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.A vapor is a specific type of gas. A vapor is a gas whose molecules gained energy and vaporized from a substance who at room temperature is either a solid or liquid. An example is water. Water is a liquid at room temperature, so when it becomes a gas it is referred to as water vapor.