Read the following and answer the questions that follow:
Rajan noticed that a new iron gate installed at his home developed a reddish-brown coating after a few months. His grandmother found that cooking oil stored in an open container had developed an unpleasant smell and taste over time. Rajan's science teacher explained that both changes result from oxidation reactions occurring in everyday life.
(i) Name the reddish-brown coating on the iron gate and state the chemical process responsible for its formation. (1 mark)
(ii) What is the term for the spoilage of fats and oils by oxidation? Name one gas whose contact with the oil causes this change. (1 mark)
(iii) Suggest one method to prevent the iron gate from corroding and one method to slow down the spoilage of the cooking oil. (1 mark)
(iv) State one way in which rusting and rancidity are similar and one way in which they differ as chemical processes. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
(i) The reddish-brown coating on the iron gate is called rust (iron oxide, Fe₂O₃·xH₂O). The chemical process responsible is corrosion, caused by the reaction of iron with moisture and oxygen from air.
(ii) The spoilage of fats and oils due to oxidation is called rancidity. The gas whose contact with oil causes this change is oxygen (O₂).
(iii) The iron gate can be protected by painting or galvanising (coating with zinc). The cooking oil can be protected by storing it in an airtight container or adding antioxidants (nitrogen flushing also slows oxidation).
(iv) Similarity: Both rusting and rancidity are caused by oxidation reactions.
Difference: Rusting affects metals (iron), while rancidity affects fats and oils (organic substances).
Source: Chapter 1, Section 1.3 (Corrosion and Rancidity); Chapter 3, Section 3.5
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Explanation
- (i) Examiners expect both the name "rust" and the process "corrosion" for full credit.
- (ii) "Rancidity" is the exact textbook term; oxygen is the gas to name (the textbook mentions nitrogen flushing to prevent it, implying oxygen causes it).
- (iii) One method each is enough — painting/galvanising for iron; airtight container/antioxidants for oil.
- (iv) The similarity (both = oxidation) and difference (metal vs. fat/oil) is the key contrast examiners look for. Keep it concise — one line each.