Shape and volume
Gases do not posses any definite shape or volume. They will completely fill all the space available to them. The tendency of a gas to fill the available volume within a container is the result of the freedom that gas particles have to move everywhere in the available space. This freedom of movement of gaseous molecules is due to very weak binding forces between molecules. In other words their intermolecular forces are very weak. Because of this the molecules of gas are in a continuous motion and are associated with high velocity and therefore high kinetic energy.
Compressibility
Particles of a gas have large intermolecular spaces among them. By the application of pressure much of this space can be reduced and the particles be brought closer. Hence the volume of a gas can be greatly reduced. This is called compressing the gas (Fig.15.1).
By increasing the pressure from 1 atmosphere to 2 atmosphere, the volume of a gas gets compressed to half whereas if the volume of water were made to decrease similarly it would decrease only by 0.00001 parts.
Reducing the temperature can also reduce the volume of a gas. When the temperature is reduced, there is less energy in the particles, their mobility is reduced and they move less apart. As a result, intermolecular attraction becomes more prominent and the particles come closer. This reduces the volume of the gas.Expansibility
When pressure is applied on a gas, it contracts. Conversely, when pressure is released the gas expands.
When the temperature is increased, the particles gain more energy, move faster and move away from each other. As a result the intermolecular attraction becomes less prominent. The volume of the gas increases.Diffusibility
The molecules of the gas are in constant motion, at very high velocity. There is a great amount of intermolecular space between the molecules. When two gases are mixed, particles of one gas can easily pass through the intermolecular space of the other gas. As a result both the gases get completely and evenly mixed. Thus, a mixture of gases always remains homogeneous.
Low density
Since gases have large intermolecular spaces, they have very large volumes when compared to their mass. Thus they have low densities. If one ml of water at 39.2oF is converted into steam at 212oF and 1 atmosphere pressure, it will occupy a volume of 1700 ml.
Exertion of pressure
Solids exert pressure only in downward direction. Liquids exert pressure downward as well as to the sides. But gases exert pressure in all directions (a good example is a balloon). This pressure is due to bombardment or the particles against the walls of the container (Fig.15.2).
