Define surface tension and write its unit.

Surface tension is defined as the force per unit length acting perpendicular to an imaginary line drawn on the surface of a liquid. It arises due to the cohesive forces between liquid molecules, which tend to minimize the surface area. These intermolecular forces (primarily hydrogen bonding or van der Waals forces) pull the surface molecules inward, creating a "tight" surface layer.

Mathematically, surface tension ($\gamma$) is expressed as: $$\gamma =\frac{F}{L}$$ where:

• $F$ = force acting perpendicular to the surface (in newtons, N),
• $L$ = length over which the force acts (in meters, m).

The SI unit of surface tension is newton per meter (N/m). Other commonly used units include:

• dyne per centimeter (dyn/cm) (where 1 N/m = 1000 dyn/cm),
• joule per square meter (J/m²) (since surface tension is also equivalent to energy per unit area).

Surface tension is responsible for phenomena like capillary action, droplet formation, and the ability of insects to walk on water. It decreases with increasing temperature due to reduced intermolecular forces.