Physics 001.001.010 Specific Latent Heat

Alignment

Learning Intentions

By the end of the lesson, students will be able to:

• Describe specific latent heat as the energy required per kilogram to change the state of a substance without changing its temperature.
• Distinguish between energy used to change temperature and energy used to change state.
• Relate specific latent heat to changes in internal energy and particle arrangement.
• Identify examples of latent heat in everyday phenomena such as melting ice, boiling water, sweating and wet clothes feeling cold.

Success Criteria

By the end of the lesson, students have successfully:

• Defined specific latent heat using appropriate physics language.
• Stated that temperature remains constant during a phase change because average particle kinetic energy remains constant.
• Explained that energy added during a phase change increases internal potential energy by changing particle separation or arrangement.
• Identified as specific latent heat of fusion and as specific latent heat of vaporisation.
• Interpreted a heating curve to identify regions where latent heat is being transferred.
• Used the relationship qualitatively to describe how mass and latent heat affect the energy required for a phase change.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 1: Heating Processes
• Phase Changes and Energy Conservation
• Describe the concept of specific latent heat.
• Solve problems involving specific latent heat using .

Phenomenon

Place a small amount of crushed ice and water in a beaker. Heat it gently while measuring temperature every 30 seconds.

Students observe that:

• The ice-water mixture can continue absorbing energy while staying close to .
• Once the ice has melted, the temperature of the liquid water begins to rise.
• Later, when water boils, the temperature stays close to even though energy is still being transferred to the water.

Guiding question:

If energy is still being added, why does the temperature sometimes stay the same?

Key Idea

Specific latent heat describes how much energy is required to change the state of of a substance without changing its temperature.

Concept

The concept and thought that best describes the cause of the phenomenon is below.

During a phase change, energy added to a substance does not increase the average kinetic energy of the particles. Since temperature is a measure of average kinetic energy, the temperature remains constant.

Instead, the transferred energy changes the internal potential energy of the particles. It is used to change the arrangement or separation of particles.

For example:

• During melting, energy is used to weaken the fixed structure of a solid.
• During boiling, energy is used to separate particles enough to form a gas.
• During freezing or condensation, energy is released as particles form stronger interactions (to repel).

Specific latent heat is represented by:

or

where:

• is the energy transferred in joules,
• is the mass in kilograms,
• is the specific latent heat in joules per kilogram,

There are two common types:

• Specific latent heat of fusion, : energy required per kilogram to change between solid and liquid.
• Specific latent heat of vaporisation, : energy required per kilogram to change between liquid and gas.

For water:

This means it takes much more energy to boil water than to melt the same mass of ice.

Convention

The key conventions associated with the concept and in the branch of established knowledge is below.

• Use only when a substance is changing state and temperature is constant.
• Use when a substance changes temperature without changing state.
• A heating curve has sloped sections when temperature changes and flat sections when phase change occurs.
• Latent heat is called “latent” because the energy transfer is hidden from temperature change.
• Positive usually means energy is added to the system.
• Negative usually means energy is removed from the system.
• Melting and boiling require energy input.
• Freezing and condensation release energy to the surroundings.
• Specific latent heat is a property of a material and the type of phase change.

Misconceptions

Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.

• Students may think that if energy is added, temperature must always increase.
• Students may think particles stop moving during melting or boiling because the temperature is constant.
• Students may think boiling water gets hotter than under normal atmospheric pressure if more heat is added.
• Students may confuse specific heat capacity, , with specific latent heat, .
• Students may think latent heat disappears, rather than recognising it changes internal potential energy.
• Students may think evaporation and boiling are identical processes.
• Students may think condensation absorbs energy, when it actually releases energy to the surroundings.

Further Reading

• Heating curves and cooling curves
• Internal energy and the kinetic particle model
• Specific latent heat of fusion and vaporisation
• Evaporative cooling and sweating
• Thermal equilibrium and energy conservation

Explicit Instruction

1. Review internal energy:

   • Internal energy is the total kinetic and potential energy of particles in a system.
   • Temperature depends on average kinetic energy.
   • Particle arrangement and separation relate to potential energy.

2. Compare two energy transfer situations:

   Temperature change without phase change

   Energy increases particle kinetic energy.

   Temperature changes.

   State change without temperature change

   Energy changes particle arrangement or separation.

   Temperature remains constant.

3. Introduce specific latent heat:

   Specific latent heat is the energy required to change the state of of a substance without changing its temperature.

4. Link to particle model:

   In a solid, particles are closely packed and vibrate about fixed positions.

   In a liquid, particles are still close but can move past each other.

   In a gas, particles are much further apart and move freely.

   During melting or boiling, energy is used to overcome interactions between particles. This increases internal potential energy, not average kinetic energy.

5. Interpret a heating curve:

   On a temperature-time graph:

   • Sloped sections show temperature change.
   • Flat sections show phase change.
   • At the flat sections, energy is still being transferred.
   • The longer the flat section, the more energy is required for that phase change.

Worked Examples

Worked Example 1

Describe what happens when ice melts at .

Ice at absorbs energy from its surroundings. However, the temperature remains at while melting occurs.

The added energy is not increasing the average kinetic energy of the particles. Instead, it is increasing the internal potential energy of the system by weakening the fixed arrangement of particles in the solid.

Therefore, melting requires latent heat of fusion.

Key description:

During melting, energy is absorbed to change particle arrangement from solid to liquid while temperature remains constant.

Worked Example 2

Explain why steam burns can be more severe than boiling water burns at the same temperature.

Steam at contains energy associated with its gaseous state. When steam touches cooler skin, it condenses into liquid water.

During condensation, steam releases latent heat of vaporisation to the skin.

This means the skin receives energy from:

• the steam condensing into water
• the hot water cooling down afterwards

Therefore, steam can transfer more energy than liquid water at the same temperature.

Key description:

Condensation releases latent heat, so steam can transfer a large amount of energy even when its temperature is the same as boiling water.

Worked Example 3

Describe the difference between and .

describes energy transfer when temperature changes but state does not change.

For example, warming liquid water from to .

describes energy transfer when state changes but temperature does not change.

For example, melting ice at or boiling water at .

Key description:

Specific heat capacity describes energy needed for a temperature change. Specific latent heat describes energy needed for a state change.

Check for Understanding

Check 1

A beaker of ice and water is heated. The temperature stays at until all the ice melts.

Question: Where is the added energy going?

Expected answer:

The added energy is increasing the internal potential energy of the particles. It is changing the arrangement of particles from solid to liquid rather than increasing their average kinetic energy.

Check 2

A student says, “The temperature is not changing, so no energy is being transferred.”

Question: What is wrong with this statement?

Expected answer:

Energy can still be transferred even when temperature does not change. During a phase change, energy changes the internal potential energy of the particles instead of increasing their average kinetic energy.

Check 3

Which equation should be used for each situation?

a. Heating liquid water from to

b. Melting ice at

c. Boiling water at

Expected answers:

a.

b.

c.

Investigation (Alternative to Explicit)

Hypothesis

If energy is added to ice-water at its melting point, then the temperature will remain approximately constant until the phase change is complete because the energy is used to change particle arrangement rather than increase average kinetic energy.

Data Collection

Equipment:

• Crushed ice
• Beaker
• Hot plate or Bunsen burner with tripod and gauze
• Thermometer or temperature probe
• Stopwatch
• Stirring rod
• Safety glasses
• Heatproof mat

Method:

1. Place crushed ice and a small amount of water in a beaker.
2. Record the initial temperature.
3. Heat gently.
4. Record temperature every 30 seconds.
5. Stir carefully between readings.
6. Continue until all ice has melted and the water temperature begins to rise.
7. Plot temperature against time.

Safety:

• Wear safety glasses.
• Avoid touching hot glassware.
• Use tongs or heatproof gloves when handling heated equipment.
• Keep electrical temperature probes away from open flames if using a Bunsen burner.

Analysis

Students identify:

• The flat section of the graph near
• The point where all ice has melted
• The section where liquid water begins increasing in temperature
• Evidence that energy was transferred even when temperature was constant

Discussion questions:

1. Why did the temperature remain constant while ice was present?
2. What happened to the internal energy of the system during melting?
3. Why did the temperature begin increasing after all ice had melted?
4. How does this support the idea of specific latent heat?

Evaluation

Students evaluate:

• Heat loss to surroundings
• Uneven heating
• Thermometer response time
• Whether the ice was already at
• Precision of temperature and time measurements
• Whether stirring improved temperature consistency

Suggested improvements:

• Use a data logger for continuous temperature readings.
• Use an insulated container to reduce heat loss.
• Use crushed ice to make melting more uniform.
• Repeat the experiment and average the results.

Problems

The following problems are designed to develop conceptual understanding of specific latent heat before formal calculation practice.

1. Define specific latent heat in one sentence.

2. Explain why temperature remains constant during melting.

3. Explain why temperature remains constant during boiling.

4. Describe the particle-level difference between melting and boiling.

5. Why does wet clothing make you feel cold when water evaporates from it?

6. A student heats a pure substance and observes a flat section on the temperature-time graph. What does the flat section indicate?

7. State whether each process absorbs or releases latent heat:

   • Melting
   • Freezing
   • Boiling
   • Condensation

8. Why is not suitable for describing melting at constant temperature?

9. Why is the specific latent heat of vaporisation of water greater than the specific latent heat of fusion of water?

10. A heating curve shows a substance warming as a solid, then melting, then warming as a liquid. Label where applies and where applies.

11. Describe why sweating cools the body using the idea of latent heat.

12. A freezer removes energy from liquid water at . Explain what happens to the water and where the energy goes.

13. Compare specific heat capacity and specific latent heat using the following sentence starters:

• Specific heat capacity describes…
• Specific latent heat describes…

14. Explain why adding more heat to boiling water does not increase its temperature under constant pressure.

15. Describe how energy conservation applies during a phase change.

Followup

Self-check

Students should be able to answer these questions without notes:

• Can I define specific latent heat?
• Can I explain why temperature does not change during a phase change?
• Can I describe what happens to particle kinetic energy during a phase change?
• Can I describe what happens to internal potential energy during a phase change?
• Can I identify melting, freezing, boiling and condensation as latent heat processes?
• Can I explain why is used for phase change?
• Can I distinguish between and ?

Next Topic

Solve problems involving specific latent heat using .