Impurity, temperature & pressure

Effect of impurities:

i) When soluble substance such as common salt (i.e., sodium chloride) is dissolved in water, the surface tension of water increases.

ii) When a sparingly soluble substance such as phenol or a detergent is mixed with water, surface tension of water decreases. For example, a detergent powder is mixed with water to wash clothes. Due to this, the surface tension of water decreases and water makes good contact with the fabric and is able to remove tough stains.

iii) When insoluble impurity is added into water, surface tension of water decreases. When impurity gets added to any liquid, the cohesive force of that liquid decreases which affects the angle of contact and hence the shape of the meniscus. If mercury gathers dust then its surface tension is reduced. It does not form spherical droplets unless the dust is completely removed. 

b) Effect of temperature:

In most liquids, as temperature increases surface tension decreases. For example, it is suggested that new cotton fabric should be washed in cold water. In this case, water does not make good contact with the fabric due to its higher surface tension. The fabric does not lose its colour because of this. 

Hot water is used to remove tough stains on fabric because of its lower surface tension. In the case of molten copper or molten cadmium, the surface tension increases with increase in its temperature. The surface tension of a liquid becomes zero at critical temperature.

Excess pressure across the free surface of a liquid:

Every molecule on a liquid surface experiences forces due to surface tension which are tangential to the liquid surface at rest. The direction of the resultant force of surface tension acting on a molecule on the liquid surface depends upon the shape of that liquid surface. This force also contributes in deciding the pressure at a point just below the surface of a liquid. 

a) Plane liquid surface: Figure below shows planar free surface of the liquid. In this case, the resultant force due to surface tension on the molecule at B is zero. The pressure at A and B is the same. 

b) Convex liquid surface:

Surface of the liquid in the figure is upper convex. (Convex, when seen from above). In this case, the resultant force due to surface tension on the molecule at B is vertically downwards and adds up to the downward force. This develops greater pressure at point B, which is inside the liquid and on the concave side of the meniscus. Thus, the pressure on the concave side i.e., inside the liquid is greater than that on the convex side i.e., outside the liquid. 

c) Concave liquid surface: 

Surface of the liquid in the figure below is upper concave (concave, when seen from above). In this case, the force due to surface tension on the molecule at B is vertically upwards. The force due to atmospheric pressure acts downwards. These two forces, thus, act in opposite direction. This develops a lesser pressure at point B, which is inside the liquid and on the convex side of the meniscus. Thus, the pressure on the concave side i.e., outside the liquid, is greater than that on the convex side, i.e., inside the liquid.