The words vapor and gas are used interchangeably throughout science. In fact physically and chemically, there is no difference. Gas is used in a more general setting, whereby the word vapor is used to talk about one certain phase of a substance. Since gas is the more general term, a vapor could be called a gas, but not all gases can be given the name of vapor.
When a substance goes from a liquid to a gaseous state, it is going through a process call vaporization, or evaporation. During this change, there comes a time when the substance is in both stages at the same time. This is called the critical temperature, or point, and this is the phase where the substance is called a vapor. Above the critical point, condensation of a gas will not occur. When molecules are at critical temperature, their kinetic energy increases. Due to this, the forces between the molecules of the substance become weakened. This means that the substance will appear as both a liquid and a gas at the same time. This can be seen when heating up water. When water reaches the critical temperature, steam is created. At this stage, water is present as both a vapor and as a liquid. This is the stage of equilibrium. If water is heated beyond this temperature, the substance turns into a gaseous form, without any of the liquid present. If this same substance, at equilibrium, were subjected to pressure, it would then turn to a liquid.
Vapors actually produce visible fumes because the liquid and the gas are both present at the same time. If the substance were in a gaseous phase, the fumes would not be visible. Gas is a single phase, where vapor is actually two phases at the same time.
When a gas is subjected to temperatures below the critical temperature it can exist with a liquid. When a vapor is subjected to room temperature, it is usually returns to its natural state, so it changes into a liquid state. What differentiates this from a gas, is that a gas would remain a gas when it is in its natural state.
Gases have not had a change in states where a vapor is a substance that is experiencing a phase change.
So even though these two terms are often used as synonyms, there is a difference between the two states. When someone talks about the stages of matter: solid, liquid, gas or plasma, they do not mention vapor. So when the word comes up in conversation the difference between gas and vapor can be separated by a simple demonstration using water and a heat source.
