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Specific latent heat of vaporisation measurement

The specific latent heat of vaporization of a liquid may be measured by a modification of the method of Ramsey and Marshall (1896). The apparatus is shown in Figure 1.

The double-walled glass vessel is fitted to a condenser and mounted vertically. The inner section contains the liquid in which is a heater made of platinum wire. When the liquid boils the vapour passes through small holes into the outer vessel and then down into the condenser. Here it condenses, runs down and is collected in the beaker. It is essential that evaporation is rapid, for then the vapour in the outer vessel acts as a heat shield and eliminates heat losses from the inner vessel.

When a steady state has been reached - that is, when liquid drips into the beaker at a constant rate - a dean beaker is placed under the condenser and the mass of liquid m condensing, and hence being evaporated, in a measured time t can be found.
The specific latent heat of vaporisation L of the liquid can then be found from the equation

where VI is the power supplied to the coil. If a joule-meter is available the energy input E may be measured directly; then

E = mL

The large specific latent heat of vaporization of water explains why it is much more painful to be scalded by steam at 100 oC than by an equal mass of liquid water at 100 oC. The steam first condenses before it cools to your body temperature and in doing so releases roughly ten times as much heat energy as it does in the cooling phase.

Measurement of the specific latent heat of fusion of ice

The simplest method for measuring this quantity is the method of mixtures. Ice is dropped into water a few degrees above room temperature, and the resulting fall in temperature is recorded after all the ice has melted. Since the water falls from a few degrees above the temperature of the surroundings to a few degrees below the heat losses may be ignored - the mixture is assumed to gain as much heat as it losses and a cooling correction need not be applied.

(See also 14-16 section experiments: Latent heat measurement)
© Keith Gibbs 2007