Melting and boiling

Key Idea: Energy Transfer Without Temperature Change

During changes of state, energy is transferred without changing the temperature of the substance.

This energy does not increase particle speed. Instead, it is used to:

• break intermolecular forces (during melting and boiling), or

• form intermolecular forces (during solidification and condensation).

This energy is often called latent energy (covered in detail later).

Melting (Solid → Liquid)

What Happens During Melting

Melting occurs when:

• a solid is heated to its melting point,

• the temperature remains constant while melting takes place.

During melting:

• energy is absorbed by the substance,

• particles gain potential energy, not kinetic energy,

• intermolecular forces holding particles in fixed positions are weakened.

Particles:

• break free from fixed positions,

• begin to slide past one another,

• form a liquid.

[Insert diagram showing solid particles becoming less ordered as melting occurs, with temperature remaining constant]

Why Temperature Does Not Change During Melting

• Temperature measures average kinetic energy.

• During melting, particle speed does not increase.

• Energy is used to separate particles, not speed them up.

Therefore, temperature stays constant until melting is complete.

Solidification (Liquid → Solid)

What Happens During Solidification

Solidification (freezing) occurs when:

• a liquid cools to its freezing point,

• the temperature remains constant during freezing.

During solidification:

• energy is released to the surroundings,

• particles lose potential energy,

• intermolecular forces form and strengthen.

Particles:

• become more ordered,

• settle into fixed positions,

• form a solid.

[Insert diagram showing liquid particles becoming more ordered during solidification at constant temperature]

Boiling (Liquid → Gas)

What Happens During Boiling

Boiling occurs when:

• a liquid reaches its boiling point,

• bubbles of vapour form throughout the liquid,

• temperature remains constant during boiling.

During boiling:

• energy is absorbed,

• particles gain potential energy,

• intermolecular forces are overcome completely.

Particles:

• separate widely,

• move freely as a gas.

[Insert diagram showing bubbles forming throughout a liquid during boiling at constant temperature]

Condensation (Gas → Liquid)

What Happens During Condensation

Condensation occurs when:

• a gas cools to its condensation point,

• temperature remains constant during condensation.

During condensation:

• energy is released to the surroundings,

• particles lose potential energy,

• intermolecular forces re-form.

Particles:

• come closer together,

• form a liquid.

[Insert diagram showing gas particles coming closer together during condensation at constant temperature]

Summary of Energy Changes (Exam-Critical)

Key Exam-Ready Statements

• During a change of state, temperature remains constant.

• Energy is used to break or form intermolecular forces.

• Kinetic energy remains constant during phase change.

• Potential energy changes during melting and boiling.

Questions

Question 1

Explain why the temperature of a substance remains constant during melting.

Question 2

Describe what happens to energy during boiling without a change in temperature.

Question 3

Compare melting and condensation in terms of energy transfer.

Solutions

Solution 1

During melting, energy is absorbed by the substance.

This energy is used to overcome intermolecular forces rather than increase particle speed.

Since kinetic energy does not increase, the temperature remains constant.

Solution 2

During boiling, energy is absorbed to completely overcome intermolecular forces.

Particles separate to form a gas, but their average kinetic energy does not change.

Therefore, temperature remains constant during boiling.

Solution 3

During melting, energy is absorbed to weaken intermolecular forces.

During condensation, energy is released as intermolecular forces form.

In both processes, temperature remains constant while energy is transferred.

Examiner-Level Guidance

• Always state “temperature remains constant” explicitly.

• Mention intermolecular forces for full marks.

• Do not say energy is used to “increase heat”.

• Link phase change to potential energy, not kinetic energy.

What Is a “Point” in Thermal Physics?

In thermal physics, a point refers to a specific temperature at which a physical change occurs under fixed conditions, usually at normal atmospheric pressure.

Melting point and boiling point are therefore definite temperatures, not ranges.

Melting Point

Meaning of Melting Point

The melting point of a substance is:

the constant temperature at which a solid changes into a liquid, at normal atmospheric pressure.

At the melting point:

• solid and liquid exist together in equilibrium,

• energy supplied does not raise the temperature,

• energy is used to overcome intermolecular forces.

Important Clarifications

• Each pure substance has a specific melting point.

• Impurities usually lower and broaden the melting point.

• The melting point is the same as the freezing point for a pure substance.

Example:

• Pure ice melts at 0 °C at normal atmospheric pressure.

[Insert diagram showing solid and liquid coexisting at melting point with constant temperature]

Boiling Point

Meaning of Boiling Point

The boiling point of a substance is:

the constant temperature at which a liquid changes into a gas throughout the liquid, at normal atmospheric pressure.

At the boiling point:

• vapour bubbles form throughout the liquid,

• liquid and gas coexist,

• energy supplied does not increase temperature,

• energy is used to completely overcome intermolecular forces.

Important Clarifications

• Boiling occurs throughout the liquid, not just at the surface.

• Boiling point depends on pressure:
  • lower pressure → lower boiling point,
  • higher pressure → higher boiling point.

• At BGCSE level, boiling point is assumed at normal atmospheric pressure unless stated otherwise.

Example:

• Pure water boils at 100 °C at normal atmospheric pressure.

[Insert diagram showing bubbles forming throughout a liquid at boiling point]

Comparison of Melting Point and Boiling Point

Key Exam-Ready Statements

• The melting point is the temperature at which a solid turns into a liquid.

• The boiling point is the temperature at which a liquid turns into a gas throughout the liquid.

• During melting and boiling, temperature remains constant.

• Both points are defined at normal atmospheric pressure.

Questions

Question 1

State the meaning of melting point.

Question 2

State the meaning of boiling point.

Question 3

Explain why the temperature remains constant at the melting point of a pure substance.

Solutions

Solution 1

The melting point of a substance is the constant temperature at which it changes from a solid to a liquid at normal atmospheric pressure.

Solution 2

The boiling point of a substance is the constant temperature at which it changes from a liquid to a gas throughout the liquid at normal atmospheric pressure.

Solution 3

At the melting point, energy supplied is used to overcome intermolecular forces between particles.

This energy does not increase particle kinetic energy, so the temperature remains constant.

Examiner-Level Guidance

• Definitions must include state change and constant temperature.

• Always assume normal atmospheric pressure unless stated otherwise.

• Do not confuse boiling with evaporation.

• Mentioning intermolecular forces strengthens explanations.

Overview

Both boiling and evaporation are processes by which a liquid changes into a gas, but they occur under very different physical conditions and involve different molecular behaviour.

Understanding the differences is essential, as these are commonly tested using comparison questions.

Boiling

Boiling is a rapid process that occurs when a liquid reaches its boiling point.

Key characteristics:

• Occurs at a fixed temperature (the boiling point).

• Takes place throughout the liquid, not just at the surface.

• Bubbles of vapour form inside the liquid and rise to the surface.

• Requires a continuous input of energy.

• Temperature remains constant during boiling.

Molecular explanation:

• Particles gain enough energy to completely overcome intermolecular forces.

• Vapour pressure inside the liquid equals external pressure.

[Insert diagram showing bubbles forming throughout a liquid during boiling]

Evaporation

Evaporation is a slow process that can occur at any temperature below the boiling point.

Key characteristics:

• Occurs at all temperatures.

• Takes place only at the surface of the liquid.

• No bubbles form within the liquid.

• Involves only the most energetic molecules escaping.

• Causes cooling of the remaining liquid.

Molecular explanation:

• High-energy molecules escape from the surface.

• Average kinetic energy of remaining molecules decreases.

[Insert diagram showing fast molecules escaping from the surface of a liquid during evaporation]

Direct Comparison (Exam-Critical)

Key Exam-Ready Statements

• Boiling occurs at a fixed temperature; evaporation can occur at any temperature.

• Boiling happens throughout the liquid; evaporation occurs only at the surface.

• Evaporation causes cooling; boiling does not.

• Boiling produces bubbles; evaporation does not.

Questions

Question 1

State two differences between boiling and evaporation.

Question 2

Explain why evaporation can occur at temperatures below the boiling point.

Question 3

A liquid cools as it evaporates but not when it boils.

Explain this difference.

Solutions

Solution 1

Two differences are:

• boiling occurs at a fixed temperature while evaporation can occur at any temperature,

• boiling occurs throughout the liquid while evaporation occurs only at the surface.

Solution 2

Evaporation occurs because some molecules in the liquid have higher kinetic energy than others.

These high-energy molecules can escape from the surface even when the liquid is below its boiling point.

Solution 3

During evaporation, high-energy molecules escape from the surface, reducing the average kinetic energy of the remaining liquid and causing cooling.

During boiling, energy supplied is used to change state without reducing the average kinetic energy of the liquid, so no cooling occurs.

Examiner-Level Guidance

• Comparison questions require paired contrasts, not isolated facts.

• Always mention temperature condition and location.

• Avoid saying evaporation is “weak boiling” — this is incorrect.

• Cooling must be linked to loss of high-energy molecules.

What Is a Cooling Curve?

A cooling curve is a graph that shows how the temperature of a substance changes with time as it loses thermal energy.

Typically:

• the vertical axis represents temperature,

• the horizontal axis represents time.

Cooling curves are used to:

• identify melting/freezing points,

• identify boiling/condensation points,

• understand energy changes during phase changes.

How to Sketch a Cooling Curve (Exam-Critical)

Axes and Shape

1. Draw a set of axes:
   • Vertical axis: Temperature (°C)
   • Horizontal axis: Time

2. The curve consists of:
   • sloping sections (temperature changing), and
   • horizontal sections (temperature constant during change of state).

[Insert fully labelled cooling curve showing gas → liquid → solid]

Interpreting the Cooling Curve (Section by Section)

Consider a pure substance cooling from a high temperature.

Section A–B: Cooling of a Gas

• The substance is in the gaseous state.

• Temperature decreases steadily with time.

• Particles lose kinetic energy and move more slowly.

• No change of state occurs.

Interpretation:

Energy is removed and temperature falls.

Section B–C: Condensation (Gas → Liquid)

• Temperature remains constant.

• The substance changes from gas to liquid.

• Energy is released to the surroundings.

• Intermolecular forces are formed.

Interpretation:

Energy removal causes a change of state, not a temperature change.

Section C–D: Cooling of a Liquid

• The substance is now a liquid.

• Temperature decreases steadily again.

• Particle motion slows further.

Interpretation:

Energy removal lowers kinetic energy.

Section D–E: Solidification / Freezing (Liquid → Solid)

• Temperature remains constant.

• The substance changes from liquid to solid.

• Energy is released as particles become fixed in position.

Interpretation:

Energy is released while the state changes.

Section E–F: Cooling of a Solid

• The substance is fully solid.

• Temperature continues to fall.

• Particles vibrate less about fixed positions.

Interpretation:

Further energy removal lowers temperature.

Key Features to Identify on a Cooling Curve

• Horizontal sections indicate changes of state.

• Sloping sections indicate temperature change.

• The length of a horizontal section indicates the amount of energy involved in the change of state.

• Cooling curves apply to pure substances with sharp melting and boiling points.

Common Examination Interpretations

Students may be asked to:

• label states of matter on the curve,

• identify the freezing point,

• identify the condensation point,

• explain why temperature remains constant during flat sections.

Key Exam-Ready Statements

• A cooling curve shows temperature against time.

• Flat sections represent phase changes.

• During phase changes, temperature remains constant.

• Energy is released during condensation and solidification.

Questions

Question 1

What information is shown by a cooling curve?

Question 2

Explain why a cooling curve has horizontal sections.

Question 3

A cooling curve shows a flat section at 0 °C.

Identify the change of state and explain what happens to energy during this section.

Solutions

Solution 1

A cooling curve shows how the temperature of a substance changes with time as it loses thermal energy.

Solution 2

Horizontal sections occur because energy is being released during a change of state.

This energy is used to form intermolecular forces rather than reduce particle kinetic energy, so temperature remains constant.

Solution 3

The flat section at 0 °C represents solidification (freezing).

During this process, energy is released to the surroundings while the substance changes from liquid to solid, and the temperature remains constant.

Examiner-Level Guidance

• Always label axes and states when sketching.

• Flat lines must be horizontal, not slanted.

• Use correct terms: condensation, solidification, not “cooling only”.

• Mention energy release explicitly for full explanation marks.

What Is Unusual About Water?

Most substances:

• contract when cooled, and

• expand when heated.

Water behaves differently over a small temperature range.

The Unusual Expansion of Water

When water is cooled from above 4 °C to 0 °C:

• it expands instead of contracting,

• its density decreases,

• it reaches maximum density at 4 °C, not at 0 °C.

This behaviour is called the anomalous expansion of water.

[Insert diagram showing volume of water decreasing down to 4 °C, then increasing from 4 °C to 0 °C]

Particle Explanation (Kinetic Model)

• At temperatures above 4 °C:
  • water molecules move freely,
  • cooling reduces motion,
  • molecules come closer together.

• Between 4 °C and 0 °C:
  • water molecules arrange into a more open structure,
  • hydrogen bonding creates extra spacing,
  • this causes expansion on cooling.

When water freezes at 0 °C:

• molecules form a fixed open lattice,

• volume increases further,

• solid ice is less dense than liquid water.

[Insert diagram comparing particle arrangement in liquid water at 4 °C and ice at 0 °C]

Key Observations (Exam-Critical)

• Water has maximum density at 4 °C.

• Ice is less dense than water.

• Ice therefore floats on water.

• Expansion occurs before freezing, not only after.

Consequences of the Unusual Expansion of Water

1. Ice Floats on Water (Environmental Importance)

Because ice is less dense:

• it floats on the surface of lakes and rivers,

• a layer of ice forms on top.

This ice layer:

• acts as an insulator,

• prevents the water below from freezing solid,

• allows aquatic life to survive in cold climates.

[Insert diagram of a frozen lake with ice on top and liquid water beneath]

2. Freezing of Lakes from the Top Down

As water cools:

• water at 4 °C sinks (most dense),

• colder water (below 4 °C) stays at the surface,

• freezing starts at the surface, not the bottom.

This is vital for:

• fish survival,

• maintaining ecosystems in winter.

3. Bursting of Water Pipes and Containers

When water freezes:

• it expands,

• large pressure is exerted on container walls.

Consequences:

• water pipes may burst in cold weather,

• sealed containers may crack.

This explains why:

• pipes are insulated,

• water systems are drained in freezing conditions.

4. Weathering of Rocks (Freeze–Thaw Action)

Water enters cracks in rocks.

• When it freezes, it expands.

• The crack widens.

• Repeated freezing and thawing breaks rocks apart.

This contributes to:

• soil formation,

• landscape weathering.

Summary Table: Behaviour and Consequences

Key Exam-Ready Statements

• Water has maximum density at 4 °C.

• Below 4 °C, water expands when cooled.

• Ice floats because it is less dense than liquid water.

• This unusual behaviour has important environmental and practical effects.

Questions

Question 1

State what is meant by the unusual expansion of water.

Question 2

Explain why ice floats on water.

Question 3

Describe two consequences of the unusual expansion of water.

Solutions

Solution 1

The unusual expansion of water is the increase in volume when water is cooled from 4 °C to 0 °C instead of contracting.

Solution 2

Water reaches maximum density at 4 °C.

Below this temperature, water expands and becomes less dense.

Ice is therefore less dense than liquid water and floats.

Solution 3

One consequence is that ice floats on lakes, insulating the water below and allowing aquatic life to survive.

Another consequence is that water pipes can burst when water freezes due to expansion.

Examiner-Level Guidance

• Always mention 4 °C explicitly.

• Use the term “maximum density” correctly.

• Consequences must be explained, not just listed.

• Environmental examples score higher than vague statements.

Why Latent Heat Is Needed

During melting and boiling, energy is supplied to a substance but its temperature does not change.

This energy is not used to increase particle speed. Instead, it is used to change the arrangement of particles.

This energy is called latent heat.

1. Latent Heat

Meaning of Latent Heat

Latent heat is:

the thermal energy absorbed or released by a substance during a change of state without any change in temperature.

Key features:

• occurs only during changes of state (melting, boiling, freezing, condensation),

• temperature remains constant,

• energy changes the potential energy of particles, not their kinetic energy.

Examples of Latent Heat

• Ice melting at 0 °C absorbs energy but remains at 0 °C.

• Water boiling at 100 °C absorbs energy but remains at 100 °C.

• Steam condensing releases energy at constant temperature.

[Insert diagram showing flat sections on a heating/cooling curve labelled “latent heat”]

2. Specific Latent Heat

Meaning of Specific Latent Heat

Specific latent heat is:

the amount of thermal energy required to change the state of 1 kg of a substance without changing its temperature.

Key points:

• it is a property of the substance,

• it does not depend on mass,

• different substances have different specific latent heats.

Types of Specific Latent Heat

There are two main types:

(a) Specific Latent Heat of Fusion

• Energy required to change 1 kg of a solid into a liquid at its melting point.

(b) Specific Latent Heat of Vaporisation

• Energy required to change 1 kg of a liquid into a gas at its boiling point.

Particle Interpretation

• During fusion:
  • energy weakens intermolecular forces,
  • particles gain freedom to move past one another.

• During vaporisation:
  • energy completely overcomes intermolecular forces,
  • particles separate widely to form a gas.

This explains why:

• specific latent heat of vaporisation is much larger than that of fusion.

[Insert diagram comparing particle spacing in solid, liquid, and gas during latent heat processes]

Key Differences Between Latent Heat and Specific Latent Heat

Key Exam-Ready Statements

• Latent heat is energy transferred without temperature change.

• Specific latent heat is energy per kilogram for a change of state.

• Latent heat changes potential energy, not kinetic energy.

• Vaporisation requires more energy than fusion.

Questions

Question 1

Define latent heat.

Question 2

Define specific latent heat.

Question 3

Explain why temperature remains constant during a change of state even though energy is supplied.

Question 4

State one reason why the specific latent heat of vaporisation is greater than that of fusion.

Solutions

Solution 1

Latent heat is the thermal energy absorbed or released during a change of state without a change in temperature.

Solution 2

Specific latent heat is the amount of thermal energy required to change the state of 1 kg of a substance without changing its temperature.

Solution 3

During a change of state, energy is used to overcome or form intermolecular forces.

This energy does not increase the kinetic energy of particles, so temperature remains constant.

Solution 4

Vaporisation requires particles to completely overcome intermolecular forces and separate widely, which needs more energy than melting.

Examiner-Level Guidance

• Always include “no change in temperature” in definitions.

• Mention 1 kg explicitly for specific latent heat.

• Avoid confusing latent heat with heat capacity.

• Use fusion and vaporisation correctly.

Correct Use of the Term Latent Heat

Latent heat refers to the energy transferred to or from a substance during a change of state without any change in temperature.

The word latent means hidden, because:

• the energy transfer is not shown by a temperature change,

• the substance may appear unchanged even though energy is absorbed or released.

Latent heat is involved in:

• melting and solidification,

• boiling and condensation.

Molecular Interpretation of Latent Heat

Key Molecular Idea

During a change of state:

• particle speed does not change,

• therefore average kinetic energy remains constant,

• the supplied or released energy changes the potential energy of particles.

This is the molecular meaning of latent heat.

1. Latent Heat During Melting (Solid → Liquid)

Molecular Interpretation

• In a solid, particles vibrate about fixed positions.

• When the solid reaches its melting point, further energy does not increase vibration speed.

• The absorbed latent heat is used to:
  • weaken intermolecular forces,
  • increase the separation between particles.

As a result:

• particles gain freedom to move past one another,

• the solid becomes a liquid,

• temperature remains constant.

[Insert diagram showing solid particles becoming less ordered during melting while temperature stays constant]

2. Latent Heat During Boiling (Liquid → Gas)

Molecular Interpretation

• In a liquid, particles are close together but can move past one another.

• At the boiling point, particles already have enough kinetic energy to move rapidly.

• The absorbed latent heat is used to:
  • completely overcome intermolecular forces,
  • separate particles widely.

As a result:

• particles escape to form a gas,

• temperature remains constant throughout boiling.

[Insert diagram showing liquid particles separating widely to form gas during boiling]

3. Latent Heat During Cooling Processes

Solidification and Condensation

During solidification and condensation:

• latent heat is released,

• particles lose potential energy,

• intermolecular forces form or strengthen,

• temperature remains constant.

This explains why:

• condensation releases heat to surroundings,

• freezing releases energy even though temperature does not rise.

Summary: Molecular View of Latent Heat

Key Exam-Ready Statements

• Latent heat is energy transferred without temperature change.

• During latent heat processes, kinetic energy remains constant.

• Latent heat changes potential energy of particles.

• It is used to break or form intermolecular forces.

Questions

Question 1

What is meant by latent heat?

Question 2

Explain, in terms of molecular motion, why temperature does not change during melting.

Solution 1

Question 3

Describe how latent heat is involved when a liquid boils.

Solutions

Latent heat is the energy absorbed or released during a change of state without any change in temperature.

Solution 2

At the melting point, particles already have fixed kinetic energy.

The absorbed latent heat is used to weaken intermolecular forces and increase particle separation.

Since particle speed does not increase, temperature remains constant.

Solution 3

During boiling, latent heat is absorbed by the liquid.

This energy is used to completely overcome intermolecular forces between particles.

Particles separate to form a gas while temperature remains constant.

Examiner-Level Guidance

• Always link latent heat → potential energy, not kinetic energy.

• Use the phrase “no change in temperature” explicitly.

• Avoid saying particles “stop moving” — this is incorrect.

• Molecular explanations must mention intermolecular forces.

Core Idea: How Refrigeration Works

A refrigerator does not create cold.

Instead, it removes heat from the inside and releases it to the surroundings.

This heat removal is made possible mainly by the latent heat of vaporisation of a working fluid called a refrigerant.

Role of Latent Heat in Refrigeration

Why Latent Heat Is Essential

Latent heat allows a substance to:

• absorb large amounts of energy,

• without changing temperature,

• during a change of state.

This makes latent heat extremely effective for transferring heat.

The Refrigeration Cycle (Latent Heat in Action)

A refrigerator operates through a continuous cycle involving evaporation and condensation of a refrigerant.

[Insert labelled diagram of a refrigerator showing evaporator, compressor, condenser, and expansion valve]

1. Evaporation Inside the Refrigerator (Cooling Stage)

• The refrigerant enters the evaporator as a liquid.

• Inside the refrigerator:
  • the refrigerant evaporates (liquid → gas),
  • it absorbs latent heat of vaporisation from the food and air.

• This energy absorption:
  • removes heat from the interior,
  • causes the inside of the refrigerator to cool.

Key latent heat idea:

The refrigerant absorbs heat without increasing in temperature.

2. Compression of the Gas (Energy Transfer)

• The gaseous refrigerant is compressed.

• Its pressure and temperature increase.

• No latent heat effect occurs here (no change of state).

3. Condensation Outside the Refrigerator (Heat Release)

• The hot, high-pressure gas enters the condenser at the back.

• It condenses (gas → liquid).

• During condensation:
  • latent heat is released to the surroundings,
  • this is why the back of a refrigerator feels warm.

Key latent heat idea:

Heat absorbed during evaporation is now released during condensation.

4. Cycle Repeats

• The liquid refrigerant returns to the evaporator.

• The process repeats continuously.

Molecular Interpretation (Exam-Critical)

During Evaporation (Inside the Fridge)

• Refrigerant molecules absorb latent heat.

• Intermolecular forces are overcome.

• Molecules separate to form a gas.

• Heat is taken from the fridge interior.

During Condensation (Outside the Fridge)

• Refrigerant molecules release latent heat.

• Intermolecular forces reform.

• Molecules come closer together.

• Heat is released to the surroundings.

Why Refrigeration Uses Latent Heat (Advantages)

• Large amounts of heat can be transferred efficiently.

• Temperature inside the fridge remains stable.

• Cooling is continuous and controlled.

• More effective than cooling by temperature rise alone.

Key Exam-Ready Statements

• Refrigeration works by removing heat, not creating cold.

• Latent heat of vaporisation allows the refrigerant to absorb heat.

• Evaporation causes cooling inside the refrigerator.

• Condensation releases heat outside the refrigerator.

• Temperature remains constant during latent heat processes.

Questions

Question 1

Explain how latent heat is involved in the cooling effect inside a refrigerator.

Question 2

Why does the back of a refrigerator feel warm during operation?

Question 3

State two reasons why latent heat is useful in refrigeration.

Solutions

Solution 1

Inside the refrigerator, the refrigerant evaporates.

During evaporation, it absorbs latent heat of vaporisation from the food and air.

This heat removal lowers the internal temperature, causing cooling without a change in the refrigerant’s temperature.

Solution 2

At the back of the refrigerator, the refrigerant condenses.

During condensation, latent heat is released to the surroundings.

This released heat makes the back of the refrigerator feel warm.

Solution 3

Latent heat allows large amounts of heat to be absorbed without temperature change.

It also provides efficient and continuous cooling.

Examiner-Level Guidance

• Always link refrigeration to evaporation and condensation.

• Use the phrase latent heat of vaporisation explicitly.

• Mention heat removal from inside and heat release outside.

• Avoid saying the fridge “produces cold” — this is incorrect.

Specific latent heat L is calculated using:

where:

• E = thermal energy transferred (J)

• m = mass that changes state (kg)

• L = specific latent heat (J kg)

  In these experiments:

  • energy is determined from electrical heating or thermal exchange, and
  • temperature remains constant during the phase change.

  ---

  Experiment: Determining the Specific Latent Heat of Fusion of Ice

  Apparatus

  • Calorimeter (or insulated container) with lid
  • Crushed ice at 0 °C (dry)
  • Thermometer
  • Balance
  • Stirrer
  • Insulation

  [Insert labelled diagram of calorimeter with ice at 0 °C and thermometer]

  ---

  Method (Procedure)

  1. Measure the mass of the empty calorimeter.
  2. Add warm water and record its mass and initial temperature.
  3. Add dry ice at 0 °C to the water.
  4. Stir gently until all the ice has just melted.
  5. Record the final temperature of the mixture.
  6. Measure the final mass to determine the mass of ice melted.

  ---

  Measurements

  • Mass of ice melted, $m_{\text{ice}}$
  • Initial and final temperatures of the water
  • Mass of water

  ---

  Calculation (Conceptual)

  • Heat lost by warm water = heat gained by ice to melt.
  • Since the ice is already at 0 °C, no temperature rise occurs before melting.

  $$E = m_{\text{ice}} L_f$$

  $$L_f = \frac{E}{m_{\text{ice}}}$$

  (where $L_f$ is the specific latent heat of fusion)

  ---

  Key Assumptions / Errors

  • Some heat is lost to surroundings.
  • Ice must be dry to avoid extra water mass.
  • Container heat capacity may be neglected at this level.

  ---

  Experiment: Determining the Specific Latent Heat of Vaporisation of Steam

  Apparatus

  • Steam generator
  • Insulated calorimeter with water
  • Thermometer
  • Balance
  • Delivery tube
  • Stirrer

  [Insert labelled diagram of steam passing into calorimeter with water]

  ---

  Method (Procedure)

  1. Measure the mass of the calorimeter with water and record the initial temperature.
  2. Pass dry steam into the water.
  3. Stir continuously.
  4. Stop when the temperature rises to a suitable value.
  5. Record the final temperature.
  6. Measure the final mass to find the mass of steam condensed.

  ---

  Measurements

  • Mass of steam condensed, $m_{\text{steam}}$
  • Initial and final temperatures of the water
  • Mass of water

  ---

  Calculation (Conceptual)

  • Heat released by steam = heat gained by water.
  • Energy released when steam condenses at constant temperature:

  $$E = m_{\text{steam}} L_v$$

  $$L_v = \frac{E}{m_{\text{steam}}}$$

  (where $L_v$ is the specific latent heat of vaporisation)

  ---

  Key Assumptions / Errors

  • Steam must be dry (no water droplets).
  • Some heat is lost to surroundings.
  • Heat absorbed by the container may be neglected or stated as a limitation.

  ---

  Comparison Summary (Exam-Ready)

  Quantity	Ice Experiment	Steam Experiment
  State change	Solid → Liquid	Gas → Liquid
  Latent heat	Fusion ($L_f$)	Vaporisation ($L_v$)
  Temperature change	None during melting	None during condensation
  Energy transfer	Absorbed	Released

  ---

  Key Exam-Ready Statements

  • Latent heat is measured when temperature remains constant.
  • Mass that changes state must be measured accurately.
  • Energy calculations use E = mL.
  • Insulation improves accuracy by reducing heat loss.

  ---

  Exam-Style Questions (Original, BGCSE-Aligned)

  Question 1

  Describe an experiment to determine the specific latent heat of fusion of ice.

  ---

  Question 2

  Describe how the specific latent heat of vaporisation of steam can be determined experimentally.

  ---

  Question 3

  State two precautions needed to improve accuracy in latent heat experiments.

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  Worked Solutions (Grade A/A* Standard)

  Solution 1

  Warm water is placed in an insulated calorimeter and its temperature and mass are measured.

  Dry ice at 0 °C is added and stirred until it just melts.

  The final temperature and mass are measured to find the mass of ice melted.

  Heat lost by the water equals the latent heat absorbed by the ice, allowing calculation of the specific latent heat of fusion.

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  Solution 2

  Steam is passed into water in an insulated calorimeter.

  The rise in temperature and increase in mass are measured.

  The mass increase gives the mass of steam condensed.

  The heat released by condensation equals the heat gained by the water, allowing calculation of the specific latent heat of vaporisation.

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  Solution 3

  Ice and steam must be dry, and the calorimeter should be well insulated to reduce heat loss.

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  Examiner-Level Guidance (Teacher Confidence Builder)

  • Always include apparatus, method, measurements, and calculation principle.
  • Use correct symbols: Lf and Lv.
  • State assumptions and sources of error clearly.
  • Temperature constancy during phase change must be mentioned.