Investigating Heating and Cooling Curves of Water

Aim

To investigate how the temperature of water changes over time as crushed ice is heated until the water boils, and how the temperature of hot water changes over time as it cools in ice.

Learning Outcome

• Measure temperature at regular time intervals.
• Record results in a table.
• Identify the phase of water during heating.
• Plot a graph of temperature against time.
• Explain why temperature remains constant during a change of state.
• Describe the difference between heating within one state and changing from one state to another.

Time Allocation

Approximately 45 minutes.

How Does It Work?

Water exists in three physical states:

• solid,
• liquid,
• gas.

When heat is added to ice, the particles gain energy and move faster.

At the melting point, the temperature remains constant while the ice changes from a solid into a liquid.

Once all the ice has melted, the temperature of the liquid water rises steadily.

At boiling point, the temperature remains constant again while the liquid changes into gas.

During cooling, heat energy is removed from the water and the temperature decreases steadily.

Scientific Background

This investigation demonstrates important concepts in Physical Sciences:

• changes of state,
• latent heat,
• heating curves,
• cooling curves,
• and temperature behaviour during physical changes.

When a substance changes state, the added heat energy is used to break the forces between particles rather than increase temperature.

This is why the temperature remains constant during melting and boiling.

Once the change of state is complete, the temperature begins changing again.

During cooling, particles lose kinetic energy and move more slowly, causing the temperature to decrease.

Hypothesis

If ice is heated, then the temperature will increase over time except during melting and boiling where the temperature will remain constant.

If hot water is cooled, then the temperature will decrease steadily over time.

Variables

Independent Variable

Time, measured at one-minute intervals.

Dependent Variables

• Temperature of the water.
• Phase of the water during heating.

Control Variables

• Constant flame from the Bunsen burner.
• Same amount of water used.
• Same apparatus arrangement.
• Same time interval between readings.

Apparatus Required

• Crushed ice
• Hot water
• 2 × glass beakers
• Thermometer
• Bunsen burner
• Tripod stand
• Gauze mat
• Retort stand
• Clamp
• Rubber stopper with hole
• Stopwatch or timer
• Graph paper
• Pencil and ruler

Safety Precautions

Method

Part A: Heating Curve

1. Fill a glass beaker with crushed ice.
2. Insert the thermometer into the crushed ice.
3. Clamp the thermometer in position using the retort stand and clamp.
4. Ensure the thermometer bulb is surrounded by ice and not touching the bottom of the beaker.
5. Record the starting temperature at time zero.
6. Place the beaker on a gauze mat above the tripod stand.
7. Position the Bunsen burner underneath the tripod stand.
8. Light the burner and keep the flame constant throughout the experiment.
9. Record the temperature every minute.
10. At each reading, record the phase of the water.
11. Continue heating until the water boils.
12. Record all results neatly in a table.

Part B: Cooling Curve

1. Fill a second container with crushed ice.
2. Place the beaker of hot water into the crushed ice.
3. Insert the thermometer into the hot water.
4. Support the thermometer using the retort stand and clamp.
5. Record the starting temperature.
6. Record the temperature every minute.
7. Continue until sufficient readings have been collected for the cooling curve.
8. Record the results in a second table.

Results Table

Heating Curve of Water

Time (min)	Temperature (°C)	Phase of Water
0
1
2
3
4
5
6
7
8
9
10

Cooling Curve of Water

Time (min)	Temperature (°C)
0
1
2
3
4
5
6
7
8
9
10

Instructions for Graphing

1. Draw a graph of temperature against time for the heating investigation.
2. Draw a second graph for the cooling investigation.
3. Label the horizontal axis as Time (min).
4. Label the vertical axis as Temperature (°C).
5. Give each graph a suitable heading.
6. Plot the points carefully and join them with a smooth line.

Expected Observations

• The crushed ice starts at approximately 0 °C.
• The temperature remains almost constant while the ice melts.
• Once all the ice has melted, the temperature rises steadily.
• The temperature remains constant again when the water boils.
• During cooling, the temperature decreases steadily.

Questions for Learners

1. What was the temperature of the crushed ice at the start of the investigation?
2. Why did the temperature remain constant while the ice was melting?
3. What happened to the temperature after all the ice had melted?
4. Why did the temperature remain constant again when the water boiled?
5. Which variable was changed during the investigation?
6. Which variable was measured?
7. Why was it important to keep the flame constant?
8. Why must the thermometer bulb not touch the bottom of the beaker?
9. What shape did the cooling curve have?
10. What does the cooling curve show about heat loss?

Teacher Notes

• This practical works well as a teacher demonstration if apparatus is limited.
• If enough apparatus is available, learners can work in groups.
• Prepare crushed ice before the lesson starts to save time.
• Have graph paper ready before the experiment begins.
• Discuss melting, boiling, and changes of state before starting the investigation.

Common Mistakes

• Thermometer bulb touching the bottom of the beaker.
• Flame intensity changing during the experiment.
• Missing temperature readings.
• Incorrect graph plotting.
• Confusing melting with heating.

Teacher Tip

Extension Activity

Ask learners to compare the heating and cooling curves and explain why one graph rises while the other falls.

Learners can also identify:

• where melting occurs,
• where boiling occurs,
• and where temperature changes within a single state.

Real-World Application

Heating and cooling curves are important in:

• food processing,
• industrial heating systems,
• refrigeration,
• weather studies,
• power stations,
• and chemical manufacturing.

Understanding heating and cooling helps explain how substances change state and how heat energy affects matter.