Q1. [3] medium thorough-understanding
Iron nails are placed in three separate test tubes: one containing ordinary water open to air, one containing boiled distilled water sealed with a layer of oil, and one containing only dry air with a desiccant. After several days, predict in which test tube rusting will be observed and in which it will not. Give reasons for your answer and state the conditions necessary for iron to rust.
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
Observation:
Rusting will be observed only in Test Tube A (ordinary water open to air). No rusting will occur in Test Tubes B and C.
Reasons:
- Test Tube A: Both air (oxygen) and water are present → rusting occurs.
- Test Tube B: Boiled distilled water removes dissolved air; oil layer prevents air from entering → only water present, no rusting.
- Test Tube C: Desiccant (anhydrous CaCl₂) absorbs all moisture → only dry air present, no rusting.
Conditions necessary for rusting:
The presence of both oxygen (from air) and water simultaneously is essential for iron to rust.
Source: Chapter 3, Section 3.5
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Explanation
- The examiner expects you to name the correct test tube (A) and give a reason for each of the three tubes — that's the core of the answer.
- Key phrase: "both air and water are necessary" — this must appear explicitly.
- Mentioning the role of boiled water (removes dissolved air) and oil layer shows deeper understanding and secures full marks.
- Anhydrous CaCl₂ as desiccant removes moisture — state this clearly for Test Tube C.