LIWA

Carbon and its Compounds

CBSE Class 10 · Science · Chapter 4

Past board exam questions

CBSE 2027 sample paper · 1 mark

Manjeet wants to wash clothes, but the water available is hard. He compares the action of soap and detergent in hard water, to decide which one to use for washing his clothes. He observes: • Soap forms scum in hard water. • Detergent produces lather easily in hard water. Which of the following inferences is correct?

  1. Ca²⁺ and Mg²⁺ ions react with soap, forming insoluble compounds, whereas detergents do not form such insoluble salts.
  2. Soap is acidic, and detergents are basic.
  3. Soap react with Na⁺ ions in hard water to form scum while detergents do not react with Na⁺ ions.
  4. Detergents form lather easily because they are ionic compounds and react with Ca²⁺ and Mg²⁺ ions while soaps do not react with hard water.
Show answer
Ca²⁺ and Mg²⁺ ions react with soap, forming insoluble compounds, whereas detergents do not form such insoluble salts.

CBSE 2027 sample paper · 1 mark

Which of the following pair of compounds are isomers?

  1. CH₃–CH₂–CH₂–CH₃ and CH₃–CH(CH₃)–CH₃
  2. CH₃–CH₂–CH₂–CH₂–CH₃ and CH₃–CH(CH₃)–CH₃
  3. CH₃–CH₂–CH₂–CH₂–CH₃ and CH₃–CH₂–CH=CH–CH₃
  4. CH₃–CH₂–CH₂–CH₂–CH₃ and CH₃–CH₂–CH₂–CH₃
Show answer
CH₃–CH₂–CH₂–CH₃ and CH₃–CH(CH₃)–CH₃

CBSE 2027 sample paper · 2 marks

Carbon forms millions of compounds whereas silicon, which is just below it in the Periodic Table, forms a few compounds. Explain the reason for the difference in their ability to form compounds.

Show answer
Carbon has a small atomic size, so it forms strong and stable C–C bonds and shows extensive catenation (it can form long chains, branched chains and rings). Silicon, being larger in size, forms weaker Si–Si bonds and cannot show catenation to the same extent (it forms chains of only about seven or eight atoms, which are reactive), so it forms far fewer compounds.

CBSE 2027 sample paper · 3 marks

An organic compound “X” with formula C2H6O is used to blend with petrol in order to reduce the dependence on fossil fuels and lower down the harmful emissions from the cars. It is also used as a solvent in medicines such as tincture iodine, cough syrups and many tonics. (i) Draw the electron dot structure of “X”. (ii) Write the name and formula of the next homologue of “X”. (iii) Write the reaction of “X” with acetic acid to form a sweet smelling substance.

Show answer
X is ethanol, CH₃CH₂OH (C₂H₅OH). (i) Electron dot structure of ethanol (each '••' is a shared pair of electrons): H H •• •• H •• C •• C •• O •• H •• •• H H The first carbon shares one electron pair with each of three H atoms and one with the second carbon; the second carbon shares a pair with each of two H atoms and one with oxygen; oxygen shares one pair with the second carbon and one with H, and carries two lone pairs. (ii) Next homologue: propanol, C₃H₇OH. (iii) C2H5OH + CH3COOH xrightarrowconc. H2SO4 CH3COOC2H5 + H2O (ethyl ethanoate, a sweet-smelling ester).

CBSE 2027 sample paper · 3 marks

Ramya performs an experiment in the following two steps: Step 1: Heats ethanol with conc. H₂SO₄ at 170°C to get product A. Step 2: Passes hydrogen gas over product A, using Ni as a catalyst and gets product B. (i) Name the products A and B. (ii) Write the reactions involved in both the steps.

Show answer
(i) Product A – ethene; Product B – ethane. (ii) Step 1 (dehydration): CH3CH2OH xrightarrow[170° C]hot conc. H2SO4 CH2=CH2 + H2O Step 2 (hydrogenation/addition): CH2=CH2 + H2 xrightarrowNi CH3CH3

CBSE 2026 board paper · 1 mark

How many bonds can one carbon atom form in a compound ?

  1. 1
  2. 2
  3. 4
  4. 6
Show answer
4

CBSE 2026 board paper · 1 mark

The number of isomers of butane is

  1. 1
  2. 2
  3. 3
  4. 4
Show answer
2

CBSE 2026 board paper · 1 mark

Which of the following is the name of compound H–C(H)=O, that is, HCHO (a carbon atom bonded to two hydrogen atoms and doubly bonded to an oxygen atom) ?

  1. Methane
  2. Methanal
  3. Methanol
  4. Methanoic acid
Show answer
Methanal