The Gaseous State

Characteristics of gases

(1) Gases or their mixtures are homogeneous in composition.

(2) Gases have very low density due to negligible intermolecular forces.

(3) Gases have infinite expansibility and high compressibility.

(4) Gases exert pressure.

(5) Gases possess high diffusibility.

(6) Gases do not have definite shape and volume like liquids.

(7) Gaseous molecules move very rapidly in all directions in a random manner i.e., gases have the highest kinetic energy.

(8) Gaseous molecules collide with one another and also with the walls of the container with perfectly elastic collisions.

(9) Gases can be liquefied if subjected to low temperatures (below critical) or high pressures.

(10) Thermal energy of gases >> molecular attraction.

(11) Gases undergo similar change with the change of temperature and pressure. In other words, gases obey certain laws known as gas laws.

 Parameters of Gases

 The characteristics of gases are described in terms of following four parameters

  • Mass
  • Volume
  • Pressure
  • Temperature


1. Mass (m):

The mass of the gas is related to the number of moles as

n = w/M 

Where  n = number of moles

w = mass of gas in grams

M = molecular mass of the gas  


2. Volume (V): 

Since gases occupy the entire space available to them, therefore the gas volume means the volume of the container in which the gas is enclosed.

Units of VolumeVolume is generally expressed in litre (L), cm3 & dm3 

1m3 = 103 litre = 103 dm3 = 106 cm3.

3. Pressure  (P): 

Pressure of the gas is due to its collisions with walls of its container i.e. the force exerted by the gas per unit area on the walls of the container is equal to its pressure.


Pressure is exerted by a gas due to kinetic energy of its molecules.

As temperature increases, the kinetic energy of molecules increases, which results in increase in pressure of the gas. So, pressure of any gas is directly proportional to its temperature.

Units of Pressure: 

The pressure of a gas is expressed in atm, Pa, Nm–2, bar and lb/In2 (psi).

760 mm = 1 atm = 10132.5 KP­a = 101325 Pa = 101325 Nm–2

760 mm of Hg = 1.01325 bar = 1013.25 milli bar = 14.7 lb/2n2 (psi)  


3. Temperature (T):

Temperature is defined as the degree of hotness. The SI unit of temperature is Kelvin.  oC and oF are the two other units used for measuring temperature. On the Celsius scale water freezes at 0°C and boils at 100°C where as in the Kelvin scale water freezes at 273 K and boils at 373 K.  

K = oC + 273.5

F = (9/5) oC + 32