Chemical changes

1. Which best describes reduction?

1. Gain of oxygen
2. Loss of oxygen or gain of electrons
3. Loss of electrons only
4. Loss of a metal

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B: Loss of oxygen or gain of electrons. Reduction is the loss of oxygen or the gain of electrons.

2. Which method is used to extract metals more reactive than carbon?

1. Heating in air
2. Reduction with carbon
3. Electrolysis
4. Displacement

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C: Electrolysis. A large amount of energy is needed to force these reactive metals out of their compounds.

3. In a displacement reaction, what happens?

1. A more reactive metal takes the place of a less reactive metal in a compound
2. Two non-metals react
3. No reaction occurs
4. Only gases are produced

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A: A more reactive metal takes the place of a less reactive metal in a compound. The more reactive metal forms a compound, releasing the less reactive one.

4. Why is gold found as the native metal in the Earth?

1. It is very reactive
2. It is very unreactive
3. It reacts with carbon easily
4. It is a non-metal

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B: It is very unreactive. Unreactive metals don't readily form compounds with other elements.

5. Which metal is extracted by reduction with carbon rather than electrolysis?

1. Potassium
2. Aluminium
3. Iron
4. Sodium

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C: Iron. Iron is less reactive than carbon, so carbon can reduce iron oxide.

6. Which gas is tested for using a lit splint that gives a squeaky pop?

1. Oxygen
2. Carbon dioxide
3. Hydrogen
4. Chlorine

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C: Hydrogen. This is the standard test for hydrogen.

7. Magnesium ribbon is added to blue copper(II) sulfate solution. What is observed?

1. Grey solid forms and the solution stays blue
2. Brown solid forms and the blue colour fades
3. Hydrogen bubbles form and the solution turns green
4. No change, as magnesium is less reactive than copper

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B: Brown solid forms and the blue colour fades. Magnesium is more reactive than copper so it displaces it. Brown copper metal forms and the blue Cu²⁺ ions are used up, so the blue colour fades.

8. Zinc powder is added to magnesium sulfate solution. What happens?

1. The solution turns blue
2. Hydrogen gas is given off
3. No reaction, as zinc is less reactive than magnesium
4. Zinc displaces magnesium, forming zinc sulfate

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C: No reaction, as zinc is less reactive than magnesium. A metal can only displace a less reactive metal from its compound. Zinc is less reactive than magnesium, so nothing happens.

9. Which half-equation shows oxidation?

1. Mg → Mg²⁺ + 2e⁻
2. 2H⁺ + 2e⁻ → H₂
3. Cu²⁺ + 2e⁻ → Cu
4. O₂ + 4e⁻ → 2O²⁻

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A: Mg → Mg²⁺ + 2e⁻. Oxidation is loss of electrons, and Mg loses 2 electrons. In the other three equations electrons are gained, so they are reductions.

10. In Fe₂O₃ + 3CO → 2Fe + 3CO₂, which substance is reduced?

1. Iron
2. Carbon monoxide
3. Carbon dioxide
4. Iron(III) oxide

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D: Iron(III) oxide. Fe₂O₃ loses oxygen, so it is reduced. CO gains oxygen to form CO₂, so it is oxidised.