University of Virginia
Physics Department

## Heat of Fusion

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.7

Objectives

Students will

• understand the phase change that occurs during the heating and cooling of water;
• measure the amount of heat absorbed by ice as it melts.

Motivation for Learning

Discrepant Event-Molecules and Temperature

Materials
• 2 600 mL pyrex beakers
• 600 mL water
• 1 container food coloring
• Hot plate or electric tea kettle.

Procedure

1. Heat 300 mL of water in a pyrex beaker on the hot plate. Pour the other 300 mL (at room temperature) into the other beaker.
2. Place the two containers in an area that can be easily seen by all of the students.
3. Add a few drops of food coloring to both containers. The coloring should allow the students to see the molecular movement in both temperatures of water. The molecules of the hot water should move much faster than those of the water at room temperature. The food coloring should disperse much more quickly in the hot water. This is what will occur in the student activity as well.

Background Information

Most matter can exist in three different states: solid, liquid or gas. In the solid state, particles are held together tightly and don't have much freedom to move around. As a liquid, particles are more loosely held together and move around more, therefore having more energy than those in a solid state. As matter changes from one phase (solid, liquid or gas) to another, its amount of internal energy also changes. For example, when ice melts, it changes from a solid to a liquid. Heat must be added for this to occur, because a liquid has more internal energy than a solid.

As ice (below 0° C) is heated, energy is added to the water molecules and they begin to move (vibrate) faster, causing the temperature to rise. The temperature will continue to increase until it reaches 0° C, at which point it stops rising. This is called the critical point-as more heat is added, the temperature will remain at 0° C but the internal energy of the molecules will increase. Eventually, they will break apart from the solid form and become a liquid. This temperature is called the melting point and is 0° C for water. Matter exists as both a solid and a liquid at this temperature.

The amount of heat energy that is necessary to convert matter from a solid to a liquid is called its heat of fusion, and is different for varying substances. This can be calculated by the following equation:

Q = mLf

where m represents the mass of the solid and Lf is the latent heat of fusion for the given substance. For water, Lf = 334 J/g.

In this activity, the students will experimentally determine the latent heat of fusion for water by measuring the temperature of a water-ice mixture as the ice melts.

### Student Activity

Materials

• 2 Styrofoam cups (large)
• Balance for mass measurement
• 50 g ice
• Thermometer
• Stirring rod
• 200 mL water
• Hot plate

Procedure

1. Heat 200 mL of water on a hot plate.
2. Mass an empty styrofoam cup. Record this measurement.
3. Keep the cup on the balance, and add ice until 50 g have been added to the empty cup.
4. Add the hot water to the second cup. Mass the cup with the water and record.
5. Measure and record the temperature of the hot water using the thermometer.
6. Add the ice to the hot water. Stir the ice until it has completely melted. DO NOT USE THE THERMOMETER TO STIR THE MIXTURE.
7. Measure and record the temperature of the water again.
8. Mass the cup containing the water and melted ice. Record this measurement.

Data Sheet

 Mass of empty foam cup (g) Mass of cup and ice (g) Mass of cup and hot water (g) Mass of water (g) Initial temperature of hot water (°C) Final temperature of water (°C) Final mass of cup and water (g)
1. Calculate the amount of heat lost by the hot water after the ice was added. Use the equation Q = mcDT where c is the specific heat capacity of water and is 4.184 J/g°C.

2. Calculate the amount of heat gained by the ice as it melted and then as the water increased to its final temperature.

3. Determine the amount of heat absorbed by the ice during the process. Use the answers to questions 1 and 2 to do this.

4. Calculate the experimental heat of fusion. Use the equation Q = mLf.

5. How does your experimental value compare to the theoretical Lf value for water? Explain any variation.

6. Write a general equation to determine the total heat necessary to melt ice into water.

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.