1  Vapor, Gas, Steam, Fluid

As far as I can tell:

• There is no significant physical or chemical difference between a vapor and a gas. This is the most important point.
• However, the words have slightly different connotations. Remember that these are just connotations. There is sometimes considerable overlap between the terms, so precise distinctions are not necessary and probably not even possible.
• Gas is the broader term. A vapor is a particular type of gas. Any vapor could technically be called a gas, although in many situations this would sound weird or awkward.
• There seem to be two main ways in which a gas can earn the “vapor” appellation:

  a) when the gaseous phase is in equilibrium with the corresponding liquid or solid, or

  b) when we wish to emphasize that we are talking about the gaseous phase, even though for some reason the condensed phase is considered “normal”.

• Here are some examples, with comments.

  Thoughtful experts refer to gaseous H2O as “water vapor” even in situations where it is nowhere near being in equilibrium with any condensed phase. Meteorology is one example, where we speak of various amounts of water vapor in the atmosphere. Astronomy is an even more extreme example, where congregations of cosmic H20 molecules are called interstellar water vapor. Since the dawn of time, these particular H2O molecules have been nowhere near saturated vapor pressure. Nobody ever speaks of water gas.

  This is the perfect illustration of rule (b). Technically and officially, “water” refers to the H2O molecule in any phase (gas, liquid, or solid) ... but unofficially by long tradition “water” connotes the liquid unless otherwise specified. Water is normally a liquid.

  If you smell fumes in the garage, you call it gasoline vapor, not gasoline gas.

  This is another illustration of rule (b). Unofficially, gasoline is normally presumed to be a liquid unless otherwise stated. Therefore the gaseous phase is called vapor, even in situations where the partial pressure is nowhere near the saturated vapor pressure.

  You can fill balloons from a tank of helium gas. It would sound quite weird to call it helium vapor.

  The gas phase is considered normal for helium, and in this case there is no liquid or solid anywhere in sight.

  In contrast, you can produce temperatures below 2K if you start with liquid helium and pump the vapor away fast enough.

  This is the perfect illustration of rule (a). The gas is called vapor to hint at the presence of the nearby liquid.

  Thoughtful experts commonly refer to the “SVP” (Saturated Vapor Pressure) to designate a vapor that is in equilibrium with a nearby condensed phase.

  This proves that rule (a) cannot be the whole story; otherwise the word “Saturated” would be redundant.

• It has been suggested that a vapor is something that you can see or smell, or something that settles to the ground – but that’s can’t possibly be right. Ammonia (NH₃) is in many situations called ammonia gas, despite its pungent odor. Water vapor is called vapor, even though it is odorless and highly transparent. Fog and clouds are visible but are neither gases nor vapors; they are aerosols, i.e. colloidal suspensions of very fine particles of liquid (or sometimes solid) H₂O in the air. The aerosol as a whole has the property of filling its container, so it behaves like a gas in this regard. The suspended particles will not settle out. (A system where the droplets are so big that they do eventually settle out should be called a mist.)
• The word “steam” applies both to transparent water vapor and to visible clouds of water suspended in air.
• The word “fluid” is a very important technical term. It encompasses both gas and liquid. That is, any gas is a fluid and any liquid is a fluid. Above the critical temperature and/or critical pressure, “fluid” is the preferred term, because there is no meaningful distinction between liquid/vapor/gas. But this is not an if-and-only-if proposition; the term fluid may perfectly well be applied to liquids and gases at temperatures and pressures below T_c and P_c.