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 9 of 23

1. Q161 CBSE 2025
   1. (i)Differentiate between saturated and unsaturated hydrocarbons by giving one example each, with a structural ​formula.
   2. (ii)Write ​the method of converting an unsaturated hydrocarbon into a saturated hydrocarbon. Name the ​industry where this reaction is commonly used.
   3. (iii)Write the name and structure of a hydrocarbon ​having ​double bond and four carbon ​atoms ​in its one ​molecule.
   Show answer

   Saturated hydrocarbons have only single bonds, unsaturated ones a double or triple bond; nickel hydrogenation makes vanaspati ghee; the four-carbon alkene is but-1-ene.

   (i) A saturated hydrocarbon has only single bonds between its carbon atoms, general formula ${\text{C}}_n{\text{H}}_{2n+2}$; example ethane, ${\text{C}}_2{\text{H}}_6$, structure ${\text{H}}_3\text{C}-{\text{CH}}_3$. An unsaturated hydrocarbon has at least one double or triple bond between carbon atoms; example ethene, ${\text{C}}_2{\text{H}}_4$, structure ${\text{H}}_2\text{C}={\text{CH}}_2$. Saturated hydrocarbons give substitution reactions and burn with a clean blue flame, unsaturated ones give addition reactions and burn with a yellow sooty flame. (ii) The method is hydrogenation: the unsaturated hydrocarbon is heated with hydrogen in the presence of a nickel (or palladium) catalyst, ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$. It is commonly used in the vegetable oil industry, to convert vegetable oils into vanaspati ghee. (iii) But-1-ene (butene), ${\text{C}}_4{\text{H}}_8$, structure ${\text{CH}}_2=\text{CH}-{\text{CH}}_2-{\text{CH}}_3$.

   Step-by-step solution →
2. Q162 CBSE 2025
   1. (i)​Conversion of ethanol to ethanoic acid is ​an oxidation reaction. ​Why ? Name the oxidizing agent in this conversion ​and write the ​chemical ​equation for this ​oxidation reaction. Explain how this reaction ​is different from the reaction in which ethanol burns in the presence of oxygen.
   2. (ii)What is a homologous series of carbon compounds ? Write the name of the first-two members of a homologous series of compounds having functional group – COOH.
   Show answer

   Ethanol gains oxygen to form ethanoic acid, a partial oxidation unlike combustion; the first two carboxylic acids are methanoic and ethanoic acid.

   (i) It is an oxidation because oxygen is added to the molecule and hydrogen is removed from it: ethanol is ${\text{C}}_2{\text{H}}_6\text{O}$ and ethanoic acid is ${\text{C}}_2{\text{H}}_4{\text{O}}_2$, so one oxygen atom is gained and two hydrogen atoms are lost, and the alcohol group $-{\text{CH}}_2\text{OH}$ becomes the acid group $-\text{COOH}$. The oxidising agent is **alkaline potassium permanganate** (${\text{KMnO}}_4$) — acidified potassium dichromate (${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$) may also be used. The equation is ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\stackrel{{\text{alkaline KMnO}}_4}{\to }{\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$. It differs from the burning of ethanol, ${\text{C}}_2{\text{H}}_5\text{OH}+3{\text{O}}_2\to 2{\text{CO}}_2+3{\text{H}}_2\text{O}$, in that the oxidation is partial and controlled — the carbon skeleton survives and a carbon compound, ethanoic acid, is obtained, the oxygen coming from a chemical oxidising agent on warming — whereas combustion is complete and rapid: the molecule is broken down entirely by atmospheric oxygen, no carbon compound remains, and a large amount of heat and light is given out. (ii) A homologous series is a family of carbon compounds having the same functional group, in which any two successive members differ by one $-{\text{CH}}_2-$ unit (that is, by $14$ u in molecular mass) and which can all be represented by the same general formula; the members show similar chemical properties and a gradual gradation in physical properties. The first two members of the $-\text{COOH}$ series are **methanoic acid** ($\text{HCOOH}$) and **ethanoic acid** (${\text{CH}}_3\text{COOH}$).

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3. Q163 CBSE 2025
   A ​carbon compound 'A' is widely used as a preservative in pickles and has a molecular formula ${\text{C}}_2{\text{H}}_4{\text{O}}_2$. ​This compound reacts with ethanol to form a sweet smelling compound 'B'.

   1. (i)Identify the compound 'A' and write its structure.
   2. (ii)Write the chemical ​equation for the reaction of 'A' with ​ethanol to form compound 'B'. State the role of presence of an acid in the reaction. ​
   3. (iii)How can we get compound 'A' back from 'B'?
   4. (iv)How can 'A' be obtained ​from ethanol?
   5. (v)Name the ​gas ​produced when compound 'A' reacts with washing soda.
   Show answer

   'A' is ethanoic acid; it forms the ester ethyl ethanoate with ethanol, is recovered by hydrolysis, and gives carbon dioxide with washing soda.

   (i) 'A' is ethanoic acid (acetic acid), ${\text{CH}}_3\text{COOH}$; its structure is a methyl group joined to a carboxyl group, the carboxyl carbon being doubly bonded to one oxygen and singly bonded to an $-\text{OH}$ group. (ii) ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{\text{conc. }{\text{H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$; 'B' is ethyl ethanoate. The acid acts as a catalyst and, being a dehydrating agent, removes the water formed and drives the reversible reaction towards the ester. (iii) By hydrolysing 'B': warming it with $\text{NaOH}$ gives sodium ethanoate and ethanol (saponification), and adding dilute $\text{HCl}$ to the sodium ethanoate liberates ethanoic acid; warming 'B' with water and a dilute mineral acid does it in one step. (iv) By oxidising ethanol: ${\text{CH}}_3{\text{CH}}_2\text{OH}\underset{\Delta }{\overset{\text{alkaline }{\text{KMnO}}_4}{\to }}{\text{CH}}_3\text{COOH}$ (acidified ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$ works equally well). (v) Carbon dioxide: $2{\text{CH}}_3\text{COOH}+{\text{Na}}_2{\text{CO}}_3⟶2{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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4. Q164 CBSE 2025
   (a) Write ​the molecular formula of ethyne and draw its electron ​dot structure. (b) ​Write chemical equation to show the reaction ​of ethanol with

   1. (i)Sodium ​metal
   2. (ii)Ethanoic acid (in ​the presence of an acid)
   3. (iii)Acidified ​potassium dichromate Write the name of the product formed in ​each case.
   Show answer

   Ethyne is ${\text{C}}_2{\text{H}}_2$ with a triple bond; ethanol gives sodium ethoxide with sodium, an ester with ethanoic acid and ethanoic acid with dichromate.

   (a) Ethyne is ${\text{C}}_2{\text{H}}_2$. In its electron dot structure each carbon shares one pair of electrons with its hydrogen atom and **three** pairs with the other carbon, written $\text{H}:\text{C}:::\text{C}:\text{H}$ (that is $\text{H}-\text{C}\equiv \text{C}-\text{H}$), so each carbon has eight electrons around it. (b) (i) $2{\text{CH}}_3{\text{CH}}_2\text{OH}+2\text{Na}\to 2{\text{CH}}_3{\text{CH}}_2\text{ONa}+{\text{H}}_2$; the products are sodium ethoxide and hydrogen gas. (ii) ${\text{CH}}_3{\text{CH}}_2\text{OH}+{\text{CH}}_3\text{COOH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$; the product is ethyl ethanoate, an ester (with water). (iii) ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\stackrel{{\text{acidified K}}_2{\text{Cr}}_2{\text{O}}_7,\Delta }{\to }{\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$; the product is ethanoic acid.

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5. Q165 CBSE 2025
   What are soaps ​? Write the structure of a soap molecule. Explain the ​cleansing action of a ​soap. Why are soaps not considered suitable for washing clothes in a ​region where ​water is ​hard ? How is ​this ​problem overcomed ?
   Show answer

   Soaps are salts of long-chain carboxylic acids that clean by forming micelles; they form scum in hard water, so detergents are used.

   Soaps are the sodium or potassium salts of long-chain carboxylic acids, made by heating a fat or an oil with alkali. A soap molecule, for example sodium stearate ${\text{CH}}_3({\text{CH}}_2{)}_{16}{\text{COO}}^{-}{\text{Na}}^{+}$, has a long hydrocarbon tail that is hydrophobic and dissolves in oil, and an ionic $-{\text{COO}}^{-}{\text{Na}}^{+}$ head that is hydrophilic and dissolves in water. In cleansing, the tails of many molecules enter the oily dirt while the heads remain in the water, so the molecules form a micelle with the dirt trapped inside; agitation frees the micelles from the cloth, they repel one another and remain suspended, and the dirt is rinsed away with the water. Soaps are unsuitable in hard water because hard water contains calcium and magnesium ions, and the calcium and magnesium salts of fatty acids are insoluble: they separate as a white curdy scum, so soap is wasted before any lather forms and the scum sticks to the clothes, as in $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{CaCl}}_2⟶({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}\downarrow +2\text{NaCl}$. The problem is overcome by using detergents, the ammonium or sulphonate salts of long-chain hydrocarbons, whose calcium and magnesium salts are soluble in water, so they lather well even in hard water; the water may also be softened first.

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6. Q166 CBSE 2025
   (i) Write the name and ​the molecular formula of ​a commercially important ​carbon compound having functional group $-\text{OH}$. ​(ii) Write chemical equations to show the reactions of ​the above mentioned compound with

   1. (1)excess conc. sulphuric acid
   2. (2)ethanoic acid in the presence of an acid catalyst
   3. (3)sodium metal
   4. (4)alkaline potassium permanganate (${\text{KMnO}}_4$) Also write the name of the product formed in ​each ​of the ​above reaction.
   Show answer

   Ethanol gives ethene with excess conc. sulphuric acid, ethyl ethanoate with ethanoic acid, sodium ethoxide with sodium and ethanoic acid on oxidation.

   (i) Ethanol, molecular formula ${\text{C}}_2{\text{H}}_6\text{O}$, usually written ${\text{C}}_2{\text{H}}_5\text{OH}$. (ii)(1) ${\text{C}}_2{\text{H}}_5\text{OH}\underset{443\text{K}}{\overset{{\text{conc. H}}_2{\text{SO}}_4}{\to }}{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$; product ethene. (2) ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{\text{acid catalyst}}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$; product ethyl ethanoate, an ester. (3) $2{\text{C}}_2{\text{H}}_5\text{OH}+2\text{Na}⟶2{\text{C}}_2{\text{H}}_5\text{ONa}+{\text{H}}_2\uparrow$; products sodium ethoxide and hydrogen gas. (4) ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\stackrel{{\text{alkaline KMnO}}_4}{\to }{\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$; product ethanoic acid.

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7. Q167 CBSE 2025
   1. (i)Give reason why carbon ​can neither form ${\text{C}}^{4+}$ cations nor ${\text{C}}^{4-}$ anions but forms covalent ​compounds.
   2. (ii)What ​is meant by functional group in carbon compounds ? Write in tabular ​form ​the structural formula ​and the functional group present ​in the following compounds : (a) ​Ethanol (b) Ethanoic acid
   Show answer

   Carbon shares electrons because gaining or losing four is too difficult; ethanol has the $-\text{OH}$ group and ethanoic acid the $-\text{COOH}$ group.

   (i) Carbon (atomic number 6, electrons 2, 4) has four valence electrons. To form a ${\text{C}}^{4-}$ anion it would have to gain four electrons, and it would be difficult for a nucleus of six protons to hold on to ten electrons. To form a ${\text{C}}^{4+}$ cation it would have to lose four electrons, which needs a very large amount of energy and would leave six protons holding only two electrons. So carbon shares its four valence electrons with other atoms, forming covalent bonds, and its compounds are covalent. (ii) A functional group is an atom or a group of atoms that replaces hydrogen in a hydrocarbon and gives the compound its characteristic chemical properties, whatever the length of the carbon chain. $$\begin{array}{lll}\text{Compound}& \text{Structural formula}& \text{Functional group}\\ \text{(a) Ethanol}& {\text{CH}}_3-{\text{CH}}_2-\text{OH}& -\text{OH}\text{(alcohol)}\\ \text{(b) Ethanoic acid}& {\text{CH}}_3-\text{COOH}& -\text{COOH}\text{(carboxylic acid)}\end{array}$$ The full structural formulae, with every bond drawn, are shown in the figure.

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8. Q168 CBSE 2025
   1. (a)Name ​two cyclic hydrocarbons and draw the structure of ​any ​one.
   2. (b)Explain the process of ​micelle ​formation on adding soap in ​water.
   Show answer

   Cyclohexane and benzene are cyclic hydrocarbons; soap molecules cluster in water with tails inward and ionic ends outward, forming micelles that trap oily dirt.

   (a) Two cyclic hydrocarbons are cyclohexane, ${\text{C}}_6{\text{H}}_{12}$, and benzene, ${\text{C}}_6{\text{H}}_6$. Structure of cyclohexane: six carbon atoms are joined by single bonds in a closed hexagonal ring, and each carbon atom is also bonded to two hydrogen atoms, so that every carbon forms four bonds (the structure is drawn in the figure; benzene has single and double bonds alternating in its ring and one hydrogen atom on each carbon). (b) A soap molecule, such as sodium stearate, ${\text{CH}}_3({\text{CH}}_2{)}_{16}{\text{COO}}^{-}{\text{Na}}^{+}$, has two ends with different properties: the ionic carboxylate end is hydrophilic and is attracted to water, while the long hydrocarbon tail is hydrophobic and dissolves in oil but not in water. When soap is added to water, the molecules at the surface line up with their ionic ends in the water and their tails sticking out of it. Inside the water, the molecules gather into clusters in which the hydrocarbon tails point inwards, away from the water, and the ionic ends face outwards into the water. Such a cluster is called a micelle. Oily dirt collects in the hydrocarbon interior of the micelles and is rinsed away with the water, and the micelles stay dispersed because their charged surfaces repel one another.

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9. Q169 CBSE 2025
   A saturated organic ​compound ‘A’ with two carbon atoms belongs to the homologous series ​of alcohols. On oxidation, it forms an organic ​acid ‘B’ with molecular mass 60 u. ​On heating ‘A’ with ​excess concentrated sulphuric acid at 443 K, an unsaturated hydrocarbon ‘C’ is formed.

   1. (i)Name A, B and ​C.
   2. (ii)Calculate molecular mass of C.
   3. (iii)What happens when a pinch of sodium carbonate is added to compound B ? Write chemical equation for the reaction.
   4. (iv)Draw ​electron dot structure of compound B.
   Show answer

   A is ethanol, B ethanoic acid, C ethene (28 u); sodium carbonate gives brisk effervescence of carbon dioxide and sodium ethanoate.

   (i) A is ethanol, ${\text{CH}}_3{\text{CH}}_2\text{OH}$ (${\text{C}}_2{\text{H}}_5\text{OH}$); B is ethanoic acid (acetic acid), ${\text{CH}}_3\text{COOH}$, whose molecular mass is $2(12)+4(1)+2(16)=60$ u; C is ethene, ${\text{CH}}_2={\text{CH}}_2$. (ii) Molecular mass of C, ${\text{C}}_2{\text{H}}_4$, $=2\times 12+4\times 1=28$ u. (iii) Brisk effervescence is seen as carbon dioxide is given off (it turns lime water milky), and sodium ethanoate (sodium acetate) is formed: $2{\text{CH}}_3\text{COOH}+{\text{Na}}_2{\text{CO}}_3\to 2{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$. (iv) In the electron dot structure of ${\text{CH}}_3\text{COOH}$ the methyl carbon shares one pair of electrons with each of three hydrogen atoms and one pair with the second carbon; the second carbon shares two pairs with one oxygen ($\text{C}=\text{O}$) and one pair with the other oxygen, which shares one pair with a hydrogen atom; each oxygen also has two lone pairs. That uses all $24$ valence electrons and gives every carbon and oxygen an octet and every hydrogen a duet (see the figure).

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10. Q170 CBSE 2025
    1. (a)Define the term “homologous series ​of carbon ​compounds”. Write ​a homologous series of compounds having functional ​group $-\text{CHO}$.
    2. (b)Design an ​experiment to ​distinguish between an alcohol and a carboxylic acid. Also ​write ​chemical equation for that case in which reaction occurs.
    Show answer

    Aldehydes (methanal, ethanal, propanal) form the $-\text{CHO}$ series; sodium hydrogencarbonate gives effervescence with a carboxylic acid but not with an alcohol.

    (a) A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain; successive members differ by a $-{\text{CH}}_2-$ unit ($14$ u), fit one general formula, have similar chemical properties and show a gradation in physical properties. Homologous series with the $-\text{CHO}$ group (the aldehydes, ${\text{C}}_n{\text{H}}_{2n}\text{O}$): methanal $\text{HCHO}$, ethanal ${\text{CH}}_3\text{CHO}$, propanal ${\text{CH}}_3{\text{CH}}_2\text{CHO}$, butanal ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{CHO}$. (b) Take the two liquids in separate test tubes and add a pinch of sodium hydrogencarbonate to each. The carboxylic acid gives brisk effervescence of a gas that turns lime water milky (carbon dioxide); the alcohol shows no change. Equation, for ethanoic acid: ${\text{CH}}_3\text{COOH}+{\text{NaHCO}}_3\to {\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$.

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11. Q171 CBSE 2025
    1. (i)What is a ​homologous ​series of carbon compounds ? ​Write the name and ​formula of three successive members of the homologous series of ​compounds having functional group – COOH.
    2. (ii)Write ​the name of two carbon compounds in which carbon atoms are arranged in the form of a ring. Draw the structure of any ​one of the two.
    Show answer

    Methanoic, ethanoic and propanoic acids form a homologous series; cyclohexane (${\text{C}}_6{\text{H}}_{12}$) and benzene (${\text{C}}_6{\text{H}}_6$) are ring compounds.

    (i) A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain; successive members differ by a $-{\text{CH}}_2-$ unit ($14$ u in molecular mass), have similar chemical properties and show a gradation in physical properties. Three successive members with the $-\text{COOH}$ group are methanoic acid, $\text{HCOOH}$; ethanoic acid, ${\text{CH}}_3\text{COOH}$; and propanoic acid, ${\text{CH}}_3{\text{CH}}_2\text{COOH}$. (ii) Cyclohexane, ${\text{C}}_6{\text{H}}_{12}$, and benzene, ${\text{C}}_6{\text{H}}_6$. In the structure of cyclohexane six carbon atoms are joined in a ring by single bonds, and each carbon carries two hydrogen atoms, so that every carbon forms four bonds (see the figure).

    Step-by-step solution →
12. Q172 CBSE 2025
    1. (a)Define the term “structural isomers”. Write ​the structures of ​two isomers of butane (${\text{C}}_4{\text{H}}_{10}$).
    2. (b)Two carbon compounds X and Y have the ​molecular formula ${\text{C}}_2{\text{H}}_4$ ​and ${\text{C}}_3{\text{H}}_8$ respectively. Which one of the two is most likely to show addition reaction ? Justify your answer. Also give (i) chemical equation to explain the process ​of addition reaction in this case and (ii) name of ​the industry where this ​reaction is applied.
    Show answer

    Ethene (X) shows addition because it has a double bond; propane (Y) is saturated. Hydrogenation with nickel is used in the vegetable oil industry.

    (a) Structural isomers are compounds with identical molecular formula but different structures. Two isomers of butane, ${\text{C}}_4{\text{H}}_{10}$: n-butane, the straight chain ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, and isobutane (2-methylpropane), the branched chain ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$, drawn in full in the figure. (b) X, ${\text{C}}_2{\text{H}}_4$ (ethene), is most likely to show an addition reaction. It fits ${\text{C}}_n{\text{H}}_{2n}$, so it is an unsaturated hydrocarbon with a carbon-carbon double bond, and atoms can add across that double bond. Y, ${\text{C}}_3{\text{H}}_8$ (propane), fits ${\text{C}}_n{\text{H}}_{2n+2}$: it is saturated, with only single bonds, so it does not show addition. (i) ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$ (ii) The vegetable oil (vanaspati ghee) industry, where vegetable oils are hydrogenated using a nickel catalyst.

    Step-by-step solution →
13. Q173 CBSE 2025
    1. (i)​Draw electron dot structure of chlorine molecule. (Atomic Number ​of Chlorine $=17$)
    2. (ii)What happens when chlorine reacts with methane in the presence of sunlight ? Write the name ​of the ​reaction.
    3. (iii)Name the two oxidising agents used for the ​conversion ​of alcohols to acids.
    4. (iv)List four differences ​in properties ​between covalent compounds and ionic compounds.
    Show answer

    Two chlorine atoms share one pair; methane undergoes substitution in sunlight; alkaline ${\text{KMnO}}_4$ and acidified ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$ oxidise alcohols; ionic and covalent compounds differ.

    (i) Chlorine ($Z=17$) is $2,8,7$, with $7$ valence electrons. Two chlorine atoms share one pair of electrons, one from each, forming a single covalent bond, Cl–Cl; in the electron dot structure each Cl has three lone pairs and the shared pair, that is $8$ outer electrons (see the figure). (ii) In sunlight chlorine replaces the hydrogen atoms of methane one by one: ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$, and further replacement gives ${\text{CH}}_2{\text{Cl}}_2$, ${\text{CHCl}}_3$ and ${\text{CCl}}_4$. The reaction is a substitution reaction. (iii) Alkaline potassium permanganate (alkaline ${\text{KMnO}}_4$) and acidified potassium dichromate (acidified ${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$). (iv) Melting and boiling points: ionic compounds have high melting and boiling points; covalent compounds made of separate molecules, such as most carbon compounds, have low ones. Electrical conduction: ionic compounds conduct electricity when molten or dissolved in water, because their ions can then move; covalent compounds are generally poor conductors, because their bonding does not give rise to ions. Solubility: ionic compounds are generally soluble in water and insoluble in solvents such as kerosene and petrol; many covalent compounds are insoluble in water but dissolve in organic solvents. Physical state: ionic compounds are solids at room temperature, somewhat hard and generally brittle; many covalent compounds are liquids or gases at room temperature — methane is a gas and ethanol a liquid.

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14. Q174 CBSE 2025
    1. (i)Give reason why ​carbon forms compounds mainly by covalent bonding.
    2. (ii)Why do ​covalent compounds have low melting and boiling points.
    3. (iii)Give ​reason ​for the following : I. Covalent compounds ​are bad ​conductors of electricity. II. Carbon shows catenation.
    Show answer

    Carbon shares electrons; covalent compounds have weak forces between molecules, so low melting points, and no ions, so no conduction; carbon shows catenation.

    (i) Carbon ($2,4$) has four valence electrons. Gaining four electrons to form ${\text{C}}^{4-}$ is difficult, because a nucleus of six protons cannot easily hold on to ten electrons, and losing four to form ${\text{C}}^{4+}$ would need a large amount of energy. So carbon completes its outer shell by sharing its four valence electrons with other atoms, that is, by covalent bonding. (ii) Most covalent compounds, including most carbon compounds, consist of separate molecules. The bonds within each molecule are strong, but the forces of attraction between the molecules are weak; melting or boiling has to overcome only these weak forces, so little energy is needed and the melting and boiling points are low. (iii) I. Covalent bonding does not give rise to ions, and the shared electrons stay in the bonds, so a covalent compound has no ions or free electrons to carry a current: it is a bad conductor of electricity. II. Carbon atoms form strong, stable bonds with other carbon atoms, because the small carbon atom holds shared electron pairs strongly, and each carbon atom can form four bonds; so carbon atoms link up into long chains, branched chains and rings. This is catenation.

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15. Q175 CBSE 2025
    ‘A’ ​and ‘B’ are two salts used for washing purposes. Salt ‘A’ ​is used for bathing also. Four test tubes I, II, III and IV ​as mentioned below are ​taken. The ​test tubes are ​shaken one ​by one ​almost identically for the same time and the lengths of foam formed in each test tube is noted.

    Contents of the four test tubes

    Test tube	Contents
    I	Rain water + solution of salt ‘A’
    II	Rain water + solution of salt ‘B’
    III	Tubewell water + solution of salt ‘A’
    IV	Tubewell water + solution of salt ‘B’

    1. (a)In which one of the four test tubes is the foam formed the minimum?[1]
    2. (b)Differentiate between salt A and salt B.[1]
    3. (c)(i)What are esters? What happens when an ester reacts with an alkali (say sodium hydroxide)? Give chemical equation for the reaction.[2]
    4. (c)(ii)OR : What is the cause of hardness of water? Sometimes it is observed that while bathing foam is formed with difficulty and an insoluble substance is formed. Name this substance and write the cause of its formation.[2]
    Show answer

    Test tube III gives least foam; A is a soap and B a detergent; an ester with alkali gives a sodium salt and alcohol.

    (a) Test tube III (tubewell water with salt A) shows the minimum foam, because the calcium and magnesium ions of hard tubewell water throw the soap out as insoluble scum. (b) Salt A is a soap - the sodium salt of a long-chain carboxylic (fatty) acid; it is mild enough for bathing and is biodegradable, but it lathers poorly in hard water and forms scum. Salt B is a detergent - the sodium salt of a long-chain benzene sulphonic acid or alkyl hydrogen sulphate; it lathers well even in hard water because its calcium and magnesium salts are soluble. (c)(i) Esters are the sweet-smelling compounds formed when a carboxylic acid reacts with an alcohol in the presence of a little concentrated sulphuric acid. With an alkali an ester is hydrolysed (saponification) to the sodium salt of the acid and the alcohol: ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}⟶{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$. (c)(ii) Hardness of water is caused by dissolved calcium and magnesium salts (hydrogencarbonates, chlorides and sulphates). The insoluble substance formed while bathing is scum - the calcium or magnesium salt of the fatty acid of the soap - formed because ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ ions displace the sodium of the soap: $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{CaCl}}_2⟶({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}\downarrow +2\text{NaCl}$.

    Step-by-step solution →
16. Q176 CBSE 2025
    All compounds ​of carbon ​which ​contain only carbon and ​hydrogen are ​called hydrocarbons. Among these, saturated hydrocarbons are called 'alkanes', unsaturated compounds ​with one or more double bonds ​are called 'alkenes' and compounds containing one or more triple bonds ​are called 'alkynes'.

    1. (a)Identify the following compound and write its name. ${\text{CH}}_3-\text{C}\equiv \text{CH}$[1]
    2. (b)How do saturated and unsaturated carbon compounds differ in terms of the flame produced by them on burning ?[1]
    3. (c)(i)Which type of hydrocarbons undergo addition reactions ? Show with an example.[2]
    4. (c)(ii)OR : What are structural isomers ? Draw two structural isomers of butane (${\text{C}}_4{\text{H}}_{10}$).[2]
    Show answer

    It is propyne, an alkyne; saturated compounds burn with a clean blue flame, unsaturated with a sooty yellow flame; alkenes and alkynes undergo addition.

    (a) The compound ${\text{CH}}_3-\text{C}\equiv \text{CH}$ contains a carbon-carbon triple bond, so it is an alkyne; with three carbon atoms (molecular formula ${\text{C}}_3{\text{H}}_4$) its name is propyne. (b) A saturated carbon compound burns with a clean blue, non-sooty flame, whereas an unsaturated carbon compound burns with a yellow flame and a great deal of black smoke, because its higher percentage of carbon does not burn completely in a limited supply of air. (c)(i) Unsaturated hydrocarbons, that is the alkenes and the alkynes, undergo addition reactions, because their double or triple bond can open. Example: ethene adds hydrogen over a nickel catalyst to give ethane, ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$. (c)(ii) Structural isomers are compounds having the same molecular formula but different structural formulae, and hence different properties. Butane, ${\text{C}}_4{\text{H}}_{10}$, has two structural isomers: n-butane, ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$ (a straight chain of four carbon atoms), and iso-butane or 2-methylpropane, ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$ (a chain of three carbon atoms with a methyl group on the middle one).

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17. Q177 CBSE 2025
    The combining capacity of various elements depends on the number of ​valence electrons. Also the reactivity of elements ​is explained ​as their tendency to attain a ​completely ​filled ​outer shell, that is, to attain a noble gas configuration. This may be either through gain of electrons or loss of electrons ​or sharing of electrons.

    1. (a)An element A has atomic number 16, how will it attain its nearest noble gas configuration ?[1]
    2. (b)Write the number of (i) single and (ii) double covalent bonds in a molecule of butene (${\text{C}}_4{\text{H}}_8$).[1]
    3. (c)(i)Explain the formation of a molecule of ammonia (${\text{NH}}_3$), using electron dot structure. (Atomic number of nitrogen is 7)[2]
    4. (c)(ii)OR : Why does carbon share its valence electrons with other atoms of carbon or with atoms of other elements ?[2]
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    Element A gains two electrons (${\text{A}}^{2-}$); butene has 10 single and 1 double bond; ammonia has three shared pairs and a lone pair.

    (a) Element A has the electronic configuration $2,8,6$, so its outermost shell is two electrons short of the argon configuration $2,8,8$. It attains its nearest noble gas configuration by gaining two electrons, becoming the negative ion ${\text{A}}^{2-}$. (b) Butene, ${\text{C}}_4{\text{H}}_8$, has $10$ single covalent bonds (eight $\text{C}-\text{H}$ and two $\text{C}-\text{C}$) and $1$ double covalent bond (one $\text{C}=\text{C}$). (c)(i) Nitrogen $(2,5)$ has five valence electrons and needs three more, while each hydrogen has one and needs one more. Nitrogen shares one electron with each of three hydrogen atoms, giving three shared pairs, that is three single covalent $\text{N}-\text{H}$ bonds, and the two electrons of nitrogen that are left over stay as a lone pair. Nitrogen then counts eight electrons and each hydrogen two. (c)(ii) Carbon $(2,4)$ has four valence electrons. It cannot gain four, because a nucleus of six protons could not hold ten electrons, and it cannot lose four, because removing four electrons from one atom needs an enormous amount of energy. It therefore shares its four valence electrons with other atoms of carbon or with atoms of other elements, forming four covalent bonds and attaining a stable octet.

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18. Q178 CBSE 2024
    Carbon ​compounds :

    1. (i)are good conductors ​of ​electricity.
    2. (ii)are bad conductors ​of electricity.
    3. (iii)have strong forces of attraction between their ​molecules.
    4. (iv)have weak forces of attraction between ​their molecules. The ​correct statements are ​:

    1. (a)(i) and (ii)
    2. (b)(ii) and (iii)
    3. (c)(ii) and (iv)
    4. (d)(i) and (iii)
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    (c) (ii) and (iv)

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19. Q179 CBSE 2024
    The name and formula ​of third ​member of ​homologous series of alkyne ​is :

    1. (a)Propyne ${\text{C}}_3{\text{H}}_6$
    2. (b)Propyne ${\text{C}}_3{\text{H}}_4$
    3. (c)Butyne ${\text{C}}_4{\text{H}}_8$
    4. (d)Butyne ${\text{C}}_4{\text{H}}_6$
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    (d) Butyne ${\text{C}}_4{\text{H}}_6$

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20. Q180 CBSE 2024
    The ​structural ​formula ​of Cyclohexane is

    1. (a)A ring of six carbon atoms with double bonds on the two upper slanting sides and on the bottom side, single bonds on the other three sides, and one H atom on each carbon.
    2. (b)A ring of six carbon atoms joined by single bonds only, with two H atoms on each carbon.
    3. (c)A ring of six carbon atoms with triple bonds on the top and bottom sides, single bonds on the four slanting sides, and two H atoms on each of the left and right carbons.
    4. (d)A ring of six carbon atoms with double bonds on all four slanting sides, single bonds on the top and bottom sides, and one H atom on each carbon.
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    (b) A ring of six carbon atoms joined by single bonds only, with two H atoms on each carbon.

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