O-Level Chemistry Boiling Points: Distinguish Molecules From Bonds

Identify the structure before explaining a property

An SGExams Chemistry discussion warns students against saying that covalent bonds break when a simple molecular substance boils. The distinction is useful, but some statements in an informal discussion can be too broad. Start from the structure and process in the actual question rather than memorising every claim in a revision post.

The 2026 Chemistry 6092 syllabus covers bonding, structure and their relationship to physical properties. This guide focuses on simple molecular substances and comparisons with giant structures. Follow the syllabus for your own course and year, and use any structural information supplied by the question.

Separate attractions within and between molecules

A molecule contains atoms joined by covalent bonds. In a liquid made of those molecules, there are also attractions between neighbouring molecules. When that substance boils without decomposing, the molecules become much more widely separated. The covalent connections within each molecule remain intact.

For example, liquid water becoming water vapour does not turn its molecules into separate hydrogen and oxygen gases. Water molecules remain water molecules. This makes a simple check for an explanation: if the answer predicts new substances when the question describes only boiling, it may have confused a physical change with a chemical reaction.

Link the attractions to energy

To explain a boiling-point comparison, identify the relevant attractions and compare the energy required to separate the particles. For otherwise comparable simple molecular substances, stronger attractions between molecules generally require more energy to overcome, leading to a higher boiling point at the same external pressure.

Avoid writing only that one substance has stronger bonds. That phrase leaves unclear whether you mean attractions between molecules or covalent bonds inside them. Use the terminology expected in your course and make the location of the attraction explicit. If the problem supplies more information, use it rather than assuming a universal comparison from a substance’s name.

Do not transfer the molecular model to every solid

A giant covalent substance is an extended network of covalently bonded atoms, not a collection of small molecules held together in the same way. Its high melting point is related to the strong covalent bonding in that network. A giant ionic lattice involves a different structure and attractions, so its explanation must also change.

Do not infer that covalent bonds can only ever be broken in giant structures. Covalent bonds in molecules can be broken during chemical reactions as well. The narrower point is that ordinary boiling of a simple molecular substance does not require breaking the covalent bonds within its molecules.

Use a structure-to-property checklist

For a practice question, write four short lines: structure, particles, attraction affected and energy consequence. Then turn them into a connected explanation. Compare your answer with the question’s requested property, since an explanation for electrical conductivity needs different reasoning from one for boiling point.

Correct one ambiguous phrase at a time. Replace bonds are weak with a statement identifying which attractions are relatively weak and where they act. Revisit a new example later to check that you can choose the correct structural model without relying on the same memorised sentence.

Choose a suitable practice question from our Secondary Pure Chemistry papers, checking the level, year and original instructions.

Sources and further reading

Checked on 7 November 2026. The linked discussion is real; worked examples and practice routines are our original guidance. Older discussions are not evidence of current examination rules.

• O LEVEL PURE CHEM!! safety tips :) — 26 October 2026.
• Official curriculum reference