The term gas is used when the substance is in its gaseous form. The
term vapor is used when the substance is in equilibrium between two
phases, usually liquid and gaseous.
All matter on earth exists in any of the three states: solid, liquid
or gaseous. ‘Gas’ refers to a substance in the gaseous state. Gases do
not have either shape or volume. However, the term ‘vapor’ or ‘vapour’
refers to a substance in equilibrium between two phases, usually liquid
and gaseous. Vapor is not a state of matter but is a specific type of
gas.
Gases are in a gaseous state at room temperature. The molecules in a
gas can expand to occupy any available volume as there is very little
inter molecular attraction. On the other hand, the molecules of a vapor
gain energy and vaporize from a substance which is either a solid or
liquid at room temperature.
A gas is a substance above its critical temperature but below its
critical pressure, while a vapor is a substance above its boiling point
temperature.
Summary:
1. A gas has one defined state at room temperature whereas a vapour is a substance that is in gaseous and liquid equilibrium at room temperature, at a given pressure. A gas can also refer to a single element of matter with its own unique physical and chemical properties, for example oxygen, nitrogen and neon.
2. Because vapour is actually the gaseous state of an element, vapour particles will be of a single element and may have a definite shape whereas gas particles when observed under microscopic view do not have a definite shape and will be a collection of atoms, ions, electrons and molecules. However both gas and vapour particles are in a random motion, colliding with each other or the walls of the containing vessel when observed under a microscope.
3.Gas is one of the 4 states of matter which are solid, liquid, gas and plasma, vapor does not classify as such. It is just a substance it is gaseous form.
4.Vapor can turn back and forth into liquid and solid states but a gas cannot
5.Gases cannot be see while vapors are visible
6.Vapors settle down on ground while gases do not
