Factors Affecting Surface Tension
Surface tension is a property of a liquid surface displayed by its acting as if it were a stretched elastic membrane. This phenomenon can be observed in the nearly spherical shape of small drops of liquids and of soap bubble. Because of this property, certain insects can stand on the surface of water. Surface tension depends mainly on the forces of attraction between the particles within the given liquid and also upon the gas, solid or liquid in contact with it. Surface tension can be expressed by the formula:
\(Surface\,Tension\left( T \right)=\frac{Force\left( F \right)}{Length\left( L \right)}\),
Impurities present in a liquid appreciably affect the surface tension. A highly soluble substance like salt increases the surface tension whereas soluble substance like soap decreases the surface tension.
(1) Temperature: The surface tension of liquid decreases with rise of temperature. The surface tension of liquid is zero at its boiling point and it vanishes at critical temperature. At critical temperature, intermolecular forces for liquid and gases become equal and liquid can expand without any restriction. For small temperature differences, the variation in surface tension with temperature is linear and is given by the relation
\({{T}_{t}}={{T}_{o}}\left( 1-\alpha t \right)\),
Where,
\({{T}_{t}}\),\({{T}_{0}}\) are the surface tensions at \({{t}^{0}}C\) and \({{0}^{0}}C\) respectively
\(\alpha \) = The temperature coefficient of surface tension.
Examples: (i) Hot soup tastes better than the cold soup.
(ii) Machinery parts get jammed in winter.
(2) Impurities: The presence of impurities either on the liquid surface or dissolved in it, considerably affect the surface tension, depending upon the degree of contamination. A highly soluble substance like sodium chloride when dissolved in water increases the surface tension of water. But the sparingly soluble substances like phenol when dissolved in water, decreases the surface tension of water.