Q: Water rises to a height of 10cm in a capillary tube and mercury falls to a depth of 3.42 cm in the same capillary tube. If the density of mercury is 13.6g/cc and the angle of contact of mercury and water are 135° and 0° respectively, the ratio of surface tension of water and mercury is

(A) 1 : 0.15

(B) 1 : 3

(C) 1 : 6.5

(D) 1.5 : 1

Solution: h =2Scosθ/ρgr

or S =hρgr/2cosθ

or S ∝ hρ/cosθ

S_{w}/S_{Hg} = h_{1}/h_{2} × ρ_{1}/ρ_{2} × cosθ_{1}/cosθ_{2}

Putting the values, we obtain 1 : 6.5

Hence, (C) is correct.