The following salts will produce a gas on reacting with hydrochloric acid except

Explanation

When certain salts react with hydrochloric acid (HCl), they can produce gases. Let us look at each salt one by one to see which ones release a gas and which one does not.

CaCO₃ (calcium trioxocarbonate(IV)) reacts with HCl to produce carbon dioxide gas (CO₂):
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑
The CO₂ gas causes bubbling or effervescence.

FeS (iron(II) sulphide) reacts with HCl to produce hydrogen sulphide gas (H₂S), which smells like rotten eggs:
FeS + 2HCl → FeCl₂ + H₂S↑

Na₂SO₃ (sodium trioxosulphate(IV)) reacts with HCl to produce sulphur dioxide gas (SO₂), which has a choking smell:
Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂↑

CuSO₄ (copper(II) tetraoxosulphate(VI)) does NOT produce any gas when it reacts with HCl. The tetraoxosulphate(VI) ion (SO₄²⁻) is very stable and does not decompose to release a gas when treated with dilute acid. Instead, copper(II) chloride and sulphuric acid are formed in solution:
CuSO₄ + 2HCl → CuCl₂ + H₂SO₄
No gas is produced in this reaction.

Therefore, CuSO₄ is the salt that will NOT produce a gas on reacting with hydrochloric acid.