The difference between a “gas” and a “vapor,” as the terms are used in the physics discipline, is subtle yet simple. The fundamental three states of matter “solid, liquid and gas” have been understood since antiquity, but the factors and variables surrounding the transition of matter from one state to another was not so clear.
In the seventeenth century, English philosopher, alchemist, and physicists Robert Boyle [known as the father of modern chemistry] established the posit that “the volume of a dry gas varies inversely with the pressure exerted on it, given the temperature remains constant.” This posit is known as Boyle’s law.
About a hundred years after Boyle, French scientists Jacques Charles considered the effect of temperature on a volume of gas and established his own postulate that “as a given volume of gas was heated, its volume increases in direct proportion to the number of degrees “K’ (Kelven temperature scale) in temperature change.” Today these two postulates are known as Boyle’s and Charles laws respectively, and are key fundamental precepts in the understanding of physical properties of all gases and vapors.
However, the thermodynamics of kinetic theory require some reference point be used to quantify the transitional relationships of matter. Subsequent to the work of Boyle and Charles, the parameters of standard pressure and standard temperature were established as constants.
Standard pressure is the weight of air at 0°C required to support a column of mercury 760mm high in a glass tube opened at one end and closed at the other, as demonstrated by Italian scientists Evangelista Torricelli in 1643. Standard temperature was established as the temperature at which water transitions from ice into a liquid, 273°K, 32°F or 0°C. The Centigrade or Celsius temperature scale is a linear scale based on the change of water from solid into liquid, and liquid into a vapor (gas).
Absolute zero[0°K] is a theoretical constant relative to kinetic theory which stipulates the point at which matter has no kinetic energy and ceases to exist. Like any other singularity, absolute zero is a hypothetical instance where the laws of physics break down.
Therefore, in terms of physics, any gas which exists in a gaseous state at zero degrees centigrade (0°C) at standard pressure, is referred to as a “gas.” The gas produced by any solid which sublimates or liquid which boils or evaporates [transitions to a gaseous state] at or above 0°C at standard pressure, is referred to as “vapor.”
