Measuring the specific latent heat of fusion of water by a domestic electric boiler [back]

Key Learning Points

1. Energy and power
2. Specific latent heat of fusion
3. Heat loss

Teachers' notes: (zip 91k)

Typical results, suggested answers, students' worksheet.

Introduction

 You can measure the specific latent heat of fusion of water with this simple setup.

Do you remember that we used a domestic electric boiler to measure the specific heat of water in the last chapter? Now we continue to use it to measure the specific latent heat of fusion of water. In fact, this simple setup can be used to find the value of the latent heat to an accuracy higher than that of the traditional experiment done in a school laboratory.

As in the previous activity, most of the apparatuses used in this activity can be found in an ordinary family. What you need in extra is some ice and an electronic balance.

Notes to teachers: This activity involves handling a lot of hot water. Students should do the activity under careful supervision and be warned not to do it at home.

Appratus

3 L domestic electric boiler, 1-2 kg of ice, a thermometer, a glass container, a measuring cup that can hold at least 1.5 L of water, electronic balance, cloth, a stopwatch or digital watch for measuring time.

Procedure

 Put the ice into a large amount of water to prepare melting ice at 0 . The mass of ice used is measured by an electronic balance.
1. Break the ice into small pieces and put them into a large amount of tap water in the glass container. Wait until the all the ice is melting.
2. Measure about 2 L of tap water. Measure the temperature of the water.
3. Heat up some water with the electric boiler. Pour out all the water after boiling.
4. Pour about 1 L of tap water into the boiler immediately. Put the boiler on the electronic balance and record the reading of the balance.
5. Dry the melting ice with the cloth and put it into the boiler. Record the increase in reading of the balance.
6. Turn on the boiler and start timing with the stopwatch. Stop timing as soon as the boiler switches automatically from the boiling mode to the warming mode.
7. As a control experiment, redo the boiling process using tap water of mass equal to that of the water and ice mixture in step 5. You may control the mass of tap water accurately by noting the reading of the balance. Record the time required to boil the tap water.
8. Fill in the worksheet, follow the steps shown to calculate the specific latent heat of fusion of water.

Discussion

1. Has all the supplied electric energy been used to melt the ice and heat up the water? If not, where has the rest of the energy gone?
2. Why do we have to redo the experiment with the same mass of tap water? Why do we have to find the difference between the times recorded in step 6 and step 7? Denote the time difference by . What physical processes does the electric energy supplied in the time  correspond to?
3. Write down an equation relating the time difference , the power rating , the temperature of tap water , the specific heat of water c, the specific latent heat of fusion of water , and the mass of ice .
4. How is the value of the specific latent heat found in this experiment compared with the standard value? What are the possible sources of error?