Science (086) — AI-generated practice question

When sodium carbonate reacts with dilute hydrochloric acid, it produces sodium chloride, water, and carbon dioxide gas. Effervescence (brisk bubbling) is observed.

Balanced equation:

$$\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

Test for the gas produced (CO₂):
Pass the gas through lime water [Ca(OH)₂ solution]. The lime water turns milky, confirming the gas is carbon dioxide.

$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$$

The white precipitate of CaCO₃ causes the milkiness.

Source: Chapter 2, Section 2.1.3 — How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

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• Mark distribution (3 marks): 1 mark for stating the products/observation, 1 mark for the balanced equation, 1 mark for the lime water test with its equation or description.
• Always include state symbols (s, aq, l, g) in balanced equations — examiners reward this.
• The lime water test is the standard test for CO₂ at this level; mentioning the white precipitate of CaCO₃ completes the answer.
• Do not confuse Na₂CO₃ (sodium carbonate / washing soda) with NaHCO₃ (baking soda) — both react with HCl to give CO₂, but the equations differ in stoichiometry.