Gases - General Properties of Gases

Gas Facts and Equations

You can perform many calculations for real gases using ideal gas laws. Typically, real gases behave like ideal gases at low pressures and ordinary temperatures.
You can perform many calculations for real gases using ideal gas laws. Typically, real gases behave like ideal gases at low pressures and ordinary temperatures. Ed Lallo, Getty Images

A gas is a form of matter that lacks a defined shape or volume. Gases share important properties, plus there are equations you can use to calculate what will happen to the pressure, temperature, or volume of a gas if conditions are changed.

Gas Properties

There are three gas properties that characterize this state of matter:

  1. Compressibility - Gases are easy to compress.
  2. Expandability - Gases expand to completely fill their containers.
  1. Because particles are less ordered than in liquids or solids, the gas form of the same substance occupies much more space. 

All pure substances display similar behavior in the gas phase. At 0° C and 1 atmosphere of pressure, one mole of every gas occupies about 22.4 liters of volume. Molar volumes of solids and liquids, on the other hand, vary greatly from one substance to another. In a gas at 1 atmosphere, the molecules are approximately 10 diameters apart. Unlike liquids or solids, gases occupy their containers uniformly and completely. Because molecules in a gas are far apart, it is easier to compress a gas than it is to compress a liquid. In general, doubling the pressure of a gas reduces its volume to about half of its previous value. Doubling the mass of gas in a closed container doubles its pressure. Increasing the temperature of a gas enclosed in a container increases its pressure.

Important Gas Laws

Because different gases act similarly, it is possible to write a single equation relating volume, pressure, temperature, and quantity of gas. This Ideal Gas Law and the related Boyle's Law, Law of Charles and Gay-Lussac, and Dalton's Law are central to understanding the more complex behavior of real gases.

Ideal Gas Law: The ideal gas law relates the pressure, volume, quantity, and temperature of an ideal gas. The law applies to real gases at normal temperature and low pressure.
PV = nRT

Boyle's Law: At constant temperature, the volume of a gas is inversely proportional to its pressure.
PV = k1

Law of Charles and Gay-Lussac: These two ideal gas laws are related. Charles's law states at constant pressure, the volume of an ideal gas is directly proportional to temperature. Gay-Lussac's law says at constant volume, the pressure of a gas is directly proportional to its temperature.
V = k2T (Charles's Law)
Pi/Ti = Pf/Tf (Gay-Lussac's Law)

Dalton's Law: Dalton's law is used to find pressures of individual gases in a gaseous mixture.
Ptot = Pa + Pb

where:
P is pressure, Ptot is total pressure, Pa and Pb are component pressures
V is volume
n is ​a number of moles
T is temperature
k1 and k2 are constants