question_answer

Water rises in a capillary tube, whereas mercury falls in the same tube. Why?

Answer:

                In a capillary tube, a liquid rises to a height h given by \[h=\frac{2\sigma \cos \theta }{r\rho g}\] For water, \[\theta \] is acute, \[\cos \theta \]is positive and hence h is positive. So water rises in the capillary tube. For mercury, \[\theta \] is obtuse, \[\cos \theta \]is negative and hence h is negative. So mercury gets depressed in the capillary tube.