Question

Explain shape of meniscus for different liquids in capillary.

Answer 1

Hint : The formation is due to the difference between the forces of attraction (adhesive forces) between the inner surface of the tube and the liquid molecules and the force of attraction (cohesive forces) between the liquid molecules.

Complete Step By Step Answer:
We all have seen a meniscus for a liquid in the capillary tube, while doing an experiment with a capillary tube in our labs.
When we pour a liquid in a tube like container, the top surface of the liquid (that is open to air) obtains a bulge like shape, either directing inwards or outwards.
This phenomenon is due to the surface tension of the liquid. Or we can also say that this is due to the difference between the forces of attraction (adhesive forces) between the inner surface of the tube and the liquid molecules and the force of attraction (cohesive forces) between the liquid molecules.
If the cohesive forces are stronger than the adhesive forces, then the molecules of the liquid will come closer and form a bulge that points outwards.
If the adhesive forces are stronger than the cohesive forces, then the molecules of the liquid will come closer to the inner surface of the tube and form a bulge that points inwards.
For liquids like water in glass, the strength of the adhesive forces and the cohesive forces are equal. As a result, we do not observe a meniscus, when we pour water in a glass tube.

Note :
We also under the bulge of the meniscus with the following formula:
$h=\dfrac{2T\cos \theta }{r\rho g}$ , where h is the height to which the liquid rises in a capillary tube, T is the surface tension, $\theta$ is the angel of contact, g is acceleration due to gravity and $\rho$ is density of the liquid.
If the value of $\theta$ is smaller, the bulge of the meniscus is very small. Or we can say that the radius of curvature of the meniscus is small.