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Difference between vapor and gas The term gas refers to a substance that is in its gaseous form or a substance that has a single defined thermodynamic state at room temperature whereas a vapor refers to a substance that is a equilibrium between two phases at room temperature, namely gaseous and liquid phase. 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. For example, steam is a water vapor that turns into water at room temperature. Oxygen, which is a gas, will still be a gas at room temperature. Gas is a state of matter. Vapor is not, it is constantly in transition. An easy way to distinguish between vapor and gas is that vapor is something that can be seen or smelled or something that settles down on the ground, while one cannot see a gas but can only smell it. However, there are some exceptions to this classification, for example, water vapor. Water vapor cannot be smelt and it does not settle down on the ground.