# Measurement of the specific latent heat of vaporisation of water

When steam, at 100 degrees celsius, is bubbled into water, it first condenses to water and then its temperature decreases as it loses heat to the surrounding water. If this occurs in an insulated container then the heat lost by the steam is equal to the heat gained by the water and the container. The specific latent heat of vaporisation of water (l) can then be calculated using the formula: ms l + ms cw(fall in temp of condensed steam) = mc cc (rise in temp of calorimeter) + mw cw (rise in temp of water) where ms, mc and mw are the masses of steam, calorimeter and water respectively and cw and cc are the specific heat capacities of water and of the material of the calorimeter. Note: ccopper = 390 J / kg / K, caluminium = 910 J / kg / K, cpolystyrene = 0.

Procedure:
1. To select different calorimeters click on the word "Copper" for other options. Mass is shown in the top pan balance.

2. Click in the Mass box and select the mass of water in the calorimeter. (min.50g, max. 90g). Press Submit.

3. Click on "Place Calorimeter" to put the calorimeter in the insulated container.

4. Record the starting temperature, mass of water, mass of calorimeter, and the material of the calorimeter.

5. Press "Add Steam" to allow steam from the steam generator to pass to the water in the calorimeter.

6. After a temperature rise of 10 to 15 degrees, the steam pipe is removed (happens automatically here) so heating is stopped. Record temperature.

7. Press "Get Total Mass" to find the mass of calorimeter + water + steam. Record.

8. Press "Reset".

9. Repeat the experiment using a variety of calorimeters and masses of water.
Mass water:

Precautions:

• Ensure that only steam (not water) enters the water in the calorimeter. Use a "steam trap" (it actually traps water) if available.
• Ensure that the calorimeter is well insulated to avoid loss or gain of heat energy.
• Stir the water throughout the experiment to ensure that the thermometer reading reflects the heat supplied.
• Use a sensitive thermometer graduated to 0.1 or 0.2 degrees. An error of 1 deg. in 10 is a large relative error.
• Use cooled water (about 5 deg. below room temp.) at the start of the experiment so that, overall, heat is neither lost nor gained from the surroundings.

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