} C H_{3} C O O C_{2} H_{5} \\ + H_{2} O \end{aligned}

(4.9)

Definition

Esterification

The reaction of a carboxylic acid with an alcohol, with an acid catalyst, to form an ester and water.

ExampleEthanoic acid + ethanol → ethyl ethanoate + water.

Esters can be split again. Boil one with sodium hydroxide solution and you get back the alcohol, but the acid comes out as its sodium salt. With the long-chain esters found in oils and fats, that sodium salt is soap, so this reaction is named saponification (Latin sapo, soap). Soaps, then, are sodium or potassium salts of carboxylic acids with long carbon chains.

Saponification

\begin{aligned} C H_{3} C O O C_{2} H_{5} \\ + N a O H \\ \overset{}{\to} C_{2} H_{5} O H \\ + C H_{3} C O O N a \end{aligned}

(4.10)

(ii) Reaction with a base. Ethanoic acid neutralises sodium hydroxide just as hydrochloric acid would. The salt formed is sodium ethanoate, better known as sodium acetate, and water is the other product.

Neutralisation

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

(4.11)

(iii) Washing soda and baking soda. Add ethanoic acid to a carbonate or a hydrogencarbonate and it fizzes: a salt, water and carbon dioxide form. Try it in Activity 4.9.

Activity 4.9NCERT Activity 4.9 · p. 74

Fizz from washing soda and baking soda

You’ll need

a spatula of washing soda (sodium carbonate)
a spatula of baking soda (sodium hydrogencarbonate)
dilute ethanoic acid or white vinegar
a test tube with a cork and delivery tube
freshly made lime water

Predict first

What will the gas from this reaction do to lime water?

NothingTurn it blueTurn it milky

Set up the test tube and delivery tube as you did for acids and carbonates in Chapter 2 (Activity 2.5).
Put the washing soda in the tube, add about 2 mL of dilute ethanoic acid and quickly fit the cork.
Bubble the gas through the lime water and watch.
Repeat with baking soda.

With sodium carbonate

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

(4.12)

With sodium hydrogencarbonate

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

(4.13)

Compare

Telling ethanol and ethanoic acid apart

Ethanol

Boils at 78 °C, freezes at −114 °C
Mild, spirit-like smell
No effect on blue litmus
No reaction with baking soda
Gives $H_{2}$ with sodium

Ethanoic acid

Boils at 118 °C, freezes at 17 °C
Sharp smell of vinegar
Turns blue litmus red
Fizzes with baking soda, giving $C O_{2}$
Also gives $H_{2}$ with sodium

Example 4.5

Identify the compounds

A neutral liquid A, $C_{2} H_{6} O$ , gives off a gas that burns with a pop when sodium is added. Warmed with alkaline potassium permanganate, A gives B, which fizzes with baking soda. A and B, warmed with a few drops of concentrated sulphuric acid, give C, which smells fruity. Identify A, B and C and write the equation for the formation of C.

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

Practice · 2 questions

Practice 4.4 (NCERT in-text questions, p. 74): questions with answers

Short answerEasyNCERT in-text Q1, p. 74
Two unlabelled bottles hold propan-1-ol and propanoic acid. Describe a chemical test that tells them apart, and what you would see.
Show solution
AnswerAdd sodium hydrogencarbonate: only the acid fizzes.
Add a pinch of sodium hydrogencarbonate (or sodium carbonate) to a little of each. Propanoic acid fizzes and gives carbon dioxide, which turns lime water milky; propan-1-ol shows no change. Blue litmus also works: only the acid turns it red.
Short answerEasyNCERT in-text Q2, p. 74
What does an oxidising agent do? Name two oxidising agents that convert ethanol into ethanoic acid.
Show solution
AnswerIt supplies oxygen; alkaline $K M n O_{4}$ and acidified $K_{2} C r_{2} O_{7}$ .
An oxidising agent supplies oxygen to another substance (or removes hydrogen from it), and so oxidises it. Alkaline potassium permanganate and acidified potassium dichromate both oxidise ethanol to ethanoic acid when warmed with it.

4.5 Soaps and Detergents

Rinse a greasy plate in plain water and the grease stays put; add a little soap and it lifts off. Why can soap do what water cannot? Begin with a simple test.

Activity 4.10NCERT Activity 4.10 · p. 74

Can soap keep oil and water mixed?

You’ll need

two small clear bottles with caps, labelled P and Q
water
two spoonfuls of mustard oil
liquid soap or soap solution

Predict first

After shaking, in which bottle will the oil take longer to separate?

P, oil and water onlyQ, with soap addedBoth the same

Half fill both bottles with water and add a spoonful of oil to each.
Add a few drops of soap solution to bottle Q only.
Cap both and shake them equally hard for the same time.
Look at them as soon as you stop, and again after five minutes. Where does a clear oil layer appear first?

Water alone cannot clean a greasy collar, because grease and water refuse to mix, and most dirt sticks to cloth through a film of grease. Soap works as a go-between. Each soap molecule is shaped like a tadpole. Its head is ionic, a $C O O^{-}$ group with a sodium or potassium ion, and is drawn to water: it is hydrophilic. Its long tail is a hydrocarbon chain that mixes with oil but shuns water: it is hydrophobic.

Put soapy water on a grease spot and the chains bury themselves in the grease while the ionic heads stay in the water. The molecules crowd round the oily droplet, heads outward, in a ball called a micelle. Rubbing and rinsing lift the micelles, with the grease trapped inside, off the cloth and wash them away.

3/4The grease is lifted off inside a ball of soap molecules.

Fig. 4.15Tails grip the grease, heads stay in the water, and the micelle carries the grease away.NCERT Figs 4.12–4.13 · p. 75

Step 1: one soap molecule, a long zigzag carbon tail that likes oil and a round ionic head, marked with a minus sign and a sodium ion, that likes water. Step 2: in water, a grease droplet sits on a cloth fibre and soap molecules push their tails into it with their heads in the water. Step 3: surrounded on all sides, the droplet lifts off the fibre as a micelle, tails inside and heads facing the water. Step 4: rinse water carries the micelle away; micelles repel one another because their heads carry the same charge.

Definition

Micelle

A cluster of soap molecules in water, with the hydrocarbon tails pointing inwards, often around oily dirt, and the ionic heads facing the water.

Now see what happens when the water is hard.

Activity 4.11NCERT Activity 4.11 · p. 76

Soft water, hard water

You’ll need

two test tubes
10 mL distilled water or rainwater
10 mL hard water from a hand-pump or tube well
soap solution and a dropper
a ruler

Predict first

In which tube do you expect more foam?

distilled waterhard waterthe same in both

Pour the distilled water into one tube and the hard water into the other.
Add three drops of soap solution to each.
Shake both equally hard for the same time and measure the height of the foam.
Look for a white, curdy solid in either tube.

Note for the teacher: if there is no hard water nearby, dissolve a pinch of calcium chloride or magnesium sulphate (Epsom salt) in tap water.

Table 4.10Foam and scum

Water	Foam height (cm)	White curdy solid?
Distilled water
Hard water

Your entries are saved automatically.

2/3Calcium and magnesium ions turn soap into insoluble scum.

Fig. 4.16Calcium ions in hard water turn soap into scum; detergents are not affected.NCERT Activities 4.11–4.12 · p. 76

A test tube of water with labels beside it. Step 1: soft water with soap, a tall column of foam and no scum. Step 2: hard water with soap, only three bubbles and white curdy flecks of scum in the water, with the equation 2RCOO− + Ca2+ → (RCOO)2Ca, an insoluble solid, where R is a long carbon chain. Step 3: hard water with detergent, a tall foam again and no scum, because the calcium salt of a detergent stays dissolved.

In towns that depend on borewells, a bath bucket often ends up with a grey, sticky ring, and the soap seems to vanish before it lathers. Borewell water is often hard: it carries dissolved calcium and magnesium ions. These ions react with soap to form insoluble salts, the scum, and only the soap left over after that can make lather.

Scum forms

\begin{aligned} 2 R C O O^{-} + C a^{2 +} \\ \overset{}{\to} (R C O O)_{2} C a ↓ \end{aligned}

(4.14)

where

$R$: a long hydrocarbon chain, such as $C_{17} H_{35}$ in sodium stearate

Detergents get round this. A detergent molecule also has a long hydrocarbon tail and a charged head, so it cleans through micelles just as soap does. The difference is in the head: most detergents are sodium salts of sulphonic acids, and some are ammonium salts with chloride or bromide ions. Calcium and magnesium cannot pull these heads out of solution, so no scum forms and the detergent keeps working in hard water. Washing powders, shampoos and dish-wash liquids are therefore mostly detergents.

Compare

Soap and detergent

Soap

Sodium or potassium salt of a long-chain carboxylic acid
Made from vegetable oils or animal fats
Forms scum in hard water
Bath soap and laundry bars

Detergent

Usually a sodium salt of a long-chain sulphonic acid
Made mostly from petroleum chemicals
Works in hard water: no scum
Shampoos, washing powders, dish-wash liquids

Practice · 2 questions

Practice 4.5 (NCERT in-text questions, p. 76): questions with answers

Short answerEasyNCERT in-text Q1, p. 76
Meera wants to check whether her tap water is hard by shaking it with a little washing powder, a detergent. Will her test work? What should she use instead?
Show solution
AnswerNo; she should use soap.
No. A detergent lathers in hard and soft water alike, because its calcium and magnesium salts dissolve, so it shows no difference. She should use soap: in hard water it gives little lather and a white, curdy scum.
Short answerMediumNCERT in-text Q2, p. 76
A washing machine keeps tumbling and churning the clothes instead of letting them soak. Why is this agitation needed to get them clean?
Show solution
AnswerIt brings soap to the dirt and carries the micelles away.
Soap molecules must reach the oily dirt held in the fibres, and the micelles that form must be pulled out of the cloth into the water. Soaking alone does this very slowly. Churning, rubbing or beating keeps bringing fresh soap solution to the dirt. It breaks the grease into small droplets that micelles can surround, and flushes the micelles out of the fabric.

Recap

Carbon and its Compounds: key points and summary

Although carbon is scarce in the crust and the air, every living thing and most everyday materials are built from its compounds.
Carbon shares electrons instead of gaining or losing four; a shared pair of electrons is a covalent bond (Section 4.1).
One, two or three shared pairs make single, double or triple bonds. Covalent compounds have low melting and boiling points and are poor conductors (Section 4.1).
Diamond, graphite and fullerenes are allotropes of carbon: the same atoms in different arrangements (Allotropes of carbon).
Catenation and tetravalency let carbon form millions of compounds: chains, branches and rings, saturated or unsaturated (Section 4.2, Section 4.2.2).
Alkanes ( $C_{n} H_{2 n + 2}$ ), alkenes ( $C_{n} H_{2 n}$ ) and alkynes ( $C_{n} H_{2 n - 2}$ ) are hydrocarbons; compounds with the same formula but different structures are isomers (Section 4.2.2).
Functional groups such as –Cl, –OH, –CHO, C=O and –COOH decide the chemistry of a compound (Section 4.2.3).
Members of a homologous series share a functional group and differ by $C H_{2}$ (14 u); their physical properties change gradually (Section 4.2.4).
An IUPAC name joins a stem for the number of carbons to a prefix or suffix for the group (Section 4.2.5).
Carbon compounds burn to carbon dioxide and water with heat and light; too little air gives a sooty flame (Section 4.3.1).
Alcohols are oxidised to carboxylic acids, unsaturated compounds add hydrogen (as in hydrogenating oils), and alkanes undergo substitution with chlorine in sunlight (Section 4.3.2, Section 4.3.3, Section 4.3.4).
Ethanol reacts with sodium and is dehydrated to ethene; ethanoic acid is a weak acid that forms esters and reacts with bases, carbonates and hydrogencarbonates (Section 4.4.1, Section 4.4.2).
Soaps and detergents clean by forming micelles around oily dirt; soap forms scum in hard water, detergents do not (Section 4.5).

Group activityNCERT Group Activity · p. 78

Models and home-made soap

You’ll need

modelling clay in two colours and toothpicks
for soap: 25 mL coconut or groundnut oil, 35 mL of 20 % sodium hydroxide solution, 8 g common salt, a steel beaker and a stirrer

Models. Roll big clay balls for carbon and small ones for hydrogen and oxygen. Push four toothpicks into each carbon, two into each oxygen and one into each hydrogen. Build methane, ethane, ethene (two toothpicks between the carbons), ethanol and the two isomers of butane. No carbon may be left with an empty toothpick.
Soap, with your teacher. Warm the oil with the sodium hydroxide solution, stirring all the time, for 10 to 15 minutes until the mixture thickens.
Stir in the salt, which makes the soap separate from the liquid. Let it cool and set, then cut it into shapes. A few drops of perfume can be stirred in before it sets.

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 Q1, p. 61	Practice 4.1 (NCERT in-text questions, p. 61) · Q1
NCERT in-text Q2, p. 61	Practice 4.1 (NCERT in-text questions, p. 61) · Q2
NCERT in-text Q1, p. 68	Practice 4.2 (NCERT in-text questions, pp. 68–69) · Q1
NCERT in-text Q2, p. 68	Practice 4.2 (NCERT in-text questions, pp. 68–69) · Q2
NCERT in-text Q3, p. 68	Practice 4.2 (NCERT in-text questions, pp. 68–69) · Q3
NCERT in-text Q4, p. 69	Practice 4.2 (NCERT in-text questions, pp. 68–69) · Q4
NCERT in-text Q5, p. 69	Practice 4.2 (NCERT in-text questions, pp. 68–69) · Q5
NCERT in-text Q1, p. 71	Practice 4.3 (NCERT in-text questions, p. 71) · Q1
NCERT in-text Q2, p. 71	Practice 4.3 (NCERT in-text questions, p. 71) · Q2
NCERT in-text Q1, p. 74	Practice 4.4 (NCERT in-text questions, p. 74) · Q1
NCERT in-text Q2, p. 74	Practice 4.4 (NCERT in-text questions, p. 74) · Q2
NCERT in-text Q1, p. 76	Practice 4.5 (NCERT in-text questions, p. 76) · Q1
NCERT in-text Q2, p. 76	Practice 4.5 (NCERT in-text questions, p. 76) · Q2
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 Carbon and its Compounds

Allotropes of carbon4 questions
4.2.2 Chains, Branches and Rings4 questions
4.2.5 Nomenclature of Carbon Compounds4 questions
4.3.4 Substitution Reaction4 questions
4.4.2 Properties of Ethanoic Acid4 questions
4.5 Soaps and Detergents4 questions

Glossary · 15

Key terms (English–Hindi)

addition reactionसंकलन अभिक्रियाsankalan abhikriya: A reaction in which atoms add across a double or triple bond, as when vegetable oils are hydrogenated.First seen in the chapter →
allotropeअपररूपaparroop: One of the forms of an element in which its atoms are arranged differently; diamond and graphite are allotropes of carbon.First seen in the chapter →

catenationशृंखलनshrinkhalan: The ability of an element's atoms to bond to one another in chains, branches or rings; carbon shows it most strongly.First seen in the chapter →
covalent bondसहसंयोजी आबंधsahsanyoji aabandh: A bond formed when two atoms share a pair of electrons; two or three shared pairs make a double or triple bond.

esterificationएस्टरीकरणesterikaran: The reaction of a carboxylic acid with an alcohol, using an acid catalyst, to give a sweet-smelling ester and water.

functional groupप्रकार्यात्मक समूहprakaaryaatmak samooh: An atom or group of atoms, such as –OH or –COOH, that decides the chemical properties of a carbon compound.First seen in the chapter →

homologous seriesसमजातीय श्रेणीsamjaatiya shreni: A family of compounds with the same functional group in which successive members differ by a $C H_{2}$ unit (14 u).First seen in the chapter →

micelleमिसेलmisel: A ball of soap molecules in water with the oily tails inside and the ionic heads outside; it can hold oily dirt.First seen in the chapter →

oxidising agentउपचायकupchaayak: A substance that adds oxygen to another substance, such as alkaline potassium permanganate.First seen in the chapter →

saponificationसाबुनीकरणsaabunikaran: The splitting of an ester by an alkali into an alcohol and the salt of the acid; it is how soap is made.First seen in the chapter →
saturated compoundसंतृप्त यौगिकsantript yaugik: A carbon compound with only single bonds between its carbon atoms, such as ethane.First seen in the chapter →
structural isomersसंरचनात्मक समावयवीsanrachnatmak samaavayavi: Compounds with the same molecular formula but different structures, such as butane and 2-methylpropane.First seen in the chapter →
substitution reactionप्रतिस्थापन अभिक्रियाpratisthaapan abhikriya: A reaction in which one atom or group in a molecule takes the place of another, such as Cl replacing H in methane.First seen in the chapter →

tetravalencyचतुःसंयोजकताchatuh-sanyojakta: Having a valency of four: each carbon atom forms four covalent bonds.First seen in the chapter →

unsaturated compoundअसंतृप्त यौगिकasantript yaugik: A carbon compound with at least one double or triple bond between carbon atoms, such as ethene.First seen in the chapter →

Carbon and its Compounds: Class 10 Science · Chapter 4

What you will learn in Carbon and its Compounds

4.1 Bonding in Carbon – The Covalent Bond

Allotropes of carbon

Practice 4.1 (NCERT in-text questions, p. 61): questions with answers

4.2 Versatile Nature of Carbon

4.2.1 Saturated and Unsaturated Carbon Compounds

4.2.2 Chains, Branches and Rings

4.2.3 Will you be my Friend?

4.2.4 Homologous Series

4.2.5 Nomenclature of Carbon Compounds

Practice 4.2 (NCERT in-text questions, pp. 68–69): questions with answers

4.3 Chemical Properties of Carbon Compounds

4.3.1 Combustion

4.3.2 Oxidation

4.3.3 Addition Reaction

4.3.4 Substitution Reaction

Practice 4.3 (NCERT in-text questions, p. 71): questions with answers

4.4 Some Important Carbon Compounds – Ethanol and Ethanoic Acid

4.4.1 Properties of Ethanol

4.4.2 Properties of Ethanoic Acid

Practice 4.4 (NCERT in-text questions, p. 74): questions with answers

4.5 Soaps and Detergents

Practice 4.5 (NCERT in-text questions, p. 76): questions with answers

Carbon and its Compounds: key points and summary

Practice questions with answers (NCERT “Exercises”)

Find your NCERT exercise question

Quick check: MCQs on Carbon and its Compounds

Key terms (English–Hindi)

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