Key Concepts: Dry air is approximately 78% N₂, 21% O₂, 0.9% Ar and 0.04% CO₂. Oxygen can be removed by reaction with metals. Combustion of elements forms oxides. Carbon dioxide is produced by thermal decomposition of carbonates and is a greenhouse gas.
Section A — Composition of Dry Air
1. State the approximate percentage by volume of the four most abundant gases in dry air. [4]
| Gas | Approximate % by volume |
|---|
| Nitrogen | |
| Oxygen | |
| Argon | |
| Carbon dioxide | |
2. Why are percentages given for dry air rather than moist air? [1]
Section B — Determining the Percentage of Oxygen in Air
3. Describe an experiment using heated copper to determine the percentage of oxygen in air. Include what you measure and how you calculate the result. [4]
4. A 100 cm³ sample of air is passed repeatedly over heated copper. The final volume is 79 cm³. Calculate the percentage of oxygen in the sample. Show your working. [2]
5. State two precautions to improve the accuracy of this experiment. [2]
Section C — Combustion of Elements in Oxygen
6. Write word equations for the combustion of magnesium, hydrogen and sulfur in oxygen. [3]
7. Write balanced symbol equations for the three combustion reactions in Question 6. [3]
8. State one observation for each combustion reaction below. [3]
a) Magnesium burning in oxygen
b) Hydrogen burning in oxygen
c) Sulfur burning in oxygen
9. Classify the products of combustion: magnesium oxide, water, sulfur dioxide. State whether each is a metal oxide or non-metal oxide. [2]
Section D — Carbonates and the Greenhouse Effect
10. Write the general word equation for the thermal decomposition of a metal carbonate. [2]
11. Write the balanced symbol equation for the thermal decomposition of calcium carbonate. [1]
12. Describe how you would test the gas produced in Question 11 to confirm its identity. [2]
13. Explain how carbon dioxide contributes to the greenhouse effect and climate change. [3]
14. State two human activities that have increased atmospheric CO₂ levels. [2]
Total marks: 34
Mark Scheme
1. N₂ ~78%; O₂ ~21%; Ar ~0.9%; CO₂ ~0.04% [4]
2. Water vapour content varies; using dry air gives consistent, reproducible results [1]
3. Pass air back and forth over heated copper in a sealed/closed apparatus; copper reacts with oxygen forming copper oxide (black); the gas volume decreases; measure the initial and final volumes; % oxygen = (decrease in volume / initial volume) × 100 [4]
4. Decrease = 100 − 79 = 21 cm³; percentage = 21/100 × 100 = 21% [2]
5. Any two: use excess copper; ensure apparatus is airtight; allow to cool before reading volume; pass air back and forth until volume is constant [2]
6. Magnesium + oxygen → magnesium oxide; hydrogen + oxygen → water; sulfur + oxygen → sulfur dioxide [3]
7. 2Mg + O₂ → 2MgO; 2H₂ + O₂ → 2H₂O; S + O₂ → SO₂ [3]
8. a) Bright white/dazzling flame; b) Burns with a blue flame, squeaky pop; c) Blue flame, choking/acidic smell [3]
9. MgO: metal oxide; H₂O: non-metal oxide (hydrogen oxide); SO₂: non-metal oxide [2]
10. Metal carbonate → metal oxide + carbon dioxide [2]
11. CaCO₃ → CaO + CO₂ [1]
12. Bubble the gas through limewater; limewater turns milky/cloudy — confirms carbon dioxide [2]
13. CO₂ absorbs infrared radiation emitted by Earth's surface; traps heat in the atmosphere (greenhouse effect); increasing CO₂ levels cause global temperatures to rise (climate change/global warming) [3]
14. Any two: burning fossil fuels; deforestation; cement manufacture; industrial processes [2]