Which of the following statements best explains the difference between a gas and a vapour?

Analyze the question: We need the best statement that distinguishes a gas from a vapour.

Concept: Both are the gaseous state of matter. The difference lies in the substance's critical temperature:

• A gas is a substance whose temperature is above its critical temperature, so it cannot be liquefied by pressure alone. You must cool it first.
• A vapour is the gaseous form of a substance that is normally a liquid or solid at room temperature. Its temperature is below the critical temperature, so it can be condensed into a liquid simply by increasing pressure (or slightly cooling it).

Examples: water vapour, ethanol vapour, and bromine vapour condense easily. Oxygen, nitrogen, and hydrogen are gases because they need very low temperatures before they can be liquefied.

Evaluate the options:

• A. Vapours are liquids at room temperature. Wrong, because a vapour is the gaseous state, not a liquid. (The substance is usually liquid at room temperature, but the vapour itself is not.)
• B. Vapour can easily be condensed into liquids. Correct. This matches the critical temperature idea: a vapour liquefies easily under pressure or mild cooling.
• C. Vapour is readily converted into solids. Wrong, since this is not a defining property.
• D. Vapours are generally denser than gases. Not a defining distinction; density depends on molar mass, temperature and pressure.

Answer: B

Exam tip: Remember "Vapour = Value for easy condensation". A vapour sits below its critical temperature, so pressure alone can liquefy it. A true gas cannot be liquefied by pressure alone at room temperature.