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The capillary tube method for finding the s.v.p. of water

This is a simple method, for which the apparatus is shown in Figure 1: a capillary tube closed at one end containing a short column of air with a water plug stands in a glass of water. The air trapped in the tube is of course saturated since there is excess water present.

Let the atmospheric pressure be A, let the air pressure in the tube be p and the s.v.p. of water at that temperature be s. Neglecting the weight of the small amount of water in the plug, we have


p = A - s

Now saturated vapours do not obey Boyle's law, because their pressure is constant at a given temperature and is independent of the volume of air. Therefore for the air in the tube we have:

(A - s)V/T = constant

where V is the volume of air in the tube and T is its temperature.

We can therefore measure the s.v.p. of water if its value at one temperature is known.

The results obtained for water are shown in Figure 2(a). Notice that the s.v.p. rises with temperature as predicted, and that its value at 100 oC is the value of standard atmospheric pressure. This fact can be explained as follows. Consider a bubble below the surface of the liquid: the pressure within the bubble is the saturated vapour pressure of that liquid, neglecting the hydrostatic pressure which is comparatively small.

Now when the liquid boils bubbles rise to the surface of the liquid and escape so the pressure within them must be equal to the external pressure (Figure 2(b).

We can say, therefore, that:

© Keith Gibbs 2007