It is well known fact liquids assume the shape of the container. Why is then small drops of mercury form sperical bead instead of spreading on the surface. Why do particles of soil at the bottom y river remain separated but they stick together whey taken out?
Why does a liquid rise (or fall) in a tin capillary as soon as the capillary touches the surface of the liquid? All these phenomena are caused due to the characteristic properties of liquid, due to the characteristic properties of liquid called surface tension.
A molecule in the intermolecular forces from all sides. The molecule, therefore does not experiences any net force. But for the molecule on the surface of liquid, net attractive force is towi the molecules below it. Since there are no molecules above it.
Liquid tend to minimize their surface area. The molecule on the surface experience a net downward force and have more energy experience any net force.
Therefore, liquids tends to have minimum number from the bulk, attractive force will have to be overcome.
This will required expenditure of energy. The energy required to increase the surface area of the liquid by one units is defined as surface energy.
It's dimensions are Jm-2. 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 gamma. It has dimension 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. Spherical shay satisfies this condition, that is why mercury drops are sperical in shape. This is the reason that sharp glass edges are heated for making them smooth. On heating, the glass melts and the surface of the liquid tends to make the rounded shape at the edges, which makes the edges smooth. This is called fire polishing of glass.
Liquid tends to rise (or fall) in the capillary because of surface tension. Liquids wets the things because they spread across their surface as thin film. Most soil grains are pulled together because surface area of thin film of water is reduced. It is surface tension which gives stretching property to the surface of a liquid. On flat surface, droplets are slightly flattened by the effect of gravity; but in the gravity free environment drops are perfectly sperical.
The magnitude of surface tension of a liquid depends on the attractive forces between the molecules.
When the attractive forces between the molecules. When the attractive forces are large, the surface tension is large. Increase in temperature increases the kninetc energy of the molecules and effectiveness of intermolecular attraction decreases, so intermolecular attraction decreases, so surface tension decreases as the temperature is raised.
