Surface Tension - Class 11 PDF Download

Surface tension is a characteristic  property of the liquids. This property is caused due to the strong intermolecular forces of attraction between the liquid molecules.

Let us understand this phenomenon by the following explanation:

Molecule P : In the above beaker, the molecule 'P' present in the body of the liquid is attracted equally in all the directions by the molecules which surround it. But the molecule 'R' present at the surface of the liquid just experiences a downward pull by the molecules because there are more molecules of the liquid below than in the air above the surface.

Hence the imbalance of the forces at the surface sets the liquid surface under tension known as surface tension. The molecules at the surface have higher energy as compared to molecules present in the interior of the liquid. Because more are the attractive forces, less is the energy and molecule in the interior is under the hold of more attraction forces. This is the law of nature that every molecule wish to acquire lower energy or more stability. Therefore the molecules from the surface tend to move from a state of higher energy to a state of lower energy associated with the interior molecules. In this way liquids tend to contract or minimize their surface area in order to have the minimum number of molecules at the surface.

so, surface tension may be defined as the force acting per unit length perpendicular to the line drawn on the surface of liquid.

It is denoted by Greek letter 'ϒ'

Dimensions = kgm^-2

S.I. Unit = Newton per meter (Nm^-1)

Surface Energy

The effect of surface tension is to reduce the surface of the liquid to minimum. But if we try to increase the area of the surface of the liquid, then we need to pull the molecule from the bulk against the high attractive forces.

so, the energy required to increase the surface area of the liquid by one unit is called surface energy of the liquid.

Dimensions = Jm^-2

Surface tension is dependent upon two factors:

1. Nature of liquid
2. Temperature

1. Nature of liquid: The magnitude of surface tension depends upon the nature of the liquid . More are the attractive intermolecular forces, more is the surface tension. This is because the surface molecules are pulled with greater attractive forces from the molecules present downwards. So, the surface of the liquid contracts more.

Surface tension ∝ Attractive forces

2. Temperature: As the temperature increases, surface tension decreases. This is because, with the rise in temperature the kinetic energy of the liquid molecules increases. This in turn decreases the intermolecular forces. It results in decrease in the inward pull functioning on the surface of the liquid. so, surface tension decreases with rise in temperature. 

Surface tension ∝ 1/ Temperature