Gases have similar physical properties. A few typical properties are discussed here.

1) Diffusion: The  Spontaneous mixing up of molecules by random motion and collisions to form a homogeneous mixture.    ‘OR’

The spontaneous movement of molecules from a region of its higher concentration to a region of its lower concentration is called diffusion.

Rate of diffusion depends upon the molecular mass of the gases. Lighter gases diffuse rapidly than heavier ones. For example, H2 diffuses four times faster than O2gasdue to its low molecular weight.

2) EFFUSION:  The  escaping of gas molecules through a tiny hole into a space with lesser pressure is called effusion

Example, when a tyre gets punctured, air effuses out.

Explanation:  Effusion depends upon molecular masses, lighter gases effuse faster than heavier gases. In effusion process gas molecules donot collide with each otherbut can escape one by one through the hole.

3) Pressure: the force(F) exerted per unit surface area (A) is called pressure.

Pressure= force/area        P = F/A.

Gas molecules are always in continuous state of motion. Hence, when molecules strike with the walls of the container or any other surface, they exert pressure.

Unit: The SI unit of force is Newton and that of area is m2. Hence pressure has SI unit of Nm-2. It is also called Pascal (Pa)

The pressure equal one newton per square meter is called pascal.

One Pascal (Pa) = 1 Nm-2

Measuring instruments(parameters): Barometer is used to measure atmospheric pressure and manometer is used to measure pressure in the laboratary.

Standard Atmospheric Pressure:

It is the pressure exerted by the atmosphere at the sea level


It is defined as the pressure exerted by atmosphere at sea level.


It is defined as the pressure exerted by a mercury column of 760 mm height at sea level.


It is sufficient pressure to support a column of mercury 760 mm in height at sea level on an area of  1 cm2 at 0Cº.

1 atm  = 760mmHg  = 760 torr. (1 mmHg = 1 torr) = 101325 Nm2 = 101325 Pa.

So 1 atm = 101325 Pa (pascal).


Gases are highly compressible due to empty spaces between their molecules. When gases are compressed, the molecules come closer to one another and occupy less volume as compared to the volume in uncompressed state.



Gas molecules are always in state of continuous motion. They can move from one place to another because gas molecules possess very high kinetic energy. They move through empty spaces that are available for the molecules to move freely. This mobility or random motion results in mixing up of gas molecules to produce a homogeneous mixture.

  Density of Gases

Mass per unit volume is called density. d (Þ) = m/v

Expression: Gas density is expressed in grams per dm3 (due to large volume). Whereas, liquid and solid densities are expressed in grams per cm3.

Explanation: Gases have low density than liquids and solids. It is due to light mass and more volume occupied by the gas molecules i.e. liquids and solids are 1000 times denser than gases. The density of gases increases by cooling because their volume decreases

 Example:  At normal atmospheric pressure, the density of oxygen gas is 1.4 g dm-3 at 20°C and 1.5 g dm-3 at 0°C. Density of water is 1gm/cm3 while density of gold is 19.3 gm/cm3.

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