What is Gas?

GAS is that form of matter that has neither fixed volume nor definite shape. Until the middle of the 17th century air was the only gas recognized. But within the next century scientists discovered most of the principal gases and noted their properties. The rare gases, however, were not found until toward the end of the 19th century.

Ten of the elements are gases at normal temperature and pressure— chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen, radon, and xenon. However, all elements can be turned to gas, or vaporized, by the application of sufficient heat. At the same time, all gases can be liquefied, or turned into fluids, if enough pressure is applied and if they are made cold enough. Every gas has a different temperature to which it must be reduced before it can be liquefied, regardless of the amount of pressure used. This temperature is called the critical temperature of that gas.

The molecules that make up a gas are in continual rapid motion in straight lines, knocking against each other and against the walls of any vessel that contains the gas. This accounts for the pressure, in all directions, that gases exert. If the temperature of the gas is raised —temperature being the average amount of kinetic energy in the molecules—the molecules have more energy and move faster, their blows are stronger, and the gas exerts more pressure. If we compress the gas into a smaller volume, the mole­cules naturally knock oftener against the containing vessel, and we say that the pressure is increased.

The two principal laws governing gases are Boyle's law and Charles's law. The first law states that if the temperature is constant, the volume of a gas is inversely proportional to the pressure on it—if we double the pressure, we halve the volume; if we halve the pressure, we double the volume. The other law states that if the pressure is kept the same, the volume of a gas increases (or decreases) by about 1/273 of its vol­ume at freezing point for every degree the temperature is raised (or lowered).