Effect of Impurity and Temperature on Surface Tension

a) Effect of impurities. (i) When a soluble substance, such as ordinary common salt (sodium chloride), is dissolved in water, the surface tension of the water increases. (ii) When a sparingly soluble substance, such as phenol, or a detergent, is mixed into water, the water's surface tension instead decreases — this is exactly why detergent powder is mixed with water to wash clothes: the lowered surface tension lets the water make much better contact with the fabric, letting it penetrate and remove tough stains far more effectively than plain water could. (iii) When an insoluble impurity is added to a liquid, its surface tension also decreases — any impurity added to a liquid weakens its cohesive force, which in turn affects the angle of contact and hence the shape of its meniscus; if mercury gathers dust, for instance, its surface tension is reduced, and it will no longer form clean spherical droplets unless the dust is completely removed first. …