Science — AI-generated practice question

The metal compound A is calcium carbonate (CaCO₃). It reacts with dilute HCl to produce CO₂ gas (which extinguishes a burning candle) and calcium chloride.

Word equation:
Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide

Balanced chemical equation:

$$\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$$

This is a double displacement (and decomposition) reaction. CO₂ is heavier than air and does not support combustion, hence it extinguishes the candle.

Source: Chemical Reactions and Equations, Chapter 1

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• Identify compound A first: The clues are — reacts with HCl, gives a gas that extinguishes a candle (CO₂), and forms CaCl₂ → compound A must be CaCO₃.
• Balancing: Note the coefficient 2 before HCl — examiners specifically check this.
• 1 mark for identifying A, 1 mark for the correct balanced equation, 1 mark for stating the gas is CO₂ and why it extinguishes the candle.
• Do not write H₂CO₃ as a product; it immediately decomposes to H₂O and CO₂.