SSC CGL 2024 Tier-I · 13 September 2024, Shift 1 · General Awareness · Question 19 of 100

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 polyatomic ionic compound is a white, crystalline powder used in fire extinguishers and to neutralise acids and bases?

(a)Sodium Bicarbonate
(b)Sodium Thiosulphate
(c)Sodium Bisulphite
(d)Sodium Chromate

Answer

Why

Correct — A. Sodium bicarbonate, NaHCO₃, is the white crystalline powder the stem describes, and its bicarbonate ion, HCO₃⁻, is the polyatomic part.

Treated with acid or heated it releases carbon dioxide, the gas that smothers a flame. That is the working principle of soda-acid and dry-powder extinguishers.

Being the salt of a weak acid with a strong base, it also reacts with acids and with bases, which is the neutralising behaviour in the stem — so option (a).

Why the others are wrong

(b)
Sodium Thiosulphate — Sodium thiosulphate (Na₂S₂O₃) is the photographer's hypo and the standard reagent in iodine titrations. It is not an extinguisher chemical and does not work as an antacid.
(c)
Sodium Bisulphite — Sodium bisulphite (NaHSO₃) is used as a reducing agent and a preservative. Its gas is sulphur dioxide, not carbon dioxide, so it cannot do the smothering job described.
(d)
Sodium Chromate — Sodium chromate (Na₂CrO₄) is a yellow chromium(VI) compound and toxic. The single word white in the stem eliminates it before its chemistry needs any thought.

Concept

A polyatomic ion is a charged group of covalently bonded atoms that travels as one unit — bicarbonate HCO₃⁻, thiosulphate S₂O₃²⁻, chromate CrO₄²⁻.

All four options are sodium salts of such ions, so that phrase in the stem is scene-setting rather than the discriminator. The two words that decide the question are white and fire extinguisher.

Sodium bicarbonate is best learnt as a cluster: baking soda in the kitchen, an antacid in the stomach, and the carbon dioxide source in an extinguisher.

The phrase 'neutralise acids and bases' reads oddly for one compound. Sodium bicarbonate does behave both ways, being the salt of a weak acid with a strong base, so take the stem at face value instead of hunting for a misprint.

Key facts

Sodium bicarbonate has the formula NaHCO₃ and is commonly called baking soda.
Sodium bicarbonate releases carbon dioxide on reaction with an acid, which is how soda-acid fire extinguishers work.
Sodium chromate is yellow, which alone rules it out of a stem specifying a white powder.
The bicarbonate ion, HCO₃⁻, is a polyatomic ion.

Study next

Sodium carbonate, washing soda, and how it differs from baking sodaSalts of strong and weak acids and bases and the pH of their solutionsCommon polyatomic ions and their charges

Common traps

Reading polyatomic ionic compound as the discriminator when every option satisfies it
Confusing sodium bicarbonate with sodium carbonate, which is washing soda

Chemistry stems describe a compound by its use and appearance and ask you to name it, or run the exchange in reverse. The same compound comes up at 18 Sep 2024, 09:00, GA Q.8, on why a milkman adds a little baking soda to fresh milk.

Related PYQs

No directly related past PYQ was found.