Definition of Capillarity

Capillarity (or capillary action) is the tendency of a liquid to rise or fall in a narrow tube

Illustration of Capillarity

Demonstration of Capillary Action

(i) Dip three capillary tubes with fine bores but with different diameters into clean water as in the figure above. You will observe that the water rises in the tubes but the narrower the bore the greater the height to which the water rises.

(ii) Repeat the experiment with soap solution. You will observe a similar situation except that the heights of soap solution are lower than in the case of water.

(iii) Repeat the experiment again with mercury. You will observe that the level of mercury falls in the three tubes. The mercury level is depressed below the level of mercury outside the tubes (i.e. in thecontainer). Again the narrower the tube the lower the level of mercury .

Explanation of Capillarity

(i) The meniscus of water or soap solution is curved upwards (concave) because the adhesion of water and soap solution to glass is greater than the cohesion of water or soap. Therefore water or soap solution wets the glass tube and so spreads a thin film of water/soap solution on the inner surface of the tube. The adhesive forces thus force the water (soap solution) to creep up the inside of the tube. The water/soap solution is held up as it creeps by surface tension forces acting around the circumference of the meniscus. The water/soap solution thus keeps rising in the tube until the weight of the column of water/soap solution balances the surface tension acting at the top of the column.

(ii) In the case of mercury the cohesion of mercury molecules which is greater than the adhesion of mercury to glass causes the mercury level to be depressed in the tube. The surface tension forces acting around the circumference of the tube holds down the mercury column as it is depressed by cohesive forces of the mercury molecules. The depression continues until the weight of the mercury column in the tube is equal to the surface tension.

Cohesive and Adhesive Forces

Force of cohesion is the force of attraction existing among molecules of the same substance while force of adhesion is the force of attraction that exists between molecules of different substances. These forces can be used to explain why water wet glass and mercury does not.

The force of adhesion of water molecules to glass molecules is stronger than the force of cohesion of water molecules. This makes water to wet the glass when it is spilled on it. The force of cohesion of mercury molecules is greater than the force of adhesion between mercury and glass molecules. Hence, when mercury is spilled on glass, it does not wet the glass but forms spherical droplets. For the same reason, the water surface in a glass vessel is convex i.e curves downward while the mercury in a glass vessel is concave.(it curves upward) as shown in the diagrams below.

EVALUATION

1. State four assumptions of the kinetic theory of gases
2. Explain why water poured in a glass tube curve downward and when the liquid in the glass is mercury, the mercury curve upward