Science (086) — AI-generated practice question

Gas produced: Carbon dioxide (CO₂)

Reaction: When sodium carbonate (Na₂CO₃) reacts with dilute hydrochloric acid (HCl), carbon dioxide gas is produced along with sodium chloride and water.

$$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2\uparrow$$

Observation with lime water: When CO₂ is passed through lime water, it turns milky (white). This happens because carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate (CaCO₃):

$$\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$$

Source: Chapter 2 (Acids, Bases and Salts); Chapter 1, Section 1.2.1

---

• 1 mark for correctly naming the gas (CO₂).
• 1 mark for the correct observation (lime water turns milky/white).
• 1 mark for the chemical equation/reason — CaCO₃ precipitate forms.
• Always write the equation for the lime water test; examiners expect it.
• The key phrase is "turns milky" — not cloudy or white precipitate alone, though both are acceptable. "Milky" is the standard CBSE term.