## Measurement of the Specific Latent Heat of Vaporisation of Water Using an Electrical Method

Apparatus:

Mains water heater, stop-watch, thermometer, top-pan balance, stirrer.

Procedure:

1. Measure the mass of the empty water heater.

2. Fill the heater with water so that it is approximately half full.

Remeasure the mass and hence determine the mass of the water.

3. Stir and measure the initial temperature of the water.

4. Start the stop-watch and switch on the heater.

5. When the water is hot BUT NOT BOILING switch off the heater and STOP-WATCH.

Record the time of heating. Over the next minute or so note the maximum temperature reached by the water. Calculate the temperature rise of the water.

6. Switch on the heater again. When the water starts to boil (100×C), start the stop-watch.

7. After five minutes turn off the heater. Leave the water / heater to cool for awhile.

You can begin the calculations below.

8. Measure the mass of the heater and remaining water and so calculate the water converted to steam.

Calculation of specific latent heat:

1. Calculate the heat energy supplied by the heater during stages 4 & 5 from:

Heat energy supplied = Mass of water x SHC water x Temperature rise

Note:

2. Calculate the power of the heater from:

Power of heater = Heat energy supplied / Time of heating (stage 4 & 5)

3. Calculate the heat supplied by the heater while boiling the water from:

Heat supplied (stage 7) = Power of heater x Time of heating (stage 7)

4. Calculate the specific latent heat of vaporisation of water from:

Heat supplied (stage 7) = Mass of water converted to steam x Specific latent heat

5. The accepted value of the specific latent heat of vaporisation of water is 2.1 MJ / kg.

Discuss why your answer is different from this.