Properties of small molecules - AQA GCSE Chemistry Exam Questions

Practise 24 exam-style questions on Properties of small molecules for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

24 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Name the type of bond that holds the two atoms together in a chlorine molecule, Cl₂. State whether this bond is strong or weak.

Model answer

A covalent bond (a shared pair of electrons). It is a strong bond.

Explanation

Students often think chlorine's low boiling point means its bonds are weak. The covalent bond inside each Cl₂ molecule is strong; it is the forces between the molecules that are weak.

Mark scheme

1 mark
covalent (bond)
allow single covalent bond / shared pair of electrons
1 mark
strong

Question 2

2 marks

The melting points and boiling points of four substances made of small molecules are:
hydrogen sulfide, H₂S: melting point −86 °C, boiling point −60 °C
ammonia, NH₃: melting point −78 °C, boiling point −33 °C
chlorine, Cl₂: melting point −101 °C, boiling point −34 °C
methane, CH₄: melting point −182 °C, boiling point −162 °C

Which substance is a solid at −80 °C? Which substance is a gas at −80 °C?

Model answer

Ammonia is a solid at −80 °C, because −80 °C is below its melting point (−78 °C).
Methane is a gas at −80 °C, because −80 °C is above its boiling point (−162 °C).

Explanation

With negative temperatures, remember that −80 °C is colder than −78 °C but warmer than −86 °C. Hydrogen sulfide and chlorine are both liquids at −80 °C, because −80 °C lies between their melting and boiling points.

Mark scheme

1 mark
solid: ammonia
allow NH₃; do not accept if more than one substance is given
1 mark
gas: methane
allow CH₄; do not accept if more than one substance is given

Question 3

3 marks

Perfumes contain substances made of small molecules. When perfume is sprayed onto skin, it evaporates quickly and can soon be smelled across a room.

Explain why perfume evaporates quickly at body temperature.

Model answer

The molecules in perfume have only weak intermolecular forces between them. Very little energy is needed to overcome these forces, so energy from the warm skin is enough for the molecules to escape from the liquid into the air, where they spread out and reach your nose.

Explanation

Substances with weak intermolecular forces have low boiling points and evaporate easily. Perfume molecules do not break apart when they evaporate; whole molecules move into the air.

Mark scheme

1 mark
there are weak intermolecular forces between the molecules
1 mark
little energy is needed to overcome these forces
allow body heat provides enough energy
1 mark
so the molecules easily escape from the liquid into the air / the substances have low boiling points
do not accept covalent bonds breaking

Question 4

5 marks

Bromine (Br₂) melts at −7 °C and boils at 59 °C, so it is a liquid at room temperature. Liquid bromine does not conduct electricity.

Explain these properties in terms of the structure and bonding of bromine.

Model answer

Bromine is made of small Br₂ molecules, in which the two atoms are joined by a strong covalent bond. Between the molecules there are only weak intermolecular forces. Little energy is needed to overcome these forces, so bromine has low melting and boiling points; its melting point is below room temperature and its boiling point above it, so it is a liquid. The covalent bonds are not broken when it melts or boils. Bromine molecules have no overall electric charge, so there are no ions or delocalised electrons free to move and carry charge, so liquid bromine does not conduct electricity.

Explanation

All the bulk properties of a molecular substance follow from two facts: intermolecular forces are weak compared with covalent bonds, and molecules are uncharged. Make sure you say which forces are overcome when the substance changes state.

Mark scheme

1 mark
bromine consists of small (Br₂) molecules
max 5
1 mark
the two atoms in each molecule are joined by a strong covalent bond
1 mark
there are weak intermolecular forces between the molecules
1 mark
little energy is needed to overcome these forces, so it has low melting and boiling points (the covalent bonds are not broken)
1 mark
the molecules have no overall electric charge
1 mark
so there are no charged particles (ions or electrons) free to move and carry charge

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