Capillary Action - Practice Questions & MCQ

•  Capillarity -

If a capillary tube is dipped in a liquid, it is found that the liquid in the capillary either ascends or descends relative to the surrounding liquid. This phenomenon is called capillarity.

 Examples of capillarity-

1. A towel soaks water.

• Ascent Formula-

When one end of the capillary tube of radius r is immersed into a liquid of density   (For example- water and capillary tube of glass), And the shape of the liquid meniscus in the tube becomes concave upwards as shown in the figure.

Then the height h up to which the liquid level rises in the capillary tube  is given by Ascent Formula 

$\text { which says } h=\frac{2 T \cos \Theta}{\rho g r}$

where 

$T$ - surface Tension
$r$ - radius of capillary tube
$\rho$ - liquid density
$\theta$ - Angle of contact
1. The capillary rise depends on the nature of liquid and solid both. I.e on $T, \rho, \theta, r$
2. For a given liquid and solid pair as $T, \rho, \theta$ are constat then $h \alpha \frac{1}{r}$. i.e., lesser the radius of capillary greater will be the rise and vice-versa.
3. Capillary action for various liquid-solid pair
I. For $\Theta<90^{\circ}$ (I.e for water and capillary tube of glass)

So Meniscus will take Concave shape and liquid in the capillary will rise/ascend.
II. For $\Theta>90^{\circ}$ ( I.e for Mercury and glass capillary tube)

So Meniscus will take Convex shape and liquid in the capillary will fall/descend.
III. For $\Theta=90^{\circ}$ (I.e for Pure water and silver-coated capillary tube.)

So Meniscus will take Plane shape and liquid in the capillary will show No rise, no fall.