Which of the following statements with regard to the reaction given below are correct? CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat 1. Quicklime is used for white-washing of walls. 2. The solution of slaked lime is used for whitewashing of walls. 3. CaO reacts slowly with CO₂ in air to form a thin layer of CaCO₃ on walls. 4. Calcium hydroxide called 'slaked lime' is an inorganic compound. Select the correct answer using the code given below.
- A.1 and 4 only
- B.2 and 4
- C.2 and 3
- D.1, 3 and 4
Correct answer
Explanation
The correct answer is B, 2 and 4. Walls are whitewashed with a solution of slaked lime, calcium hydroxide, which is an inorganic compound; quicklime itself is not applied, and the shiny layer comes from Ca(OH)₂, not CaO.
When quicklime (calcium oxide) is added to water it reacts vigorously and gives out much heat, forming slaked lime (calcium hydroxide). This is a combination reaction and is exothermic. The slaked lime solution is painted on walls. Over two or three days the calcium hydroxide reacts slowly with carbon dioxide in the air: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, leaving a thin, shiny coat of calcium carbonate.
A is wrong: statement 1 is false, since quicklime is first slaked and the solution, not quicklime, is used.
C is wrong: statement 3 is false, because it is calcium hydroxide, not CaO, that reacts with CO₂ on the wall.
D is wrong: it includes the false statements 1 and 3.
Exam tip: CaO quicklime, Ca(OH)₂ slaked lime, CaCO₃ limestone or marble; whitewash shines because Ca(OH)₂ turns into CaCO₃.
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