SSC CGL 2024 Tier-I · 18 September 2024, Shift 3 · General Awareness · Question 13 of 99

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 10 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. The questions and options here are reproduced as printed on a candidate's own response sheet for this shift — the Commission's text, unchanged. Each answer is SSC's tentative key as marked on that sheet; SSC allowed challenges to it and never publishes a final key. The solutions are written by UnlockIAS.

Which of the following chemical reactions takes place when quick lime reacts with water?

(a)CaO(s) + H₂O(l) → Ca + H₂(aq)
(b)CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)
(c)C(s) + O₂(g) → CO₂(g)
(d)CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Answer

Why

Correct — D. Quick lime is calcium oxide, CaO. With water it forms calcium hydroxide, slaked lime:

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat

That is option (d), the equation NCERT Class 10 gives for this reaction. Two reactants make a single product, so it is a combination reaction. The large amount of heat released makes it exothermic.

Why the others are wrong

(a)
CaO(s) + H₂O(l) → Ca + H₂(aq) — Not balanced: CaO and H₂O bring two oxygen atoms, and the right side, Ca + H₂(aq), has none. Quick lime gives no hydrogen with water. It takes the water up and becomes Ca(OH)₂.
(b)
CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g) — CH₄ + 2O₂ → CO₂ + 2H₂O is the burning of natural gas, methane. NCERT uses it as an example of an exothermic reaction, but neither lime nor water is a reactant.
(c)
C(s) + O₂(g) → CO₂(g) — C + O₂ → CO₂ is the burning of coal. It is a combination reaction like the correct answer, but its reactants are carbon and oxygen, not quick lime and water.

Concept

Quick lime is calcium oxide, CaO. It is made by heating limestone, CaCO₃, which decomposes into calcium oxide and carbon dioxide.

Quick lime reacts vigorously with water to give slaked lime, calcium hydroxide, releasing a large amount of heat.

A solution of slaked lime is used for whitewashing. Over two to three days it reacts with carbon dioxide in the air and leaves a thin, shiny layer of calcium carbonate on the wall.

Options (b), (c) and (d) are all equations from NCERT Class 10, Chapter 1: the burning of natural gas, the burning of coal and the slaking of lime. The item rewards matching an equation to the reactants the stem names.

Key facts

Quick lime is calcium oxide, CaO, and slaked lime is calcium hydroxide, Ca(OH)₂.
CaO + H₂O → Ca(OH)₂ is a combination reaction that releases a large amount of heat.
Heating limestone, CaCO₃, decomposes it into quick lime and carbon dioxide.
In whitewashing, calcium hydroxide reacts with carbon dioxide in air to form a shiny layer of calcium carbonate.

Study next

Types of chemical reactions — combination, decomposition, displacement and double displacementExothermic and endothermic reactions with their textbook examplesLimestone, quick lime and slaked lime, and how each turns into the next

Common traps

Writing hydrogen as a product because metals and water often give hydrogen, when calcium oxide gives a hydroxide.
Mixing up quick lime, CaO, with slaked lime, Ca(OH)₂, or with limestone, CaCO₃.

The step that makes quick lime, the decomposition of calcium carbonate, is tested at 09 Sep 2024, 09:00, GA Q.11, as a decomposition that is not a redox reaction. Reaction types are asked directly at 09 Sep 2024, 12:30, GA Q.6, on passing electricity through water.

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 152 SSC CGL questions on it, every solved shift.