Variation of surface tension with temperature

As might be expected, the coefficient of surface tension decreases with increasing temperature. It becomes zero at the critical temperature.
The variation of surface tension with temperature can be studied by Jaeger's method using the apparatus shown in Figure 1.

Water is allowed to drip slowly into the large flask, so forcing bubbles of air out of the capillary tube which dips into a beaker of water. The lower end of the capillary tube is a depth h₁ below the water surface. It can be shown that the bubble will break free from the end of the tube when its radius is equal to the internal radius of the tube.

Using a manometer the total pressure within the apparatus may be found; this is equal to the hydrostatic pressure (h₁r₁g) plus the excess pressure within the air bubble due to the surface tension of the water. The total pressure is given by the equation:

h₂r₂g = h₁r₁g + 2T/r

where r₁ is the density of water, r₂ the density of the liquid in the manometer, r the radius of the capillary tube and h₂ the difference in levels within the manometer. The coefficient of surface tension of the water (T) can therefore be found.

Heating the water enables the value of T to be determined at a range of temperatures.

Student investigation
Using the above apparatus, investigate the effect on the surface tension of the liquid of adding known amounts of methylated spirits to the water.

Plot a graph of surface tension against the concentration of the water-meths mixture.