Question

Due to capillary action, a liquid will rise in a tube if the angle of contact is

• A. Acute
• B. Obtuse
• C. ${90}^{\circ }$
• D. ${180}^{\circ }$

Answer 1

The correct option is A Acute

The rise of liquid in a capillary tube when placed in a vessel full of liquid is known as capillary action. The height of the rise of liquid is given by the formula:
$h=\frac{2T\text{cos}\theta }{rg\rho }$
Here, $\theta$ is the angle of contact,
$T$ is surface tension of the liquid

Only if $\theta$ is acute, $\text{cos}\theta$ will be $+ve$, which means that the height will be positive. Hence liquid will rise. It also represents that adhesive forces are stronger in this case.

If $\theta$ is obtuse, $\text{cos}\theta$ will be $-ve$, which means that the height will be negative. Hence liquid will fall. It also represents that cohesive forces between molecules of mercury are stronger in this case.

$\therefore$ The level of liquid in the capillary tube will fall in case of mercury.

For $\theta ={90}^{\circ }$, $\cos \theta$ will be zero, meaning the height will be zero and liquid will neither rise nor fall. $\theta ={180}^{\circ }$ is not possible, as it represents a perfectly non-wetting fluid (Limiting case of non-wetting characteristics).