CAPF AC 2021 Q17: Science & Technology – Chemistry
- (a) The reaction of calcium with sulphuric acid is not feasible at room temperature
- (b) Formed calcium sulphate is insoluble in water and covers the calcium metal, preventing it from reacting any further
- (c) Formed calcium sulphate requires two molecules of water to crystallize as gypsum
- (d) Formed calcium sulphate is unstable
Answer: (b) Formed calcium sulphate is insoluble in water and covers the calcium metal, preventing it from reacting any further
Explanation
Calcium and sulphuric acid
How to think: the reaction starts and then stops.
So it is feasible, and something must be blocking the metal afterwards.
Look for the option that describes a coating.
The reaction: Ca + H₂SO₄ → CaSO₄ + H₂.
| Step | What happens |
|---|---|
| Start | Calcium reacts with the acid and hydrogen gas bubbles out |
| Product | Calcium sulphate forms, which is only sparingly soluble in water |
| Coating | It settles on the metal as a layer |
| Stop | The acid can no longer reach the calcium, so the reaction stops |
Same idea elsewhere:
| Case | Insoluble coating |
|---|---|
| Marble (CaCO₃) with dilute sulphuric acid | Calcium sulphate |
| Lead with dilute sulphuric acid | Lead sulphate |
| Aluminium in air | Aluminium oxide |
Answer: (b) insoluble calcium sulphate covers the metal.
Why each option is right or wrong
- (a) Incorrect. The reaction does start at room temperature; calcium is a reactive metal.
- (b) CORRECT. Calcium sulphate is sparingly soluble and coats the metal, cutting it off from the acid.
- (c) Incorrect. Gypsum is CaSO₄·2H₂O, a true fact, but it does not explain why the reaction stops.
- (d) Incorrect. Calcium sulphate is a stable compound; it is found in nature as gypsum.
Key point
An insoluble product can coat a reactant and stop the reaction. Calcium sulphate is sparingly soluble, so calcium (and marble) stop reacting with sulphuric acid after a short while.
Remember
Insoluble calcium sulphate coats the calcium and stops the reaction.
Common mistakes
- Choosing (c) because the fact about gypsum is true; it is not the reason asked for.
- Thinking calcium is too unreactive for the acid; it is above hydrogen in the series.
- Assuming all sulphates dissolve; calcium, barium and lead sulphates do not dissolve well.
Exam tip
Educated guess: the question says the reaction stops, so (a) is out because it says it never starts. A true but unrelated fact, as in (c), is a classic trap.
Sources
- NCERT Class X Science — Metals and Non-metals (reaction of metals with acids)
- NCERT Class X Science — Acids, Bases and Salts