What is observed when hydrated ferrous sulphate crystals are heated in a dry boiling tube ? Give balanced chemical equation(s) of the reactions(s) that occur(s).
Generated by claude-sonnet-4-6 · 2026-06-14 10:30 · grounding rag
Model Answer
Observations:
- The green colour of hydrated ferrous sulphate (FeSO₄·7H₂O) crystals changes (first loses water, then turns reddish-brown).
- A characteristic smell of burning sulphur (SO₂/SO₃ gases) is observed.
Chemical equations:
Step 1 – Loss of water on heating:
$$\text{FeSO}_4 \cdot 7\text{H}_2\text{O}(s) \xrightarrow{\Delta} \text{FeSO}_4(s) + 7\text{H}_2\text{O}(g)$$
Step 2 – Thermal decomposition:
$$2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)$$
This is a thermal decomposition reaction.
Source: Chapter 1, Section 1.2.2 – Decomposition Reaction
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Explanation
- Examiners expect two observations: colour change and smell of sulphur gases.
- Both equations must be written; some mark schemes award 1 mark for observations and 1 mark for the equation(s).
- The key equation tested is the decomposition one (eq. 1.19 from the textbook); including the dehydration step shows completeness.
- State symbols and the heat arrow (Δ) add accuracy and are good practice.