Class 10 Science · Chapter 4

Carbon and its Compounds

442 Class 10 Science questions on Carbon and its Compounds from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 21 of 23

1. Q401 CBSE 2017
   Ethanoic acid reacts with absolute ethanol in the ​presence ​of Conc. ${\text{H}}_2{\text{SO}}_4$ to ​form a compound

   1. (i)Write the ​smell and class of compounds to which this compound belong.
   2. (ii)Write the chemical equation ​for the ​reaction and state the role of Conc. ${\text{H}}_2{\text{SO}}_4$ in the reaction.
   3. (iii)​Write one use of the product of this reaction.
   Show answer

   The product is ethyl ethanoate, a sweet-smelling ester; conc. ${\text{H}}_2{\text{SO}}_4$ is the acid catalyst, and the ester is used in perfumes.

   (i) The compound formed is ethyl ethanoate, ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5$. It has a sweet (fruity) smell and belongs to the class of compounds called esters. (ii) ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$ (ethanoic acid + ethanol gives ethyl ethanoate + water; the reaction is esterification). Conc. ${\text{H}}_2{\text{SO}}_4$ acts as the acid catalyst; it also takes up the water formed, which helps the reaction go forward. (iii) The ester is used in making perfumes and as a flavouring agent.

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2. Q402 CBSE 2017
   What ​is a homologous series of ​carbon compounds ? List ​its ​any ​two characteristics. Write ​the ​name and formula of next higher homologous of HCOOH.
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   In a homologous series members have the same functional group and differ by $-{\text{CH}}_2-$; the next homologue of HCOOH is ethanoic acid.

   A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, so that successive members differ by a $-{\text{CH}}_2-$ unit. Any two characteristics: (1) Successive members differ by a $-{\text{CH}}_2-$ unit, i.e. by $14$ u in molecular mass. (2) All members have the same functional group and hence similar chemical properties. (Also: all members share one general formula, and their physical properties, such as melting and boiling points, change gradually with molecular mass.) Next higher homologue of HCOOH (methanoic acid): ethanoic acid (acetic acid), ${\text{CH}}_3\text{COOH}$.

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3. Q403 CBSE 2017
   Explain ​the reasons why ​carbon forms compounds ​mainly ​by covalent bonds. State two main reasons responsible for ​carbon making a large number of compounds. Also explain why compounds formed ​by carbon with most other ​elements ​are exceptionally stable.
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   Carbon shares its four valence electrons to form covalent bonds; catenation and tetravalency give many compounds; its small size makes the bonds strong.

   Covalent bonding: carbon (atomic number 6, configuration 2, 4) has four valence electrons and needs four more to reach a noble-gas configuration. It could gain four electrons to form ${\text{C}}^{4-}$, but it would be difficult for a nucleus with six protons to hold ten electrons; or it could lose four to form ${\text{C}}^{4+}$, but that would need a large amount of energy, leaving six protons holding only two electrons. So carbon shares its valence electrons with other atoms (of carbon or of other elements) and forms covalent bonds, as in methane, where it shares one pair with each of four hydrogen atoms. Two main reasons for the large number of compounds: (i) catenation, the ability of carbon atoms to bond with one another to form long straight chains, branched chains and rings, with single, double or triple bonds; (ii) tetravalency, a valency of four, so that each carbon atom can bond with four other atoms of carbon or of elements such as hydrogen, oxygen, nitrogen, sulphur and chlorine. Stability: the bonds carbon forms with most other elements are very strong because the carbon atom is small, so its nucleus holds the shared pairs of electrons strongly; elements with bigger atoms form much weaker bonds.

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4. Q404 CBSE 2017
   Write two ​points of difference ​between saturated and unsaturated hydrocarbons ​giving one example of ​each. Write the general formula of alkyne series. Write the name and ​structure of second member ​of ​this series.
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   Saturated hydrocarbons have single bonds and give substitution; unsaturated ones have double or triple bonds and give addition; alkynes are ${\text{C}}_n{\text{H}}_{2n-2}$; second member propyne.

   Differences between saturated and unsaturated hydrocarbons: (1) Bonding: in a saturated hydrocarbon the carbon atoms are joined by single bonds only; an unsaturated hydrocarbon has at least one double or triple bond between carbon atoms. (2) Reactivity: saturated hydrocarbons are fairly unreactive and undergo substitution reactions (for example with chlorine in sunlight); unsaturated hydrocarbons are more reactive and undergo addition reactions (for example with hydrogen in the presence of a nickel or palladium catalyst). Examples: saturated, ethane (${\text{C}}_2{\text{H}}_6$); unsaturated, ethene (${\text{C}}_2{\text{H}}_4$). General formula of the alkyne series: ${\text{C}}_n{\text{H}}_{2n-2}$. Second member: propyne, ${\text{C}}_3{\text{H}}_4$, with the structure $\text{H}-\text{C}\equiv \text{C}-{\text{CH}}_3$, in which the last carbon carries three hydrogen atoms (drawn in full in the figure).

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5. Q405 CBSE 2017
   Soaps and detergents are both types of ​salts. State the ​difference between the two. Write ​the ​mechanism of the cleansing action of ​soaps. Why do soaps not form lather (foam) with hard ​water ? Mention ​any two problems that arise due to ​the use of detergents instead of soaps.
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   Soaps are carboxylic acid salts and detergents sulphonic acid salts; soap forms micelles around dirt and scum with hard water; detergents are often non-biodegradable.

   Difference between soaps and detergents: Soaps: sodium or potassium salts of long-chain carboxylic (fatty) acids, with the ionic end $-{\text{COO}}^{-}{\text{Na}}^{+}$ | Detergents: generally sodium salts of long-chain sulphonic acids, with the ionic end $-{\text{SO}}_3^{-}{\text{Na}}^{+}$ (or ammonium salts with chloride or bromide ions). Soaps: form an insoluble scum with hard water, so they do not work well in it | Detergents: form no scum and work well in both soft and hard water. Soaps: biodegradable | Detergents: many are not biodegradable. Cleansing action: a soap molecule has an ionic end that dissolves in water and a long hydrocarbon tail that dissolves in oil. When soap is applied to oily dirt, the tails of many soap molecules dissolve in the oil and the ionic ends stay in the water, so the dirt is enclosed in clusters called micelles. The micelles remain suspended in the water as an emulsion, and the dirt is washed away on rinsing (see the diagram). Hard water: it contains calcium and magnesium salts. Soap reacts with the ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ ions to form insoluble calcium and magnesium salts (scum), e.g. $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{CaCl}}_2\to ({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}\downarrow +2\text{NaCl}$. The soap is used up in forming scum, so lather does not form until these ions are removed, and much soap is wasted. Problems with detergents: (1) many detergents are not biodegradable, so they persist in rivers and lakes, cause foaming and harm aquatic life; (2) detergents containing phosphates cause excessive growth of algae in water bodies, which uses up the dissolved oxygen when the algae decay and kills fish.

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6. Q406 CBSE 2017
   What are esters ? How are ​esters prepared ​? Write the chemical equation for the reaction ​involved. What happens when an ester ​reacts ​with ​sodium hydroxide ? Write the ​chemical equation for the reaction and also state the name and use of this reaction.
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   Esters are sweet-smelling compounds formed from an acid and an alcohol; with sodium hydroxide they undergo saponification, used to make soap.

   Esters are sweet-smelling carbon compounds formed by the reaction of a carboxylic acid with an alcohol; they contain the ester group $-\text{COO}-$, for example ethyl ethanoate, ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5$. Preparation (esterification): a carboxylic acid is warmed with an alcohol in the presence of a little concentrated sulphuric acid as catalyst: ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{\text{conc. }{\text{H}}_2{\text{SO}}_4,\text{warm}}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$. When an ester reacts with sodium hydroxide, it is converted back into the alcohol and the sodium salt of the carboxylic acid: ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}⟶{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$. Name: saponification (alkaline hydrolysis of an ester). Use: making soap, by heating fats or oils, which are esters of long-chain fatty acids, with sodium hydroxide; this gives the sodium salts of the fatty acids, which are soap.

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7. Q407 CBSE 2017
   Why are certain compounds called hydrocarbons ? Write the general formula for homologous series ​of alkanes, alkenes and alkynes and ​also draw the structure ​of the first ​member ​of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the ​necessary conditions for the reaction ​to occur.
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   Hydrocarbons contain only carbon and hydrogen; alkenes become alkanes by hydrogenation with a nickel catalyst.

   Hydrocarbons: compounds made up of only carbon and hydrogen are called hydrocarbons. General formulae: alkanes ${\text{C}}_n{\text{H}}_{2n+2}$; alkenes ${\text{C}}_n{\text{H}}_{2n}$; alkynes ${\text{C}}_n{\text{H}}_{2n-2}$. First members: methane, ${\text{CH}}_4$, a carbon atom with single bonds to four hydrogen atoms; ethene, ${\text{C}}_2{\text{H}}_4$, ${\text{H}}_2\text{C}={\text{CH}}_2$; ethyne, ${\text{C}}_2{\text{H}}_2$, $\text{H}-\text{C}\equiv \text{C}-\text{H}$ (structures in the figure). Reaction: hydrogenation, an addition reaction, which needs a catalyst such as nickel (with heating) or palladium: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$.

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8. Q408 CBSE 2017
   1. (a)What are isomers ​? Write the structures of two isomers of butane (${\text{C}}_4{\text{H}}_{10}$).
   2. (b)Write the name and ​chemical equation ​for the reaction that takes place when 5% solution of alkaline ${\text{KMnO}}_4$ is added drop by drop to warm ethanol.
   3. (c)Name the product formed when ethanol is heated at 443K with ​excess conc. ${\text{H}}_2{\text{SO}}_4$ and state ​the role of conc. ${\text{H}}_2{\text{SO}}_4$ in ​the ​reaction.
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   Isomers have the same formula but different structures; alkaline ${\text{KMnO}}_4$ oxidises ethanol to ethanoic acid; excess hot concentrated ${\text{H}}_2{\text{SO}}_4$ dehydrates ethanol to ethene.

   (a) Isomers are compounds that have the same molecular formula but different structures. The two isomers of ${\text{C}}_4{\text{H}}_{10}$ are butane (n-butane), ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, a straight chain, and isobutane (2-methylpropane), ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$, a branched chain (full structures in the figure). (b) The reaction is oxidation: alkaline ${\text{KMnO}}_4$ oxidises ethanol to ethanoic acid, and its purple colour disappears while ethanol is left. ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{alkaline KMnO}}_4+\text{heat}}{\to }{\text{CH}}_3\text{COOH}$ (with the oxygen shown: ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\to {\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$). (c) The product is ethene, ${\text{CH}}_2={\text{CH}}_2$. Concentrated ${\text{H}}_2{\text{SO}}_4$ acts as a dehydrating agent, removing water from ethanol: ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{hot conc. H}}_2{\text{SO}}_4,443\text{K}}{\to }{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$.

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9. Q409 CBSE 2017
   1. (a)Write a chemical test to distinguish between saturated and unsaturated hydrocarbons.
   2. (b)Name ​the products formed when ethane burns ​in ​air. Write chemical ​equation for the ​reaction showing various types of energies liberated.
   3. (c)Write the reaction between methane and chlorine in the presence of ​sunlight. ​Why is this reaction considered a substitution reaction ?
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   Unsaturated hydrocarbons decolourise bromine water; ethane burns to carbon dioxide and water giving heat and light; methane and chlorine undergo substitution.

   (a) Bromine water test: add a few drops of bromine water (orange-brown) to the hydrocarbon and shake. An unsaturated hydrocarbon, with a $\text{C}=\text{C}$ or $\text{C}\equiv \text{C}$ bond, decolourises the bromine water, because bromine adds across the multiple bond; for example, ${\text{CH}}_2={\text{CH}}_2+{\text{Br}}_2\to {\text{CH}}_2\text{Br}-{\text{CH}}_2\text{Br}$ (colourless). A saturated hydrocarbon leaves the orange-brown colour unchanged (in the absence of bright sunlight). (b) Ethane burns in air to form carbon dioxide and water, releasing heat and light: $2{\text{C}}_2{\text{H}}_6+7{\text{O}}_2\to 4{\text{CO}}_2+6{\text{H}}_2\text{O}+\text{heat}+\text{light}$. (c) ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. It is a substitution reaction because a hydrogen atom of methane is replaced by a chlorine atom: one type of atom takes the place of another.

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10. Q410 CBSE 2017
    (a) Write ​a chemical equation of each of the following types of chemical reactions ​of organic compounds.

    1. (i)​Oxidation ​reaction
    2. (ii)​Addition reaction
    3. (iii)Substitution reaction (b) What is ethanol ? What happens when it is heated with excess Conc. ${\text{H}}_2{\text{SO}}_4$ at 443 K ​? ​Write the role of Conc. ${\text{H}}_2{\text{SO}}_4$ in ​this reaction.
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    Equations for oxidation, addition and substitution; ethanol is dehydrated to ethene at 443 K, with concentrated sulphuric acid as the dehydrating agent.

    (a) (i) Oxidation (ethanol to ethanoic acid): ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\stackrel{{\text{alkaline KMnO}}_4\text{, heat}}{\to }{\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$. (ii) Addition (ethene to ethane): ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$. (iii) Substitution (methane to chloromethane): ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. (b) Ethanol, ${\text{C}}_2{\text{H}}_5\text{OH}$ (ethyl alcohol), is the second member of the homologous series of alcohols, with the functional group $-\text{OH}$. When it is heated at 443 K with excess concentrated sulphuric acid it is dehydrated to ethene: ${\text{CH}}_3{\text{CH}}_2\text{OH}\underset{443\text{K}}{\overset{{\text{hot conc. H}}_2{\text{SO}}_4}{\to }}{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$. The concentrated sulphuric acid acts as a dehydrating agent: it removes water from ethanol.

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11. Q411 CBSE 2017
    Select ​the correct ​pair of properties ​of ​acetic ​acid.

    1. (a)Smell-like vinegar; turns red litmus to blue
    2. (b)Smell-like vinegar; turns blue litmus to red
    3. (c)Smell-like orange; turns blue litmus to red
    4. (d)Smell-like orange; turns red litmus to blue
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    (b) Smell-like vinegar; turns blue litmus to red

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12. Q412 CBSE 2017
    Soaps ​are ​formed ​by ​alkaline hydrolysis of

    1. (a)Inorganic salts
    2. (b)Carboxylic acids
    3. (c)Esters of long chain fatty acids
    4. (d)Esters of small chain fatty acids
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    (c) Esters of long chain fatty acids

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13. Q413 CBSE 2017
    You ​have four test tubes, A, B, C and D containing sodium carbonate, ​sodium chloride, lime water and blue litmus ​solutions ​respectively. Out ​of these the material of which test ​tube / test tubes would be suitable for the correct test of acetic / ethanoic ​acid ?

    1. (a)only A
    2. (b)A and B
    3. (c)B and C
    4. (d)A and D
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    (d) A and D

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14. Q414 CBSE 2017
    Which one of the following ​sets of materials ​can ​be ​used ​to ​prepare ​soap ?

    1. (a)Neem oil and calcium hydroxide
    2. (b)Castor oil and sodium hydroxide
    3. (c)Mineral oil and sodium hydroxide
    4. (d)Neem oil and magnesium hydroxide
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    (b) Castor oil and sodium hydroxide

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15. Q415 CBSE 2017
    When you add a few drops ​of ​acetic ​acid ​to a test-tube containing sodium bicarbonate powder, ​which one ​of the following is your observation ​?

    1. (a)No reaction takes place
    2. (b)A colourless gas with pungent smell is released with brisk effervescence
    3. (c)A brown coloured gas is released with brisk effervescence
    4. (d)Formation of bubbles of a colourless and odourless gas
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    (d) Formation of bubbles of a colourless and odourless gas

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16. Q416 CBSE 2017
    Select the ​correct statement ​about ​dilute solution of ​acetic ​acid ​:

    1. (a)smells like onion ; soluble in water
    2. (b)smells like vinegar ; soluble in water
    3. (c)smells like orange ; insoluble in water
    4. (d)smells like rose, insoluble in water
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    (b) smells like vinegar ; soluble in water

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17. Q417 CBSE 2017
    A ​student gently ​adds distilled ​water to ​a ​test ​tube filled ${\frac{1}{4}}^{\text{th}}$ with acetic acid and observes that

    1. (a)a separate layer of water is formed in the test tube.
    2. (b)a white precipitate is formed in the test tube.
    3. (c)a clear solution is formed in the test tube.
    4. (d)bubbles of a colourless and odourless gas are released from the test tube.
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    (c) a clear solution is formed in the test tube.

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18. Q418 CBSE 2017
    In ​the ​preparation ​of ​Soap addition of Sodium ​Chloride causes

    1. (a)Complete saponification
    2. (b)Complete hydrolysis
    3. (c)Complete precipitation
    4. (d)Complete neutralisation
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    (c) Complete precipitation

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19. Q419 CBSE 2017
    For demonstrating the preparation of soap ​in the laboratory which of the following ​combinations ​of ​an oil and ​a base would ​be ​most suitable ​?

    1. (a)Mustard oil and calcium hydroxide
    2. (b)Castor oil and calcium hydroxide
    3. (c)Turpentine oil and sodium hydroxide
    4. (d)Mustard oil and sodium hydroxide
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    (d) Mustard oil and sodium hydroxide

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20. Q420 CBSE 2017
    You ​are asked to prepare ​hard water in your laboratory. ​Select a group ​from the following groups of salts, ​any salt ​of which you may dissolve in distilled water to ​obtain hard water.

    1. (a)$\text{NaCl}$; ${\text{Na}}_2{\text{SO}}_4$; $\text{KCl}$
    2. (b)$\text{NaCl}$; ${\text{CaCl}}_2$; $\text{KCl}$
    3. (c)${\text{CaCl}}_2$; ${\text{CaSO}}_4$; ${\text{MgSO}}_4$
    4. (d)${\text{Na}}_2{\text{SO}}_4$; ${\text{CaSO}}_4$; ${\text{MgSO}}_4$
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    (c) ${\text{CaCl}}_2$; ${\text{CaSO}}_4$; ${\text{MgSO}}_4$

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