Properties of gases

Properties of gases

Matter is known to exist in three states - solid, liquid and gas. A substance may be made to exist in any one of the three states by varying the temperature or pressure or both. Dynamic motion of molecules therefore is an inherent property in gaseous and liquid states of matter. The energy of motion known as kinetic energy is present in the gaseous molecules. Therefore the basic theory which explains the behaviour of gases is called as kinetic theory of gases.

A gaseous state can be described in terms of four parameters which are known as measurable properties such as the volume, V; Pressure, P; Temperature, T and Number of moles, n of the gas in the container.

Pressure effect

A gas may be considered to consist of a large number of molecules moving haphazardly all around in a vessel. Due to their constant motion, the molecules may not collide against one another very frequently, but can strike against the walls of the containing vessel. The molecular collisions are regarded as ideal (ie) perfectly elastic, so that there is no loss of energy in these collisions. Pressure is defined as force per unit area. This depends upon the number of molecules that strike per unit area of the walls of the container in one second. The greater the number of molecules striking per unit area of the walls in one second, the greater would be the pressure exerted by the gas. Thus for example, when we pump air into a bicycle tube, the number of molecules within the tube increases and hence the number of collisions of the molecules with the walls per second increases and the pressure goes up.

Temperature effect

The kinetic energy of molecules is given by 1/2 mv² where m is the mass of the molecule and v is the velocity of its motion. When a gas is heated, its temperature increases. Although the mass of the molecule remains constant, its velocity increases. This causes an increase in kinetic energy. Therefore the molecules strike the wall of the containing vessel more frequently. In this case there is no change in the number of molecules, but the number of collisions against the walls of the container in a given time increases. Therefore the pressure of the gas increases with rise in temperature when the amount and its volume remain constant.

Volume effect

The volume of the container is considered as the volume of the gas sample. This is considered from the postulates of kinetic theory of gases. That is, the volume of gas molecules themselves are negligible compared to the container volume. Volume of gas is determined by its pressure, temperature and number of moles at any instant.

Number of moles (n) effect

Effects of pressure and volume of a gas bear a direct proportionality with number of moles. When `n' increases the number of molecules colliding against the wall of container increases. This effect increases the pressure of the gas. When the amount of gas increases the volume occupied by themselves also, increases.