Firstly, although the words may be used interchangably by the average non-scientist person, there seems to be agreement in the scientific literature that gases and vapors are definitely not the same thing. For example, there is such a thing as a “gas-vapor phase equilibrium”. In order to discuss the difference between gas and vapor it is necessary to do a quick rehash of physics 101.
Phases
All substances on earth exist in three kinds of conditions, or states-of-being, if you want. They are either solids, or liquids, or gases. These three conditions are called “phases” in physics. A substance can transition from one phase to another by a change in energy. In the solid phase a substance has the lowest energy, in the liquid phase more, and it has the most energy in the gaseous phase (this is a really simple way of putting it). A fourth phase called plasma applies to ionized gases at very high temperatures and is generally not encountered in day-to-day living.
Water
To illustrate, let’s look at the substance water (H2O). It most commonly exists in the liquid phase. However, if you lower the temperature and remove kinetic energy from the molecules, it transitions into the solid phase which we call ice. Conversely, if you add energy to it by increasing the temperature, it becomes a gas, which we cannot see. Energy changes can also be brought about by changes in pressure, but let’s leave that for the time being.
So what is water vapor or steam that we see coming from the kettle’s spout? Water vapor is a mass of very small droplets (liquid phase) that disperses in the air in a visible cloud. Steam or water vapor, is NOT the gas phase of water. If you look closely you will see that the steam or water vapor, starts a few millimetres away from the actual edge of the spout. Water in its gaseous phase leaves the spout, is immediately cooled by contact with the ambient air and returns to the liquid phase in droplets (this is called condensation) that are yet too small to fall, so we call it steam. It seems that the semantic convention is to call it steam when it is man-made but water vapor when it occurs naturally.
The critical point
The critical point of substances or matter is a fixed point, determined by a combination of critical temperature, which is that temperature above which a gas can no longer be liquefied regardless of the pressure applied, and the critical pressure, which is the minimum pressure needed to liquefy a gas at the critical temperature. For example, the critical point of water is at a temperature of 374 degrees Celsius and a pressure of 217.7 atmospheres. Critical points become complicated to calculate when more than one substance is involved (multi-component systems).
Gas or vapor?
It appears that all vapors are gases, but not all gases are vapors.
Vapors are gases whose mean temperature is BELOW the critical point. That means that the gas is present together with the liquid – as in water vapor.
Phase equilibrium
When a substance exists in both the gas and liquid phase simultaneously, this is not the same as phase equilibrium, where the amount of liquid that is evaporated into gas is the same as the amount of gas that is condensed into liquid. A substance can therefore be in phase equilibrium without being at the critical point. A bowl of water standing on a desk will eventually evaporate completely without being at the critical point, nor in phase equilibrium. However, if the humidity of the air is very high, the water may reach phase equilibrium and not evaporate beyond a certain point. It will, however, not reach the critical point ever, because of the natural atmospheric pressure and range of ambient temperatures existing naturally on earth.
The difference between gas and vapor therefore boils down to (excuse the pun) a gas being “pure” i.e. without liquid in it, whereas a vapor does have liquid in it.
