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Primary General Studies: Air, Combustion and Rusting (Paper 1 (basics))
Primary General Studies: Air, Combustion and Rusting (Paper 1 (basics))
Paper one moves from the composition of air to the conditions for rusting, going from easy sums to harder ones that can all be checked with the four operations.
Direct answers Write down the 21 or 78 per cent step before a gas question, ask whether oxygen and water are both present before a rusting question, and check that the answer is sensible.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: Primary General Studies: Air, Combustion and Rusting |
| Level | parts of a closed circuit, conductors against insulators, series against parallel, adding up battery voltage, and electrical safety |
02 Questions (10 items)
- FoundationWhich gas is the most abundant in air?
- FoundationWhich gas in the air does a fuel need in order to burn?
- FoundationWhich two conditions must be present at the same time for iron to rust?
- IntermediateAbout how many litres of oxygen are there in 100 litres of air?
- IntermediateA sealed 500 mL bottle uses up all its oxygen. How much oxygen is that?
- IntermediateA 20 g nail becomes 23 g after rusting. Where do the extra 3 g come from?
- ChallengeTwo nail experiments: the salt water group shows 12 rust spots in 3 days and the dry group shows none. What is the difference?
- ChallengeAbout how many litres of oxygen are in 250 litres of air?
- ChallengeAfter all the oxygen in a sealed 300 mL bottle is burnt away, how much other gas is left?
- Error redoA classmate says iron rusts only because of water and has nothing to do with air. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Nitrogen, about 78 per cent of the air. | Nitrogen is about 78 per cent, oxygen about 21 per cent, and the rest is carbon dioxide and other gases. |
| 2 | Oxygen. | Oxygen supports burning and makes up about 21 per cent of air, so a fire goes out without it. |
| 3 | Oxygen and water. | Iron does not rust when either oxygen or water is missing, so both must be present together. |
| 4 | About 21 litres. | 100 times 0.21 is 21, because oxygen is about 21 per cent of air. |
| 5 | That is 105 mL. | 500 times 0.21 is 105. |
| 6 | From oxygen in the air. | Oxygen joins the iron to form rust, so the mass grows by 3 g. |
| 7 | The difference is 12 spots. | 12 minus 0 is 12, because salt water speeds up rusting. |
| 8 | About 52.5 litres. | 250 times 0.21 is 52.5. |
| 9 | 237 mL is left. | 300 times 0.79 is 237, and most of that is nitrogen. |
| 10 | Rusting also needs oxygen, so iron with water but no oxygen does not rust. | A nail sealed in boiled distilled water does not rust, because boiling drives the oxygen out of the water. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
After paper one, take paper two and sort out series against parallel in one pass.Tutor Registration →
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