Factors affecting the surface tension of a liquid
Surface tension for a given liquid varies in following situations
(1) The presence of any contamination or impurities considerably affects the force of surface tension depending upon the degree of contamination.
(2) The presence of dissolved substances can also affect the value of surface tension. For example, a highly soluble substance like sodium chloride (NaCl) when dissolved in water (H20) increases the surface tension of water. But the sparingly soluble substance like phenol or soap solution when mixed in water decreases the surface tension of water.
(3) Electrification affects the surface tension. When a liquid is electrified, surface tension decreases. Since external force acts on the liquid surface due to electrification, area of the liquid surface increases which acts against the contraction phenomenon of the surface tension. Hence, it decreases.
(4) Temperature plays a very crucial role in altering the surface tension of a liquid. Obviously, the surface tension decreases linearly with the rise of temperature. For a small range of temperature, the surface tension at Tt at t ºC is Tt = T0 (1− α t)
Where, T0 is the surface tension at temperature 0ºC and α is the temperature coefficient of surface tension. It is to be noted that at the critical temperature, the surface tension is zero as the interface between liquid and vapour disappear. For example, the critical temperature of water is 374ºC. Therefore, the surface tension of water is zero at that temperature. van der Wall suggested the important relation between the surface tension and the critical temperature as
Generalizing the above relation, we get
which gives more accurate value. Here n, varies for different liquids and t and tc denotes the temperature and critical temperature in absolute scale (Kelvin scale), respectively.