With the help of an appropriate example, justify that some of the chemical reactions are determined by
(a) Change in temperature,
(b) Evolution of a gas, and
(c) Change in colour
Give chemical equation for the reaction involved in each case.
Generated by claude-sonnet-4-6 · 2026-06-14 10:27 · grounding rag
Model Answer
(a) Change in temperature: When zinc reacts with dilute sulphuric acid, heat is evolved, showing a rise in temperature (exothermic reaction).
$$\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\uparrow$$
(b) Evolution of a gas: The same reaction above also produces hydrogen gas, which can be observed as bubbles around the zinc granules.
(c) Change in colour: When potassium iodide solution is added to lead nitrate solution, a yellow precipitate of lead iodide is formed, indicating a change in colour.
$$\text{Pb(NO}_3)_2 + 2\text{KI} \rightarrow \text{PbI}_2 + 2\text{KNO}_3$$
Source: Chapter 1, Section 1.1 (Activities 1.2 and 1.3)
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Explanation
- Examiners look for one clear example per observation with a correct chemical equation.
- For (a) and (b), the same Zn + H₂SO₄ reaction is acceptable for both, as it shows both temperature change and gas evolution — but state each observation separately.
- For (c), the lead nitrate + potassium iodide reaction is the textbook example (yellow PbI₂ precipitate).
- Make sure arrows (↑ for gas, ↓ for precipitate) are included — they fetch marks.
- Each sub-part carries 1 mark; losing the equation loses half the mark for that part.