University of Virginia
Physics Department

## Change of State

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.7

Objectives

Students will

• study the three states of a substance: solid, liquid and gas;
• measure the temperatures at which these states exist in water;
• understand the energy flow that results in changes of state.

Motivation for Learning

Visualizing Vaporization

Materials
• Water
• 2 600 mL beakers
• Heat resistant gloves or pot holders
• Safety goggles
• Hotplate
• Metal sheet

Procedure

1. Turn on the heat source and allow to heat up.
2. Put a small amount of water in one beaker
3. Heat the beaker on the hotplate. After a minute or two, when the water boils off, remove the beaker from the heat and show the class that it is empty.
4. Pick up the second beaker and add a small amount of water to it. Using a potholder, hold the metal sheet above the opening of the beaker.
5. Repeat step 3. After a few minutes, remove the beaker from the hotplate and lift the metal sheet from its opening. Show the metal sheet to the class (it should have droplets of water on its surface).
6. Discuss vaporization with the class. Ask them to explain the appearance of the droplets of water on the metal sheet.

Background Information

Matter can undergo a physical change from one state to another: solid, liquid or gas. When a material changes from a solid to a liquid, the process is called melting, and occurs at a temperature called the melting point, which is specific to the given substance. The liquid can change back into a solid by freezing; this same temperature is called the freezing point. When a liquid boils, it vaporizes and becomes a gas; at that temperature, called the boiling point, the gas can also become a liquid. This process is called either condensation (gas to liquid) or vaporization (liquid to gas). Under certain conditions, it is possible for a substance to change from a solid to a gas or vice versa in a process called sublimation.

The particles (atoms or molecules) of a material in each of these three states have different properties. In a solid, they are held together very tightly and have little room to move; in a liquid, the particles have more energy than in a solid and are able to move around more; in a gas, particles are not held together at all and are able to move around completely. They have much more energy and move more quickly than those in the other two states. When the temperature of a solid rises, energy is added and the kinetic energy of its individual particles increase. Eventually, the temperature reaches a point at which the added energy breaks forces between individual particles, allowing for more movement. This is the melting point, and the solid becomes a liquid. The amount of energy necessary to induce this change is called the heat of fusion. A similar process occurs when a liquid becomes a gas, and the required energy is called the heat of vaporization.

In this experiment, the students will study these three phases of water by melting ice and then boiling water. They should record the temperature of the liquid over a period of time and plot this data in order to visualize the two changes of state. At each point, the temperature should stabilize briefly as the heat energy is absorbed and a portion of the graph will be horizontal. From this plot, the students should be able to determine the freezing, melting and boiling points of water.

### Student Activity

Materials

• Glass beaker, 600 mL
• 5-10 ice cubes
• 200 mL water
• Thermometer
• Hot plate
• Clock or wristwatch timer
• Graph paper
• Small clamp

Procedure

1. Fill a 600 mL beaker about half full with ice.
2. Pour 200 mL water in the beaker along with 5-8 ice cubes. Wait several minutes, and measure the temperature of the mixture.
3. Turn on the hot plate to medium. Place the beaker with the ice and water on the hot plate and warm it slowly.
4. Every minute read the temperature of the mixture and record it. Use the small clamp to hold the thermometer off the bottom of the beaker.
5. Watch the mixture and record the time and temperature. Describe the mixture and note when all the ice has melted.
6. Continue to heat the water and record its temperature.
7. Note the time and temperature at which the water boils. Continue to record the temperature each minute for five minutes after it begins boiling. Stop before all of the water has boiled. Do not let the thermometer rest on the bottom of the beaker.
8. Plot your results on a sheet of graph paper. Label the x-axis "Time" and the y-axis "Temperature." Label what happened at various points on the graph.

Data Sheet

 Temperature of Water Description of Mixture Time (minutes) Temperature of Water Description of Mixture Time (minutes)

1. Using your graph, describe how the temperature of the water changed over time.

2. Explain any horizontal portions of your plot. At what temperature did they occur?

3. At what temperature did the ice completely melt? At what temperature did the water begin to boil?

4. What was the maximum temperature reached by the water? Explain this limit.

5. At what temperature did the added energy contribute to the heat of fusion? The heat of vaporization?

Students with Special Needs

All students should be able to participate in this acitivity.

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

Assessment

Data sheet to be completed during the laboratory.