Pearson Edexcel IGCSE Chemistry: Reactivity and Group Trends — Practice Questions (4CH1)

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Reactivity and group trends study guide

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Section A

1. State whether reactivity increases or decreases going down Group 1, and going down Group 7. [2]

2. Name the four most abundant gases in dry air (approximate order). [4]

Section B

3. Potassium is more reactive than lithium. Explain this trend in terms of electronic configuration. [3]

4. Chlorine is bubbled through a solution of potassium bromide. Describe and explain what would be observed, including a word equation. [4]

5. A student determines the percentage of oxygen in air using the reaction of iron with air in a sealed system. Describe how this experiment could be carried out and what result would be expected. [5]

6. Place the following metals in order of reactivity (most to least reactive): copper, zinc, iron, magnesium. Justify your order using one reaction-based piece of evidence. [4]

7. Define oxidation and reduction using both the oxygen-based and electron-based definitions, using the reaction between magnesium and copper oxide as your example. [6]

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Answers

1. Group 1: reactivity increases down the group [1]. Group 7: reactivity decreases down the group [1].

2. Nitrogen (approx. 78%), oxygen (approx. 21%), argon (approx. 0.9%), carbon dioxide (approx. 0.04%) [1] [1] [1] [1].

3. Potassium has one more electron shell than lithium, so its single outer electron is further from the nucleus and more shielded by inner shells [1] [1]. This makes the outer electron easier to lose, making potassium more reactive [1].

4. A colour change would be observed — the solution would turn from colourless to orange/brown as bromine is formed [1] [1]. This is because chlorine is more reactive than bromine (higher up Group 7), so chlorine displaces bromine from potassium bromide [1]. Word equation: chlorine + potassium bromide → potassium chloride + bromine [1].

5. Iron (e.g. as iron wool/filings) is placed in a sealed container of a known volume of air, with water able to enter as oxygen is used up [1] [1]. The iron rusts, reacting with the oxygen in the air (and water) over several days [1]. As oxygen is removed from the air, the volume of gas decreases, and water rises to fill the space previously occupied by oxygen [1]. The expected result is that approximately one-fifth (around 20%) of the original air volume is used up, consistent with oxygen being roughly 21% of air [1] [1].

6. Most to least reactive: magnesium, zinc, iron, copper [1]. Justification: magnesium reacts vigorously with dilute acid, while copper does not react with dilute acid at all, showing copper is below hydrogen in reactivity and magnesium is well above it; zinc and iron’s positions are confirmed by displacement reactions — zinc displaces iron from iron sulfate solution, showing zinc is more reactive than iron [1] [1] [1].

7. Oxygen-based: magnesium is oxidised because it gains oxygen (forms magnesium oxide); copper oxide is reduced because it loses oxygen (forms copper) [1] [1]. Electron-based: magnesium is oxidised because it loses electrons (forms Mg²⁺); copper ions are reduced because they gain electrons (form Cu atoms) [1] [1]. Both definitions describe the same reaction correctly — the choice depends on which convention the question uses [1] [1].

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Exam technique for this topic

Group 1 and Group 7 trends are frequently tested by giving an unfamiliar element from the same group (e.g. rubidium, or astatine) and asking candidates to predict its properties by extrapolating the known trend — always state the direction of the trend explicitly (increasing or decreasing) before applying it to the unfamiliar element, since simply naming “the trend” without a direction earns no marks. For redox questions, practise writing both the oxygen-based and electron-based explanation for the same reaction side by side until translating between them becomes automatic, since a question may specify either convention and candidates who can only reason in one direction lose marks unnecessarily.

Linking this topic to Topic 1

Reactivity trends in this topic are ultimately explained by the atomic structure and bonding concepts established in Topic 1 (Principles of Chemistry) — the ease of losing or gaining an outer electron, governed by the number of electron shells and the shielding this creates, is exactly the same underlying idea whether applied to Group 1’s increasing reactivity or Group 7’s decreasing reactivity down the group. Revising this topic with explicit reference back to electronic configuration, rather than as a standalone set of memorised trends, makes the “explain in terms of electronic configuration” questions (marked C, bold content) far more manageable, since the explanation is the same reasoning pattern applied to two different groups rather than two separate facts to memorise.

Where marks are usually lost

• Applying Group 1’s “reactivity increases down the group” rule to Group 7, or vice versa.
• Reciting the reactivity series with a metal out of place, especially around the middle of the sequence.
• Giving only the oxygen-based OR electron-based definition when a question credits both, or muddling the two together.
• Forgetting that a positive result for the oxygen-in-air experiment depends on removing air’s oxygen specifically, not just “using up the air.”