Iron left in a humid atmosphere develops a reddish-brown coating over days, while magnesium ribbon, when ignited, produces a white powder almost instantly. Both involve oxygen, yet one is a slow process and the other is rapid. State the common criterion that allows BOTH to be classified as chemical reactions, and identify the type of chemical reaction each represents.
Generated by claude-sonnet-4-6 · 2026-06-26 00:53 · grounding rag
Model Answer
Common criterion: A chemical reaction has taken place when there is a change in the chemical nature of the substance, resulting in the formation of one or more new substances with different properties. Observable indicators include change in colour, state, temperature, or evolution of gas.
Both rusting of iron and burning of magnesium produce new substances (iron oxide and magnesium oxide respectively), confirming both are chemical reactions.
Type of reaction:
- Rusting of iron — It is a corrosion (slow oxidation) reaction:
$4\text{Fe} + 3\text{O}_2 + x\text{H}_2\text{O} \rightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$
- Burning of magnesium — It is a rapid oxidation (combination) reaction:
$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$
Both are oxidation reactions, but differ greatly in rate.
Source: Chapter 1, Section 1.1; Chapter 3, Section 3.2.1
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Explanation
- Key criterion (1 mark): The examiner wants you to state that formation of a new substance with changed properties = chemical reaction. The textbook lists indicators: change in colour, state, temperature, gas evolution.
- Rusting (1 mark): Identify it as corrosion/slow oxidation. Mentioning it occurs in humid atmosphere matches the textbook ("The surface of some metals, such as iron, is corroded when they are exposed to moist air for a long period of time").
- Burning Mg (1 mark): Identify it as rapid oxidation or combination reaction — textbook explicitly calls it burning in oxygen forming magnesium oxide.
- Both are oxidation/combination reactions, but the rate difference is what the question highlights — always note this contrast.
- Writing balanced equations earns full credit and demonstrates understanding; don't skip them in a 3-mark answer.