CAPILLARITY

A tube with fine and uniform bore throughout its length, is called capillary tube. When one end of glass capillary tube open at both the ends, is dipped in a liquid like water, the liquid rises or falls in the capillary tube above or below the liquid level in container. The phenomena of rise and fall of liquid in a capillary tube is called capillarity.

ABOUT THE HEIGHT RAISED BY LIQUID

1- Greater the surface tension, greater will be the height.

2- Greater the angle of contact, smaller will be the height.

3- Greater the radius of capillary tube, smaller will be the height.

4- Greater the density of liquid, smaller will be the height.

APPLICATIONS OF CAPILLARY ACTION

• The kerosene oil in the lantern and the melted wax in a candle, rise in the capillaries formed in the cotton wick and burns.

• Coffee powder is easily soluble in water because water immediately wets the fine granules of coffee by the action of capillarity.
• The water given to the field rises in the innumerable capillaries formed in the stems of plants and trees and reaches the leaves.

 

• Writing nib is split in the middle so that a fine capillary is formed in it. When it is dipped in ink, the ink rises in the capillary.

• Blotting paper is used to absorb spilled liquid by the help of capillary action.
• Sponges and brushes are use to absorb soap water to clean the vessel.

• Mercury rises in the thermometer to show temperature is the example of capillary action.
• Absorbtion of sweat from the skin by the fabric or towel.

 

ANGLE OF CONTACT

The surface of liquid near the solid surface is curved (concave or convex). The angle of contact between the liquid and solid in contact is defined as the angle enclosed between the tangents to the liquid surface at a point of contact and the solid surface inside the liquid.

IMPORTANT POINTS

• The angle of contact is acute in case of liquids, which wet the walls of the container and is obtuse for the liquids which do not wet the walls of the container.
• The angle for pure water and clear glass surface is 0 degree
• For alcohol and clean glass, the angle of contact is 0 degree.
• For ordinary water and glass, the angle of contact is about 8
• For mercury and glass, the angle of contact is 140 degree.
• The angle of contact increases with increase in temperature of liquid.
• The angle of contact decreases on adding soluble impurity to the liquid.
• If the liquid wets the sides of the vessel containing liquid, the shape of the liquid meniscus is concave upwards.
• If the liquid does not wet the sides of the vessel containing liquid, the shape of the liquid meniscus is convex upwards.
• The shape of liquid meniscus depends upon the molecular forces and is independent of gravitational pull.
• Liquids having acute angle of contact rise in the capillary.
• Liquids having obtuse angle of contact fall in the capillary.
• Rise of liquid in a capillary tube does not violate law of conservation of energy.
• The height of the liquid column in a capillary tube on the surface of the moon is 6 times than that on the earth.