Answering Compare and Contrast Questions in IGCSE Chemistry

Compare and contrast questions appear on Paper 4 when two substances, reactions, or methods need to be evaluated side by side. These questions test the same content as describe and explain questions but require a different answer structure. Students who write two separate descriptions instead of a comparison lose marks even when every fact is correct.

The compare structure: paired statements

The mark scheme for a compare question awards marks for paired points. A paired point links both items in the same idea:

“Sodium reacts vigorously with water, whereas potassium reacts even more vigorously.”

An unpaired point describes only one item:

“Sodium reacts vigorously with water.” (No reference to potassium.)

The first earns a comparison mark. The second earns nothing in a compare question, even though the fact is correct.

Linking words that signal comparison

Use these to make your comparisons explicit:

• whereas
• while
• but
• however
• in contrast
• both… but…
• unlike

Example: “Both sodium and potassium react with water to produce hydrogen, but potassium reacts more vigorously and catches fire, whereas sodium melts into a ball and fizzes.”

Types of compare question

Type 1: Compare properties

“Compare the properties of diamond and graphite.”

Model answer:

• Both diamond and graphite are forms of carbon with very high melting points because both have giant covalent structures with many strong covalent bonds (1).
• Diamond is very hard, whereas graphite is soft and slippery (1).
• Diamond does not conduct electricity because all four outer electrons are used in bonding, whereas graphite conducts because each carbon atom uses only three electrons for bonding and the fourth is delocalised and free to move (1).

Each point pairs a property of one substance with the corresponding property of the other.

Type 2: Compare reactions

“Compare the reactions of magnesium and copper with dilute hydrochloric acid.”

Model answer:

• Magnesium reacts with dilute hydrochloric acid, producing bubbles of hydrogen gas, whereas copper does not react (1).
• Magnesium is more reactive than hydrogen, so it can displace hydrogen from the acid. Copper is less reactive than hydrogen, so no displacement occurs (1).

Type 3: Compare methods or processes

“Compare simple distillation and fractional distillation.”

Model answer:

• Both methods involve heating a liquid mixture and condensing the vapour (1).
• Simple distillation separates a liquid from a dissolved solid or from a liquid with a very different boiling point, whereas fractional distillation separates two or more miscible liquids with similar boiling points (1).
• Fractional distillation requires a fractionating column, which simple distillation does not (1).

The contrast question

Contrast means focus on differences only. Do not waste time on similarities unless the question asks for them.

“Contrast the bonding in sodium chloride and in methane.”

Model answer:

• Sodium chloride has ionic bonding formed by the transfer of electrons from sodium to chlorine, whereas methane has covalent bonding formed by the sharing of electrons between carbon and hydrogen (1).
• Sodium chloride forms a giant ionic lattice, whereas methane exists as simple molecules (1).
• Sodium chloride conducts electricity when molten or dissolved, whereas methane does not conduct in any state (1).

Every sentence contains both substances and a difference. No sentence describes only one substance.

How the mark scheme works

The mark scheme typically lists possible comparison points, each worth one mark. To earn the mark, your answer must reference both items and the specific point of comparison. Writing about only one item, even correctly, earns zero for that point.

Some mark schemes have a “must be comparative” note, meaning the examiner will not award the mark unless both items are mentioned together. Others list facts that can be credited individually but award a bonus mark for the explicit comparison.

Common errors

1. Writing two separate descriptions. “Sodium is a soft metal. It reacts with water…” followed by “Potassium is also a soft metal…” is two descriptions, not a comparison. Rewrite as paired statements.
2. Comparing the wrong feature. If the question asks you to compare the reactivity of two metals, do not compare their appearance or melting points.
3. Using “more” without stating what “more” is relative to. “Chlorine is more reactive” earns less than “Chlorine is more reactive than bromine.” Always name both substances.
4. Not enough points. A 4-mark compare question needs four paired points. Three pairs earn three marks.
5. Describing similarities when asked to contrast. Contrast means differences only.

Practice technique

Take any topic with two substances to compare (Group I vs Group VII, ionic vs covalent, strong acid vs weak acid) and write three paired comparison statements. Check that every sentence includes both items and a linking word. Do this for five different pairs and the habit becomes automatic.

For more guidance on command words, see command words. If structuring answers for compare questions is costing marks, a trial lesson can practise the paired-point technique on your weakest topics.