A capillary tube of 3 mm inner diameter is inserted into fluid of density 900 kg/m³, surface tension 0.1 N/m and contact angle 30°. Find capillary rise due to surface tension.
Balance vertical surface tension $\pi d\gamma\cos\theta$ against liquid-column weight $\rho g(\pi d^2/4)h$. Thus $h=4\gamma\cos\theta/(\rho gd)=4(0.1)\cos30^\circ/[900(9.81)(0.003)]\approx13.08$ mm. None of the choices matches. The official key awards **marks to all**, retained here.
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