Ncert Exercises & Solutions

Q. 16 The sap in trees, which consists mainly of water in summer, rises in a system of capillaries of radius r = 2.5 $\times 10^{- 5} m .$ The surface tension of sap is T=7.28 $\times 10^{- 2} N m^{- 1}$ and the angle of contact is $0 °$ . Does surface tension alone account for the supply of water to the top of all trees?

Hint: Use the concept of capillary rise.

Step 1: Find the value of capillary rise due to surface tension.

Given, radius, (r) = 2.5 x

10^{- 5} m

Surface tension, (S) = 7.28 x

10^{- 2} N / m

The angle of contact,

(θ) = 0 °

The maximum height to which Sap can rise in trees through capillarity action is given by,

h = \frac{2 S \cos θ}{r ρ g}

, where S=Surface tension,

ρ

=Density, r=Radius

= \frac{2 \times 7.28 \times 10^{- 2} \times \cos 0 °}{2.5 \times 10^{- 5} \times 1 \times 10^{- 3} \times 9.8} = 0.6 m

This is the maximum height to which the Sap can rise due to surface tension. Since, many trees have heights much more than this, the capillary action alone cannot account for the rise of water in all trees.

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