Science — CBSE Class 10 board question

(i) Change in colour:

When lead nitrate solution is added to potassium iodide solution, a yellow precipitate of lead iodide is formed.

$$\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \rightarrow \text{PbI}_2(s)\downarrow + 2\text{KNO}_3(aq)$$

(Yellow precipitate of PbI₂ formed)

(ii) Change in temperature:

When zinc granules react with dilute sulphuric acid, hydrogen gas is evolved and temperature of the flask rises (exothermic reaction).

$$\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)\uparrow$$

(iii) Formation of precipitate:

When sodium sulphate solution is mixed with barium chloride solution, a white precipitate of barium sulphate is formed.

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s)\downarrow + 2\text{NaCl}(aq)$$

(White precipitate of BaSO₄ formed)

Source: Chemical Reactions and Equations, Chapter 1 (Activities 1.2, 1.3, 1.10)

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• Each sub-question carries 1 mark: one equation with the observation stated is sufficient.
• Always mention the colour change / temperature change / precipitate colour alongside the equation — examiners award the mark for the observation too, not just the equation.
• Use state symbols (aq), (s), (g) and ↓ or ↑ arrows where applicable; these show precision.
• The Pb(NO₃)₂ + KI reaction is the standard textbook example for colour change (yellow PbI₂); Zn + H₂SO₄ for temperature rise; Na₂SO₄ + BaCl₂ for white precipitate.