NCERT Solutions Class 10 Science Chapter 4 In-text Questions — Carbon and its Compounds

NCERT Class 10 Science — Carbon and its Compounds, the five in-text QUESTIONS boxes on pages 61, 68, 71, 74 and 76. All 13 questions solved.

Also here. Solutions to the end-of-chapter questions for this chapter and the activities are on their own pages.

The boxes follow the chapter’s structure closely: bonding first, then chains and how to name them, then the reactions of carbon compounds, and finally soaps. Four things carry all thirteen questions:

• Carbon is tetravalent and catenates. Those two properties together are why there are so many carbon compounds — Question 2 of the page-68 box asks for them by name.
• Isomers are compounds with the same molecular formula and different structures; counting them means finding every distinct skeleton, and no more.
• Oxidation is the gain of oxygen, which is why turning ethanol into ethanoic acid counts as one.
• A soap molecule has a hydrocarbon tail and an ionic head, and both the detergent question and the agitation question follow from that.

Key insight. Draw before you count. Every structure question here — the electron dot structures, the pentane isomers, cyclopentane, the four named compounds — becomes straightforward once the carbon skeleton is on paper and each carbon is given its four bonds. Trying to answer from the molecular formula alone is what makes isomer questions feel like guesswork.

Questions, page 61. The box that closes Section 4.2, on the covalent bond and the structure of carbon compounds.

Page 61, Question 1

What would be the electron dot structure of carbon dioxide which has the formula CO₂?

Solution. Count the valence electrons: carbon has 4 and needs 4 more; each oxygen has 6 and needs 2 more.

One shared pair per oxygen would leave carbon two short and each oxygen one short. So carbon shares two pairs with each oxygen — a double bond on each side — and then carbon counts 8 electrons and each oxygen counts 8 as well.

$$\text{O} = \text{C} = \text{O}$$

Each oxygen keeps its two remaining pairs as lone pairs.

Page 61, Question 2

What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint — The eight atoms of sulphur are joined together in the form of a ring.)

Solution. Sulphur has six valence electrons and needs two more for an octet. In a ring of eight, each sulphur atom has exactly two neighbours, so it shares one pair with each neighbour — two single bonds.

That accounts for two of its six own electrons, leaving four, which stay as two lone pairs on every atom.

Check the count on any one atom: 6 own electrons, of which 2 are in bonds, plus 2 gained by sharing = 8. The octet is complete, and the same is true all the way round the ring.

$$\text{S}_8: \quad \text{a ring of eight sulphur atoms, each joined by a single bond to its two neighbours, each carrying two lone pairs}$$

Questions, pages 68 and 69. The box that closes Section 4.2.5, on isomers and nomenclature. It runs over the page break — its questions 4 and 5 are printed on page 69.

Page 68, Question 1

How many structural isomers can you draw for pentane?

Solution. Pentane is $\text{C}_5\text{H}_{12}$. Isomers differ in how the five carbons are joined, so the way to count them without missing or repeating any is to work down by the length of the longest chain.

Longest chain of five — all five carbons in a row. That is n-pentane:

$$\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3$$

Longest chain of four — four in a row, with the fifth carbon as a branch. The branch must go on an inner carbon, since putting it on an end carbon would simply make the chain five long again. That gives 2-methylbutane:

$$\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3$$

Longest chain of three — three in a row with two branches, both on the middle carbon. That is 2,2-dimethylpropane:

$$\text{C}(\text{CH}_3)_4$$

There is no chain of two, since that would need three branches on a two-carbon skeleton and carbon has only four bonds. So the count stops at three.

Page 68, Question 2

What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Solution. Catenation — carbon’s ability to bond with other carbon atoms, forming long straight chains, branched chains and rings. No other element does this to anything like the same extent, because carbon–carbon bonds are exceptionally strong and stable. This alone allows skeletons of any length and shape.

Tetravalency — carbon has four valence electrons and so forms four bonds. That leaves free bonds along every chain for hydrogen and for other elements such as oxygen, nitrogen, sulphur and chlorine, so each skeleton can carry many different combinations of atoms.

Together they multiply: any number of skeletons, each able to carry many arrangements of other atoms.

Page 68, Question 3

What will be the formula and electron dot structure of cyclopentane?

Solution. Cyclopentane is five carbon atoms joined in a ring. Each carbon uses two of its four bonds on its two neighbours in the ring, leaving two bonds for hydrogen.

So each carbon carries two hydrogens:

$$5 \text{ carbons} \times 2 \text{ hydrogens} = 10 \text{ hydrogens} \quad\Longrightarrow\quad \text{C}_5\text{H}_{10}$$

Note that this is not $\text{C}_5\text{H}_{12}$: closing the chain into a ring uses up two bonds that would otherwise hold hydrogens, which is why a cyclic alkane has two fewer hydrogens than the straight-chain one with the same number of carbons.

Electron dot structure. Each carbon shares one electron pair with each neighbouring carbon (two C–C bonds) and one pair with each of its two hydrogens (two C–H bonds) — four shared pairs in all, so every carbon has a complete octet and every hydrogen a full shell.

Page 69, Question 4

Draw the structures for the following compounds. (i) Ethanoic acid (ii) Bromopentane (iii) Butanone (iv) Hexanal. Are structural isomers possible for bromopentane?

Solution. Build each from its name: the stem gives the number of carbons, the suffix or prefix gives the functional group.

(i) Ethanoic acid, $\text{CH}_3\text{COOH}$ — two carbons, with a $-\text{COOH}$ group:

$$\text{CH}_3-\text{COOH}$$

(ii) Bromopentane, $\text{C}_5\text{H}_{11}\text{Br}$ — a five-carbon chain with a bromine in place of one hydrogen:

$$\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2\text{Br}$$

(iii) Butanone, $\text{CH}_3\text{COCH}_2\text{CH}_3$ — four carbons with a ketone group, which must be on an inner carbon:

$$\text{CH}_3-\text{CO}-\text{CH}_2-\text{CH}_3$$

(iv) Hexanal, $\text{CH}_3(\text{CH}_2)_4\text{CHO}$ — six carbons with an aldehyde group, which is always at the end of the chain:

$$\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CHO}$$

Are structural isomers possible for bromopentane? Yes. The bromine can sit at three different positions along the straight chain — 1-bromopentane, 2-bromopentane and 3-bromopentane — and further isomers exist on the branched pentane skeletons. That is exactly why the name “bromopentane” is incomplete: without a number it does not say which compound is meant.

Page 69, Question 5

How would you name the following compounds? (i) CH₃—CH₂—Br (ii) the compound with a $-\text{CHO}$ group on a single carbon (iii) CH₃—CH₂—CH₂—CH₂—C≡C—H

Solution. Name in three steps: count the carbons in the longest chain, identify the functional group, then apply its prefix or suffix.

(i) $\text{CH}_3-\text{CH}_2-\text{Br}$. Two carbons, so the stem is “eth-“. Bromine is shown by the prefix bromo-, not a suffix. The chain is saturated, so the name is bromoethane.

(ii) One carbon, so the stem is “meth-“. The $-\text{CHO}$ group is an aldehyde, shown by the suffix -al. The name is methanal.

(iii) $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{C}\equiv\text{CH}$. Count the carbons: four in the saturated part and two more in the triple bond — six, so the stem is “hex-“. A carbon–carbon triple bond is an alkyne, shown by the suffix -yne. The name is hexyne (more fully hex-1-yne, since the triple bond is at the end of the chain).

Questions, page 71. The box that closes Section 4.3, on the chemical properties of carbon compounds — combustion, oxidation, addition and substitution.

Page 71, Question 1

Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

Solution. Compare the two formulae. Ethanol is $\text{C}_2\text{H}_5\text{OH}$ — that is $\text{C}_2\text{H}_6\text{O}$. Ethanoic acid is $\text{CH}_3\text{COOH}$ — that is $\text{C}_2\text{H}_4\text{O}_2$.

The carbon count is unchanged, but the compound has gained an oxygen atom and lost two hydrogens. By the definition used from Chapter 1 onwards, gain of oxygen (or loss of hydrogen) is oxidation — so this conversion is an oxidation reaction on both counts.

It is brought about by an oxidising agent, which supplies that oxygen:

$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\text{or acidified K}_2\text{Cr}_2\text{O}_7 + \text{heat}]{\text{alkaline KMnO}_4 + \text{heat}} \text{CH}_3\text{COOH}$$

Page 71, Question 2

A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Solution. Welding needs an extremely hot flame — hot enough to melt steel — and how hot the flame gets depends on how completely and how fast the fuel burns.

Air is only about one-fifth oxygen; the rest is mostly nitrogen, which does not burn. Burning ethyne in air therefore gives an incomplete combustion, producing a sooty yellow flame that is not hot enough for welding — and the nitrogen present carries away heat without contributing any.

Pure oxygen supplies all the oxygen the fuel needs, so combustion is complete:

$$2\text{C}_2\text{H}_2 + 5\text{O}_2 \longrightarrow 4\text{CO}_2 + 2\text{H}_2\text{O} + \text{heat}$$

The result is a clean blue oxy-acetylene flame at about 3000 °C — hot enough to melt and join metals, and free of the soot that would spoil the weld.

Questions, page 74. The box that follows Section 4.4, on the reactions of ethanol and ethanoic acid.

Page 74, Question 1

How would you distinguish experimentally between an alcohol and a carboxylic acid?

Solution. The difference to exploit is that a carboxylic acid is an acid and an alcohol is not, so any test for acidity will separate them. The clearest is the carbonate test.

Sodium hydrogencarbonate test. Take the two liquids in separate test tubes and add a pinch of sodium hydrogencarbonate to each.

• With the carboxylic acid there is brisk effervescence, as carbon dioxide is released:

$$\text{CH}_3\text{COOH} + \text{NaHCO}_3 \longrightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$$

• With the alcohol, nothing happens at all.

The gas can be confirmed by passing it through freshly prepared lime water, which turns milky.

Litmus test, as a simpler check: the carboxylic acid turns blue litmus red; the alcohol leaves litmus unchanged.

Page 74, Question 2

What are oxidising agents?

Solution. An oxidising agent is a substance that is capable of adding oxygen to another substance — that is, of oxidising it.

The chapter’s own example is the conversion of an alcohol to an acid. Alkaline potassium permanganate, or acidified potassium dichromate, supplies the oxygen that turns ethanol into ethanoic acid, and is therefore acting as an oxidising agent:

$$\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\ \text{alkaline KMnO}_4 + \text{heat}\ } \text{CH}_3\text{COOH}$$

In the process the oxidising agent itself loses oxygen — it is reduced — which is why oxidation and reduction always happen together.

Questions, page 76. The last box, after Section 4.5 on soaps and detergents.

Page 76, Question 1

Would you be able to check if water is hard by using a detergent?

Solution. No. The test for hard water depends on something failing, and a detergent does not fail.

Soap works as a test because it forms insoluble calcium and magnesium salts — scum — with the ions that make water hard. So soap lathers freely in soft water and gives scum with little lather in hard water, and the difference is easy to see.

A detergent is different by design. Its charged ends do not form insoluble precipitates with calcium and magnesium ions, which is exactly why detergents were developed for use in hard-water areas. A detergent therefore lathers well in hard and soft water alike, and gives no scum in either.

With no difference to observe, the detergent cannot distinguish the two. Soap is what the test needs.

Page 76, Question 2

People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Solution. Soap does part of the job by itself and cannot do the rest.

When soap is added, its molecules arrange themselves around the oily dirt with their hydrocarbon tails buried in the oil and their ionic heads out in the water, forming micelles with the dirt inside. But those oily droplets are still stuck to the fibres of the cloth. Nothing in the chemistry pulls them off.

Agitation supplies the mechanical energy that does. Beating, scrubbing or tumbling loosens the dirt particles from the cloth so that the micelles, already formed around them, can carry them away into the water. Once suspended, they stay suspended — the like-charged heads on the outside of each micelle repel one another — and the rinse water takes them away.

Without agitation the soap would surround the dirt and the dirt would stay where it is.

Common mistakes

• Counting more than three isomers of pentane. Drawing the same skeleton at a different angle looks like a new isomer but is not. Working down by the length of the longest chain — 5, then 4, then 3 — gives exactly three and no repeats.
• Writing cyclopentane as $\text{C}_5\text{H}_{12}$. Closing the ring uses two bonds that would otherwise hold hydrogens, so a cyclic alkane has two fewer: $\text{C}_5\text{H}_{10}$.
• Answering the ethyne question with “air is cheaper”. The reason is the temperature of the flame: air is only about a fifth oxygen, so combustion is incomplete and the flame is too cool and sooty to weld with.
• Saying a detergent can test for hardness. It cannot, precisely because it works in hard water. The test needs something that fails in hard water, which is soap.
• Giving only single bonds in the $\text{CO}_2$ dot structure. Each oxygen needs two electrons, so each bond must be a double bond, or carbon is left four electrons short.
• Naming (iii) of the page-69 box “butyne” by counting only the saturated carbons. The two carbons of the triple bond count too — six in all, so hexyne.

Practise next

• The end-of-chapter Exercises for this chapter, where Questions 5 and 13 return to these same structures, and Questions 8, 10 and 15 develop the soap chemistry of the last box.
• Chapter 3, Metals and Non-metals, whose in-text questions cover the ionic bond — the exact contrast to the covalent bonding this chapter is built on.