CBSE Class 11-science Answered

IMBALANCE OF THE FORCES AT THE SURFACE OF THE LIQUID (SURFACE TENSION)

Surface tension is defined as the force acting per unit length perpendicular to the line drawn on the surface
of liquid. It is denoted by Greek letter γ (Gamma). It has dimensions of kg s^–2 and in SI unit it is expressed as N m^–1. The lowest energy state of the liquid will be when surface area is minimum.

A molecule in the bulk of liquid experiences equal intermolecular forces from all sides. The molecule, therefore does not experience any net force. But for the molecule on the surface of liquid, net attractive force is towards the interior of the liquid, due to the molecules below it. Since there are no molecules above it. Liquids tend to minimize their surface area. The molecules on the surface experience a net downward force and have more energy than the molecules in the bulk, which do not experience any net force. Therefore, liquids tend to have minimum number of molecules at their surface. If surface of the liquid is increased by pulling a molecule from the bulk, attractive forces will have to be overcome. This will require expenditure of energy. The energy required to increase the surface area of the liquid by one unit is defined as surface energy.