Chapter 4: Carbon and its Compounds Questions & Exercise Answers
1What would be the electron dot structure of carbon dioxide which has the formula CO₂?
Answer & Reference:
Carbon dioxide has a linear structure: O=C=O. Carbon shares two electrons with each oxygen atom, forming two double bonds.
Fig. Electron dot structure of CO₂ (Page 61)
2What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?
Answer & Reference:
Sulphur forms a ring of 8 atoms (S₈). Each S atom is bonded to two neighboring S atoms by single covalent bonds.
Fig. S₈ ring structure (Page 61)
3How many structural isomers can you draw for pentane?
Answer & Reference:
Pentane (C₅H₁₂) has three structural isomers:
(i) n-pentane: C–C–C–C–C
(ii) isopentane (2-methylbutane)
(iii) neopentane (2,2-dimethylpropane)
(Page 68)
4What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer & Reference:
(i) Catenation: Carbon can form bonds with other carbon atoms to form long chains, branched chains, or rings.
(ii) Tetravalency: Carbon has a valency of four, allowing it to bond with four other atoms (including other elements like H, O, N, etc.).
(Page 62)
5What will be the formula and electron dot structure of cyclopentane?
Answer & Reference:
Formula: C₅H₁₀
It is a pentagonal ring of 5 carbon atoms, each bonded to two H atoms.
Fig. Cyclopentane (C₅H₁₀) (Page 68)
6Draw the structures for the following compounds: (i) Ethanoic acid (ii) Bromopentane (iii) Butanone (iv) Hexanal.
8Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
Answer & Reference:
Because oxygen is added to ethanol (CH₃CH₂OH → CH₃COOH), which is the definition of oxidation. Oxidising agents like alkaline KMnO₄ or acidified K₂Cr₂O₇ are used.
(Page 71)
9A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Answer & Reference:
Air contains only ~21% oxygen, leading to incomplete combustion and a sooty flame. Pure oxygen ensures complete combustion, producing a very hot (~3000°C) clean flame suitable for welding.
(Page 71)
10How would you distinguish experimentally between an alcohol and a carboxylic acid?
Answer & Reference:
Add sodium carbonate (Na₂CO₃) or NaHCO₃:
- Carboxylic acid produces CO₂ gas (effervescence, turns lime water milky).
- Alcohol shows no reaction.
(Page 74, Activity 4.9)
11What are oxidising agents?
Answer & Reference:
Substances that add oxygen to other substances or remove hydrogen. Examples: alkaline KMnO₄, acidified K₂Cr₂O₇.
(Page 71)
12Ethane, with the molecular formula C₂H₆ has how many covalent bonds?
13Butanone is a four-carbon compound with which functional group?
Answer & Reference: (c) ketone — Butanone has the structure CH₃COCH₂CH₃.
(Page 77)
14While cooking, if the bottom of the vessel is getting blackened, it means that:
Answer & Reference: (b) the fuel is not burning completely — Incomplete combustion produces soot (carbon particles).
(Page 77)
15Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Answer & Reference:
In CH₃Cl, carbon shares:
- 1 electron with Cl
- 1 electron each with 3 H atoms
All atoms achieve noble gas configuration by sharing electrons (no ions formed).
(Page 60–61)
16Draw the electron dot structures for (a) ethanoic acid (b) H₂S (c) propanone (d) F₂.
Answer & Reference:
(a) Ethanoic acid: CH₃–C(=O)–OH
(b) H₂S: H–S–H (S has 2 lone pairs)
(c) Propanone: CH₃–C(=O)–CH₃
(d) F₂: F–F (each F has 3 lone pairs)
(Page 78)
17What is a homologous series? Explain with an example.
Answer & Reference:
A series of compounds with the same functional group, successive members differing by –CH₂–.
Example: Alcohols – CH₃OH, C₂H₅OH, C₃H₇OH...
Physical properties change gradually; chemical properties remain similar.
(Page 66–67)
18How can ethanol and ethanoic acid be differentiated on the basis of physical and chemical properties?
Answer & Reference: Physical: Ethanoic acid has vinegar-like smell; ethanol has pleasant smell. Chemical: Ethanoic acid reacts with NaHCO₃ to give CO₂; ethanol does not.
Ethanoic acid turns blue litmus red; ethanol does not.
(Page 72–74)
19Why does micelle formation take place when soap is added to water? Will a micelle form in ethanol?
Answer & Reference:
Soap has hydrophobic (tail) and hydrophilic (head) parts. In water, tails cluster inward to avoid water, forming micelles.
In ethanol (organic solvent), no micelle forms because the hydrophobic part is soluble.
(Page 75)
20Why are carbon compounds used as fuels?
Answer & Reference:
They release large amounts of heat and light on combustion (e.g., methane, propane, ethanol).
(Page 69)
21Explain scum formation when hard water is treated with soap.
Answer & Reference:
Hard water contains Ca²⁺/Mg²⁺ ions. Soap (sodium stearate) reacts to form insoluble calcium/magnesium stearate (scum).
(Page 76)
22What change will you observe if you test soap with litmus paper?
Answer & Reference:
Soap is basic (salt of strong base + weak acid), so it turns red litmus blue.
(Implied from Page 75: soap is sodium salt of fatty acid)
23What is hydrogenation? What is its industrial application?
Answer & Reference:
Addition of H₂ to unsaturated oils (e.g., vegetable oil) in presence of Ni catalyst to form saturated fats (vanaspati ghee).
Used to convert liquid oils into solid fats.
(Page 71)
Answer & Reference:
Only unsaturated hydrocarbons: C₃H₆ (propene) and C₂H₂ (ethyne).
(Page 71)
25Give a test to differentiate between saturated and unsaturated hydrocarbons.
Answer & Reference:
Add bromine water:
- Unsaturated: decolourises Br₂ water
- Saturated: no decolourisation
(Page 71, implied from addition reactions)
26Explain the cleaning action of soaps.
Answer & Reference:
Soap molecules form micelles in water. Hydrophobic tails attach to oil/dirt; hydrophilic heads face water. Micelles emulsify dirt and rinse away.
Fig. Micelle structure (Page 75, Fig. 4.12)
(Page 75)
27Would you be able to check if water is hard by using a detergent?
Answer & Reference: No, because detergents do not form scum with Ca²⁺/Mg²⁺ ions. They lather equally in soft and hard water.
(Page 76)
28Why is agitation necessary to clean clothes?
Answer & Reference:
Agitation helps dislodge dirt and allows soap micelles to trap and suspend oily particles in water for rinsing.
(Page 76)