One of the special properties of water is that it has a high surface tension. High surface tension makes rain droplets round and water "bead" on many surfaces.

What does a "high surface tension" mean?

Where water and air meet there is a "surface". Water molecules at the surface have only air above them, and thus nothing to form H-bonds with; the "red" H-bonds on the diagram below represent the "missing" H-bonds at the surface of water. The stars label the "high energy" surface molecules.

This means that water molecules at the surface have a higher energy than water molecules completely within the liquid. Nature always wants to minimize the energy and so there is a strong drive to minimize the number of water molecules on the surface, that is to minimise the surface area of water.

Surface tension is just a way to measure or quantify this property, surface tension is the energy per unit area. A high surface tension means there is a strong energy drive to mimimize the surface area.

Spheres have the smallest surface area and so water forms spherical droplets. Scroll to the bottom of this page to see the mathematical calculations that demonstrate this is true (for a sphere, a cylinder and a cube).

Droplets form when the surface energy is very high, when the surface energy is small, the droplet can spread out and "wet" the surface. This property of water is the basis for water repellant fabrics and surfaces, fabrics which do not like to interact with water cause beading, fabrics that interact with water get wet!

Another way to think about surface tension is to consider the forces acting on water molecules at the surface.

Molecules on the surface feel the pull of molecules from below and to the sides via the H-bonds, but nothing from above (where there is air). Although there are fewer H-bonds these can be a bit stronger and thus the surface molecules feel unbalanced forces pulling them tightly together.

You can think of this as like a stretched sheet of rubber or "cling film" on the surface of the water, the surface is under tension.

Surface tension is also defined as the force per unit length, a high surface tension means there are stronger forces pulling the molecules together. A low surface tension means the molecules are more relaxed the forces between them are smaller.