Question

what is a difference between gas and vapour

Answer 1

A gas refers to a substance that has a single defined thermodynamic state at room temperature whereas a vapor refers to a substance that is a mixture of two phases at room temperature, namely gaseous and liquid phase. This also means that a vapor is a substance that has experienced some sort of phase change at room temperature. A gas is a substance whose molecules are in constant free motion past each other and can be compressed, which can also be referred to as a compressible fluid. When no liquid or solid can form at the temperature of the gas it is called a fixed gas.
It is worth noting that when referring to the four states of matter there is a ‘gas phase’ which does not necessarily refer to a gas as a distinct element but rather represents differences in interrelationships of molecules. A gas has its single gas particles vastly separated, making a gas invisible to the eye.

When a substance is at a temperature below its critical temperature it is in a ‘gas phase’ and therefore will be a vapour. A vapour can co-exist with a liquid or solid when they are in equilibrium state. Therefore from this we can infer that a vapour is a gas state of a substance at a temperature where it can co-exist with its liquid or solid state so for a liquid or solid to become vapour it does not have to first boil.

Answer 2

Difference Between Vapor and Gas. A gas refers to a substance that has a single defined thermodynamic state at room temperature whereas a vapor refers to a substance that is a mixture of two phases at room temperature, namely gaseous and liquid phase