CBSE Class 10 Science Carbon And Its Compounds Worksheet Set 10

This is Worksheet Set 10 for CBSE Class 10 Science Chapter 4, Carbon and Its Compounds, a quick printable practice sheet with the answer and a Teacher's Note under every question. It includes counting covalent bonds, naming functional groups, general formulae and next homologues, saturated and unsaturated hydrocarbons, and simple facts about ethanol, flames and combustion.

Bonds, Functional Groups and Homologous Series

Question 1. Write the number of covalent bonds in the molecule of ethane. [1 mark]
Answer:
Ethane is CH₃–CH₃: one C–C bond and six C–H bonds, so there are seven covalent bonds.
In simple words: 1 C–C + 6 C–H = 7 bonds.

Teacher's Note:
1. Draw the structure to count.
2. For C_nH_2n+2 the number of bonds is 3n + 1.

Question 2. Write the number of covalent bonds in the molecule of propane, C₃H₈. [1 mark]
Answer:
Propane is CH₃–CH₂–CH₃: two C–C bonds and eight C–H bonds, so there are ten covalent bonds.
In simple words: 2 C–C + 8 C–H = 10 bonds.

Teacher's Note:
1. Use 3n + 1 with n = 3.
2. Each carbon must have four bonds.

Question 3. Write the number of covalent bonds in the molecules of butane C₄H₁₀. [1 mark]
Answer:
Butane is CH₃–CH₂–CH₂–CH₃: three C–C bonds and ten C–H bonds, so there are thirteen covalent bonds.
In simple words: 3 C–C + 10 C–H = 13 bonds.

Teacher's Note:
1. Use 3n + 1 with n = 4.
2. Count C–H bonds as equal to the number of H atoms.

Question 4. Write the name of each of the following functional groups: (a) —OH (b) —CO— [1 mark]
Answer:
(a) –OH is the alcohol (hydroxyl) group.
(b) –CO– (the C=O group inside a chain) is the ketone group.
In simple words: –OH is alcohol and –CO– is ketone.

Teacher's Note:
1. Ketone group is always in the middle of the chain.
2. Aldehyde group is –CHO.

Question 5. Why do the atoms take part in bonding ? [1 mark]
Answer:
Atoms bond to attain the electronic configuration of the nearest noble (inert) gas, that is, a full outer shell (octet or duplet), which makes them stable.
In simple words: Atoms bond to get a stable full outer shell.

Teacher's Note:
1. Bonds form by transfer or sharing of electrons.
2. Noble gases are already stable.

Question 6. Name the bond present in the molecules of N₂. [1 mark]
Answer:
Each nitrogen atom (2, 5) needs three electrons, so the two atoms share three pairs. The bond in N₂ is a triple covalent bond, N≡N.
In simple words: Nitrogen atoms share three pairs of electrons.

Teacher's Note:
1. Valency of nitrogen is three.
2. Oxygen forms a double bond.

Question 7. Why are covalent compounds generally poor conductors of electricity ? [1 mark]
Answer:
Covalent compounds are made of molecules, not ions, and have no free electrons, so they do not conduct electricity in solution or when molten.
In simple words: No ions, no free electrons, so no conduction.

Teacher's Note:
1. Compare with ionic compounds.
2. Graphite is an exception.

Question 8. The molecular formula of the consecutive members of a homologous series are C₆H₁₄ and C₇H₁₆. Write the molecular formulae of members having 9 and 11 carbon atoms of this homologous series. [2 marks]
Answer:
The series follows C_nH_2n+2 (alkanes).
9 carbon atoms: H = 2 × 9 + 2 = 20, so C₉H₂₀.
11 carbon atoms: H = 2 × 11 + 2 = 24, so C₁₁H₂₄.
In simple words: Use C_nH_2n+2 with n = 9 and n = 11.

Teacher's Note:
1. Check with C₆H₁₄: 2 × 6 + 2 = 14.
2. Alkanes have only single bonds.

Question 9. Write the valency of each carbon atom in (i) an alkane and (ii) an alkyne. [1 mark]
Answer:
(i) In an alkane the valency of each carbon atom is 4 (four single bonds).
(ii) In an alkyne the valency of each carbon atom is also 4 (for example, one triple bond and one single bond).
In simple words: Carbon is always tetravalent.

Teacher's Note:
1. Valency is the number of bonds, not the number of atoms attached.
2. A triple bond counts as three.

Ethanol, Flames and Combustion

Question 10. What is alcohol content in rectified spirit ? [1 mark]
Answer:
Rectified spirit contains about 95% ethanol (and 5% water).
In simple words: Rectified spirit is 95% alcohol.

Teacher's Note:
1. Pure ethanol is called absolute alcohol.
2. Ethanol and water cannot be completely separated by simple distillation.

Question 11. Why should both sodium and ethyl alcohol be dry when these are made to react ? [1 mark]
Answer:
Sodium reacts violently with water, forming sodium hydroxide and hydrogen gas, which can catch fire. If water is present, sodium reacts with it instead of with ethanol, so the reaction with ethanol cannot be studied properly.
In simple words: Water would react with sodium first, and dangerously.

Teacher's Note:
1. Sodium is stored in kerosene.
2. Use a small piece and keep away from water.

Question 12. Name the fourth (4th) member of alkene series. [1 mark]
Answer:
The alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀). The fourth member is pentene, C₅H₁₀.
In simple words: Count from ethene: ethene, propene, butene, pentene.

Teacher's Note:
1. The first alkene is ethene.
2. Formula C_nH_2n with n = 5.

Question 13. Name the functional group present in propanone (CH₃COCH₃). [1 mark]
Answer:
The functional group in propanone is the ketone group, –CO–.
In simple words: Propanone is a ketone.

Teacher's Note:
1. Ending -one means ketone.
2. It is also called acetone.

Question 14. Name the functional group present in each of the following compounds: C₃H₇OH, CH₃–CO–CH₃ [1 mark]
Answer:
C₃H₇OH is propanol and has the alcohol group (–OH). CH₃–CO–CH₃ is propanone and has the ketone group (–CO–).
In simple words: Alcohol group in C₃H₇OH and ketone group in propanone.

Teacher's Note:
1. Look at the atoms bonded to the carbon chain.
2. Ketone has C=O in the middle.

Question 15. Name the products obtained on complete combustion of ethanol. [1 mark]
Answer:
Ethanol burns to give carbon dioxide and water, with heat and light: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
In simple words: Complete combustion of ethanol gives CO₂ and water.

Teacher's Note:
1. Check the balancing: 2 C, 6 H and 7 O on each side.
2. Ethanol is a clean fuel.

Question 16. "Saturated hydrocarbons burn with a blue flame while unsaturated hydrocarbons burn with a sooty flame." Why ? [2 marks]
Answer:
Saturated hydrocarbons have a low percentage of carbon and burn completely in sufficient air, giving a clean blue flame.
Unsaturated hydrocarbons have a high percentage of carbon and burn incompletely, so unburnt carbon particles glow yellow and form soot.
In simple words: Complete burning gives blue flame; incomplete burning gives sooty flame.

Teacher's Note:
1. Flame colour is a test for unsaturation.
2. More air gives a blue flame.

Question 17. Which product is formed when ethanol is heated with dilute solution of alkaline KMnO₄ ? [1 mark]
Answer:
Ethanol is oxidised to ethanoic acid, CH₃COOH. The purple colour of KMnO₄ disappears.
In simple words: Alkaline KMnO₄ turns ethanol into ethanoic acid.

Teacher's Note:
1. KMnO₄ acts as an oxidising agent.
2. Two carbon atoms remain in the product.

Question 18. Select saturated hydrocarbons from the following: C₃H₆; C₅H₁₀; C₄H₁₀; C₆H₁₄; C₂H₄. [1 mark]
Answer:
Saturated hydrocarbons (alkanes) follow C_nH_2n+2. C₄H₁₀ (n = 4: 2 × 4 + 2 = 10) and C₆H₁₄ (n = 6: 14) fit.
C₃H₆, C₅H₁₀ and C₂H₄ follow C_nH_2n and are unsaturated (alkenes).
Saturated: C₄H₁₀ and C₆H₁₄.
In simple words: C₄H₁₀ and C₆H₁₄ fit the alkane formula.

Teacher's Note:
1. Test each formula in C_nH_2n+2.
2. Ring compounds like cyclohexane (C₆H₁₂) are an exception.

Question 19. Write the name and formula of the 2nd member of homologous series having general formula CnH₂n–2. [1 mark]
Answer:
C_nH_2n−2 is the general formula of alkynes. The first member is ethyne (C₂H₂), so the second member is propyne, C₃H₄.
In simple words: The second alkyne is propyne.

Teacher's Note:
1. Alkynes have a triple bond.
2. The first alkyne has n = 2.