Question

Wax is coated on the inner wall of a capillary tube and the tube is then dipped in water. Then, compared to the unwaxed capillary, the angle of contact $\theta$ and the height $h$ upto which water rises change. These changes are :

• A. $\theta$ increases and $h$ also increases
• B. $\theta$ decreases and $h$ also decreases
• C. $\theta$ increases and $h$ decreases
• D. $\theta$ decreases and $h$ increases

Answer 1

Answer: (C)

Angle of contact $\theta$
$cos \,\theta=\frac{T_{SA}-T_{SL}}{T_{LA}}$
when water is on a waxy or oily surface
$T_{SA} < T_{SL} \,cos \,\theta$ is negative i.e.,
$90^{\circ} < \theta < 180^{\circ}$
i.e., angle of contact $ \theta$ increases
And for $\theta > 90^{\circ} $ liquid level in capillary tube fall, i.e., $h$ decreases