9. What is difference between gas and vapour?

The main difference between gas and vapour can be found in their states at room temperature.

1. Gas: A gas is a state of matter that has no fixed shape or volume. It tends to expand to fill the space available to it. Gases are usually molecules or atoms which have a lot of energy and are in constant, random motion. At room temperature, substances that exist in a gaseous state are called gases. Examples include oxygen, nitrogen, and carbon dioxide.

2. Vapour: Vapour is a state of matter that is a gaseous phase of a substance which normally exists as a liquid or solid at room temperature. It is formed by the evaporation or sublimation of the substance. For example, water vapour is formed when water (which is a liquid at room temperature) is heated and changes its state to a gas.

In summary, the term 'gas' is used for substances that exist in the gaseous state at room temperature, while 'vapour' is used for the gaseous state of substances that are liquid or solid at room temperature.