JEE Main 2023 · 10 April · morning · Single correct · +4 / −1
Lime reacts exothermally with water to give 'A' which has low solubility in water. Aqueous solution of 'A' is often used for the test of CO₂, a test in which insoluble B is formed. If B is further reacted with CO₂ then soluble compound is formed, 'A' is :
- (a) Quick lime
- (b) White lime
- (c) Lime water
- (d) Slaked lime
Answer and solution
Answer (d) Slaked lime
Solution
Chemistry ma’am · 4 steps
Lime plus water is slaking, so 'A' is slaked lime. Its clear solution in water is what we call lime water.
- Slaking lime. Lime (quick lime) is CaO. It reacts with water and gives out a lot of heat, forming calcium hydroxide, which is only sparingly soluble in water.
CaO + H₂O → Ca(OH)₂ (exothermic)
- Test for CO2. A clear solution of Ca(OH)₂ (lime water) turns milky with CO₂ because insoluble CaCO₃ forms: Ca(OH)₂ + CO₂ → CaCO₃ v + H₂O. So B is CaCO₃.
- Excess CO2. With more CO₂ the milkiness disappears, because soluble calcium hydrogencarbonate forms: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. Every clue fits Ca(OH)₂ as A.
- So. 'A' is Ca(OH)₂, the compound named slaked lime, option (d).
The idea to keep. Keep the lime family straight: CaO is quick lime, Ca(OH)₂ is slaked lime, and its clear aqueous solution is lime water. Milky with CO₂, clear again with excess CO₂.
Answer from the NTA final answer key. Solution by PrepWiser.