Question

It is difficult to fill a capillary tube with mercury than with water since:

• A. Angle of contact between glass & mercury is more than ${90}^{\circ }$ and the angle of contact between glass and water is less than ${90}^{\circ }$.
• B. Angle of contact between glass and mercury is less than ${90}^{\circ }$ and the angle of contact between glass and water is more than ${90}^{\circ }$.
• C. Angle of contact is same for both water and mercury.
• D. Mercury is less dense than water.

Answer 1

The correct option is A Angle of contact between glass & mercury is more than ${90}^{\circ }$ and the angle of contact between glass and water is less than ${90}^{\circ }$.

In a capillary tube, liquid will rise up due to capillary forces acting such that liquid column is pulled up. It happens when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.

In case of water filled in a capillary tube, angle of contact formed is $\theta <{90}^{\circ }$ and meniscus formed is concave. Hence, water will rise in the capillary tube due to stronger adhesive force between water molecules and the wall of the tube.

In case of mercury, due to higher angle of contact between glass & mercury i.e $\theta >{90}^{\circ }$, the meniscus formed is convex. Thus, cohesive force between mercury molecules is stronger, and mercury column will tend to shrink rather than rising upwards.
Hence, it is difficult to fill a capillary tube with mercury than with water.