(a) What is surface tension? Explain the phenomenon in terms of intermolecular forces
(b) Describe a simple experiment to demonstrate the surface tension of a liquid
(c) State three examples to illustrate the effects of surface tension
(d) Why does water wet a clean glass surface whereas mercury does not?
(e) State two methods by which the surface tension of a liquid may reduced.
(a) Surface tension is the property of a liquid which makes its surface behave as though covered with an elastic sink. Or it is the tangential force on the surface of a liquid. Surface tension is force per unit length on the surface of a liquid
Consider two molecules A and B as shown above. The molecule A will be attracted uniformly in all directions trot pulled in any one particular. direction. On the other hand, B is attracted more by molecules within the body of the liquid and it therefore experiences a resultant inward force at right angles to the surface. There will therefore be a tendency for the surface of the liquid to contact and have as little an area as possible. The surface is thus in a tension.
Diagram or description link
NB: Water and needle shown but not labelled
A beaker is filled to the brim with water as shown above. A dry needle is placed on a small piece of filter paper, which is then gently placed on the water surface. It would be observed that the filter paper sinks to the bottom of the beaker while the needle is left floating. The surface thereby acts like an elastic skin covering the liquid which is able to support the weight of the needle
(c) – When water is allowed to drip slowly from a water tap, the shape of each small drop is spherical.
- When mercury spills on a glass sheet, it forms spherical droplets.
- Soap bubbles are spherical in shape. - Insects (water - skaker) area able to walk across a water surface etc.
- Toy duck on liquid
- A flowing or floating needle on water.
(d) The adhesion of water molecules to glass molecules is stronger than the cohesion between water molecules. Hence, when water spills on a clean glass surface it spreads and wets the glass. On the other hand, the cohesion between mercury molecules is greater than the adhesion between mercury and glass molecules. Therefore when mercury spills on glass it forms small spherical droplets on larger flattened drops
(e) By adding
- detergent/soap
- alcohol
- oil
- By heating the liquid/increasing the temperature of the liquid
- adding camphor.
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