Simple molecular structures
2 marks · EasyExplain why methane (CH₄) has a much lower boiling point than water (H₂O), using ideas about molecular mass and intermolecular forces.
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Methane has a lower molecular mass and weaker intermolecular forces than water, so less energy is needed to separate its molecules and boil it.
Also accept: Methane is a smaller molecule with weaker intermolecular forces than water.
Common mistakes: Confusing covalent bond strength with intermolecular forces; Ignoring difference in molecular mass
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Marking points
M1 · 1 mark · Mentions difference in molecular mass or size between CH₄ and H₂O.
A1 · 1 mark · Links lower mass to weaker intermolecular forces and lower boiling point for methane.
Worked solution
Methane (CH₄) has a lower molecular mass than water (H₂O). This means the intermolecular forces between methane molecules are weaker than those in water. As a result, less energy is required to overcome these forces and boil methane, so it has a much lower boiling point.