CAPF AC 2025 Q97: Science & Technology – Chemistry
Which of the following statements relating to carbon and its compounds are correct?
1. 1-Propyne has three double bonds.
2. The boiling point of acetic acid is higher than methane.
3. Ethanol reacts with sodium metal and produces hydrogen gas.
4. Esters give effervescence on reaction with sodium bicarbonate.
Select the answer using the code given below.
- (a) 1 and 3
- (b) 2 and 3 only
- (c) 1 and 2
- (d) 2, 3 and 4
Answer: (b) 2 and 3 only
Explanation
Carbon and its compounds
How to think: check each statement with a simple fact.
| Statement | Fact | Verdict |
|---|---|---|
| 1. 1-Propyne has three double bonds | Propyne (CH≡C-CH₃) has one triple bond and no double bond | Wrong |
| 2. The boiling point of acetic acid is higher than methane | Acetic acid boils at about 118 °C, methane at about -162 °C | Correct |
| 3. Ethanol reacts with sodium metal and gives hydrogen gas | 2C₂H₅OH + 2Na gives 2C₂H₅ONa + H₂ | Correct |
| 4. Esters give effervescence with sodium bicarbonate | It is carboxylic acids (such as acetic acid) that give CO₂, not esters | Wrong |
Statements 2 and 3 are correct.
Bank key corrected: the stored key was (c) 1 and 2; the right answer is (b).
Answer: (b) 2 and 3 only.
Why each option is right or wrong
- (a) Incorrect. Statement 1 is wrong because propyne has a triple bond.
- (b) CORRECT. Statements 2 and 3 are correct; 1 and 4 are wrong.
- (c) Incorrect. It includes statement 1, which is wrong.
- (d) Incorrect. Statement 4 is wrong: esters do not fizz with sodium bicarbonate.
Key point
Propyne: one C≡C triple bond. Acetic acid bp 118 °C; methane bp -162 °C. Ethanol + Na gives sodium ethoxide + H₂. Carboxylic acids + NaHCO₃ give CO₂ (effervescence); esters do not.
Remember
Acids fizz with NaHCO₃; esters do not. -yne means triple bond.
Common mistakes
- Treating 'propyne' as having double bonds; '-yne' means a triple bond.
- Giving the sodium bicarbonate test to esters instead of acids.
- Forgetting the boiling point comparison between a liquid acid and a gas.
Exam tip
Educated guess: 'propyne' ends in -yne, and three double bonds in a 3-carbon chain is impossible, so statement 1 is false. That leaves (b) and (d); esters are famous for their fruity smell, not for fizzing, so (b).
Sources
- NCERT Class X Science, Chapter 4 (Carbon and its Compounds: properties of ethanol and ethanoic acid)
- NCERT Class XI Chemistry, Part II, Chapter 12 (Organic Chemistry: Some Basic Principles and Techniques) and Chapter 13 (Hydrocarbons)