Reviewed CSCA Chemistry question · Standard

A student prepares a gas by adding dilute sulfuric acid to solid sodium sulfite. Which test correctly identifies the gas produced?

A glowing splint relights
The gas turns limewater milky
The gas bleaches damp litmus paper
The gas gives a pop sound with a lighted splint

Show the answer and explanation

Correct answer

C. The gas bleaches damp litmus paper

Principle or equation

Sodium sulfite reacts with dilute acid to produce sulfur dioxide (SO2). Sulfur dioxide is a reducing and bleaching agent; it bleaches damp litmus paper (turns it white).

Why this answer is correct

The reaction is Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2. Sulfur dioxide gas is identified by its pungent smell and its ability to bleach damp litmus paper. It does not relight a glowing splint (oxygen does), does not turn limewater milky (carbon dioxide does), and does not give a pop with a lighted splint (hydrogen does).

Example

Na2SO3(s) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + SO2(g). SO2 bleaches damp litmus.

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