Which statement correctly distinguishes a gas from a vapour?

• A. Gas is a state of matter; vapour is the gaseous form of a substance that is typically liquid or solid at room conditions.
• B. Gas and vapour are the same in all conditions.
• C. Vapour is a solid.
• D. Gas is only found in nature.

Answer: (A)

The main idea being tested is how we distinguish gas from vapour. A gas is one of the fundamental states of matter that has no fixed shape or volume and fills the space available. Vapour, on the other hand, is the gaseous form of a substance that is normally a liquid or solid at room conditions. So water is a liquid at room temperature, but when it boils it becomes water vapour. Gases that are naturally in the gaseous state at room temperature, like nitrogen or oxygen, are simply called gases, not vapour, because they don’t need a special “gaseous form” label for a normally liquid/solid substance. Vapour is not a solid, and the idea that vapour is always different from gas doesn’t apply; vapour specifically refers to the gas phase of materials that aren’t typically gases under everyday conditions. Also, gas is not restricted to anything natural—synthetic and industrial gases are common too.