Examples of Surface Tension (Due to formation of surface film):
When a glass rod is dipped in water and taken out it is found that the drop of water is sticking to the end of the rod. The molecules of water appear to be contained in a bag whose surface is made up of a film of water molecules themselves. This example shows that free surface of a liquid behaves like a film which is under tension.
When water falls slowly in drops from a tap, each drop gradually forms at the tip of the nozzle. As the drop of water grows in size, it looks like a hanging elastic bag whose surface area is expanding as it fills up.
Water spiders are able to walk on the surface o the water because their feet produce dimples on the surface film without rupturing the film. As soon as a foot is lifted the film which is under tension again becomes flat.
The feet of spider produce dimples on the surface film without rupturing the film. Due to which the surface tension acts in an inclined manner. The vertical component of the surface tension supports the weight of the insect.
As soon as a foot is lifted the film which is under tension again becomes flat.
A safety razor blade, when placed gently with its flat surface on water floats on it even though the density of steel, is nearly eight times greater than that of water.
The surface film forms due to surface tension support the needle/blade and their thickness is not sufficient to break the film.
When a paint brush is immersed in paint, there is no free surface area available for liiquid ion the paint and the hair are spread. When the brush is taken out, its bristles are drawn close together due to the surface tension in the film formed between adjacent hair.
A liquid tries to acquire minimum surface area on account of its surface tension. For a given volume sphere has the smallest surface area. Hence rain water falling down acquires spherical shape i.e. rain drops are spherical.
When irregular shaped camphor piece is dropped on water surface, it gets dissolved gradually. Thus the surface tension of water around it decreases. Due to irregular shape the unbalanced forces makes it move haphazardly on the surface of water. Thus the irregular shaped camphor piece dances on water surface.
Oil does not spread on cold water but spreads on hot water because when water is cold the surface tension of oil is less than that of cold water but when water is hot the surface tension of oil is greater than that of hot water.
High tides in seawater are reduced by sprinkling oil on the water surface.
Rainwater does not pass through tiny holes in the fabrics of umbrellas, tents, raincoats, etc. because, on account of surface tension, the holes are blocked by fine films of water.
The surface tension of liquid prevents it from spreading over a solid surface. But lubricating oil must readily spread on the solid surface which is to be lubricated. Therefore, by blending different types of oil, the surface tension of a lubricating oil is made as low as possible.
When soap dissolves in water, the surface tension of soap solution is greatly reduced and this enables soap solution to quickly spread over large areas. This is one of the reasons why soap is used in washing.
Soap is used in toothpaste to enables the paste to quickly spread over a large area and give a good mouthwash.
Soap is used as wetting agent in dying.
Ice skating is a very enjoyable, because, the tiny droplets of water, formed under the skates, act almost like rigid ball bearings over which the skates glide very smoothly.
Surface tension is used for diagnosis of jaundice.
Factors Affecting Surface Tension:
Presence of Impurity:
If the impurity added in a liquid is highly soluble, then the surface tension of liquid increases.
If the impurity added in a liquid is partly soluble, then the surface tension of liquid decreases.
The surface tension at t °C is given by T = T0(1 – α t), Where T0 = Surface tension at 0O C and α = Temperature coefficient of surface tension. Its value depends on the nature of the liquid.
Generally surface tension of liquid decreases with increase in temperature. it is due to increase in kinetic energy of liquid molecules.
The temperature at which surface tension of a liquid becomes zero is called a critical temperature of the liquid.
Surface tension of a liquid at boiling point and at critical temperature is zero.
Surface tension of molten cadmium and copper increases with increase in temperature.
The presence of dust particles in liquid decreases its surface tension.
Use of Surface Tension in Washing of Clothes:
The surface tension of water is such that water does not spread on oily or greasy clothes.
The surface tension of liquid decreases with increase in temperature. By adding detergent the surface tension of water decreases and it increases the wetting of the cloth by water. The water penetrates into the pores of the cloth and removes the dirt. The dirt is then suspended in detergent solution.
Similarly, the detergent solution suspends the dirt in the solution and does not allow the dirt to settle back on the cloth.
Explanation of Some Phenomenon Associated with Surface Tension / Surface Film:
Antiseptics Have Low Surface Tension:
Antiseptic is a medicine used externally on the wounded or injured surface of the body. The antiseptic should spread on body surface easily to get a proper result.
Due to lower surface tension the antiseptic can spread easily on body surface.
It is possible to produce a fairly vertical film of soap solution but not of water:
As we add impurity or any soluble substance ion water, the surface tension of water decreases.
Thus the surface tension of soap solution is very less compared to water itself.
Oil spreads over the surface of the water while water does not do so:
The surface tension of oil is less than that of water.
Thus when oil is dropped on water it can not break the surface film of water hence spreads on water. While when water is poured on oil it is capable of breaking the surface film of oil very easily.
Ends of a glass rod become round on heating:
On heating glass melts. The molten glass behaves like a liquid to obtain minimum surface area on account of its surface tension.
For a given volume sphere has the smallest surface area.
Thus the molten glass at the end of the tube acquires spherical shape.
Thus the end of a glass rod becomes round on heating.
Surface tension is a joint property of the interface separating two substances at least one of which is a fluid. It is not the property of a single fluid alone.
Surface film is the thin film of liquid near its surface and having the thickness equal to the molecular range of attraction for that liquid. The phenomenon of surface tension is associated intimately with this film.