Anything that occupies space and has weight is called matter. In general, matter exists in three states (i) solids, having definite volume, a definite shape, and are rigid. (ii) Liquids have a definite volume, but no definite shape and are nonrigid. (iii) Gases have neither definite shape, nor definite volume and are nonrigid.


When the molecular forces of attraction between the particles of a matter are minimum, the particles exist in a state known as gaseous state or a state of matter in which molecules are far away from each other and free to move in available space is called gaseous state.

Parameters of Gases

The characteristics of gases are described in terms of four measurable parameters and it is also called measurable properties of gases which are

        (a)   Mass                                   (b)   Volume

        (c)   Pressure                             (d)   Temperature

(a) Mass (m): The mass of gas is denoted by m, which is related to the no of moles ‘n

or, m = nM

(b)  Volume V :

        (i)    Gases occupy all the space available to them. The volume occupied by a gas is simply the volume of the container in which it is filled.

        (ii)   The volume of a gas is denoted by ‘V’ and it is measured in units of liters (or) cubic meter or

        (iii) I litre = 1dm3 = 1000 cm3=1000 ml = 10-3m3

(c)  Pressure (P) :

        (i)    It is a force acting per unit area. A confined gas exerts uniform pressure on the walls of its container in all directions.

        (ii)   It is denoted by ‘P’ and specified in pascal (Pa)

        (iii) Other units of pressure are atm, cm Hg, mmHg, N/m2 , bar , torr.

       (iv)   1 atm = 76 cm Hg = 760 mm Hg =  =  = 1.013 x 105N/m2 bar = 1.013 x 105Pa = 760 torr.

(iv)  Pressure exerted by a gas is due to kinetic energy (K.E. =½) of the gas molecules.

        (vii)  K.E of the gas molecule increases as the temperature increases. So, the pressure of a gas is directly proportional to temperature, i.e.,

(d)  Temperature (T) :

        (i)    The temperature of a gas is denoted by ‘T’ and it is measured in the unit of Kelvin (K)

        (ii)   Other units of temperature are 0C,0F, K.

        (iii)         K =0C, 0F, K

Properties of Gases

(a) They do not have definite shape and volume.

(b) They can occupy the whole space available to them.

(c) Gases have unlimited dispensability and high compressibility.

(d) They have very low densities because of negligible intermolecular forces.

(e) Gases exert pressure on the walls of the container with perfectly elastic collisions.

  (f) They diffuse rapidly through each other from a homogenous mixture against the electric, magnetic and gravitational field.


There are certain laws that relate to the four parameters called gas laws.

Boyle’s Law

(a) It states that at a constant temperature, the volume of a given mass of gas is inversely proportional to the pressure.

Charles’s Law

(a)  This law states that at constant pressure, the volume of a given mass of a gas is directly proportional to its absolute temperature.                                 (Absolute temperature,

(b) Mathematically: (at constant pressure and number of moles)

                V = Volume of gas

                T = Absolute temperature

                V = KT or             

Hence, if the volume of a gas of mass is  at temperature  change to  at, pressure remains constant, then

or log V– log T = constant

(c) For each degree change in temperature, the volume of a sample of a gas changes by a fraction of its volume at 0C.

(d) Graphical representation