Changes of state

What you'll learn

How the particle model explains solids, liquids, gases, and changes of state.
Why mass is conserved during melting, freezing, evaporation, condensation and sublimation.
The difference between temperature, internal energy, specific heat capacity and specific latent heat.
How to calculate energy changes using the two key OCR J249 equations.

Particles and the three states of matter

Matter is anything that has mass and takes up space. In GCSE Physics, we often model matter as being made from tiny particles.

Definition

Particle

A particle is a tiny part of a substance. Depending on the substance, particles could be atoms, molecules or ions — they are not always single atoms.

The particle model is a simplified way to explain how matter behaves:

In a solid, particles are close together in a regular arrangement and vibrate about fixed positions.
In a liquid, particles are close together but arranged randomly, and they can move past each other.
In a gas, particles are far apart and move randomly in all directions.

This diagram shows the changes of state and how the particle arrangement changes.

Particle model showing solids, liquids, gases and changes of state

Common Mistake

The gaps are not filled with air

In the particle model, the spaces between particles are usually just empty space. Do not say the gaps are filled with “air” unless the question is specifically about a mixture containing air.

The names of changes of state

A change of state is when a substance changes between solid, liquid and gas without becoming a new substance.

The main changes are:

Melting: solid to liquid.
Freezing: liquid to solid.
Evaporation: liquid to gas from the surface, at temperatures below boiling as well as at boiling point.
Boiling: liquid to gas throughout the liquid, at the boiling point.
Condensation: gas to liquid.
Sublimation: solid directly to gas.
Deposition: gas directly to solid, the reverse of sublimation.

Key Idea

Changes of state are physical changes

Changes of state are physical changes because no new substance is made. If the change is reversed, the material recovers its original properties.

Physical changes and chemical changes

A physical change changes the form, shape or state of a substance, but not what the substance is made of. Melting ice is physical because it is still water.

A chemical change produces one or more new substances. Burning a fuel is chemical because new substances such as carbon dioxide and water vapour are made.

State changes are physical changes because the particles themselves stay the same; it is their arrangement and movement that change.

Mass is conserved during changes of state

Mass is the amount of matter in an object or substance. During a change of state, particles are not created or destroyed, so the mass stays the same.

Definition

Conservation of mass

Conservation of mass means the total mass stays constant, as long as no particles enter or leave the system being measured.

A system is the object or collection of objects you are focusing on. A sealed container is a closed system because particles cannot escape.

Example

Mass in a closed and open container

A sealed flask contains 80.0 g of liquid water. It is heated until some water evaporates into steam inside the flask. An open beaker also contains 80.0 g of water and is heated.

For the sealed flask, choose the system as the whole flask and everything inside it. The liquid water and steam are both still inside the system.
Apply conservation of mass: no particles can escape, so the total mass of water in the flask remains 80.0 g.
For the open beaker, some water vapour can leave the beaker. The balance reading for the beaker and remaining water may decrease, but the missing mass is in the escaped vapour, not destroyed.

Heating, temperature and internal energy

When energy is transferred to a substance by heating, the energy stored inside the substance changes.

Definition

Internal energy

The internal energy of a material is the total energy stored by its particles due to their movement and their positions relative to each other.

Temperature tells you how hot something is. In the particle model, a higher temperature usually means the particles have a higher average kinetic energy, so they move or vibrate more vigorously.

Heating can do two different things:

It can raise the temperature if the substance stays in the same state.
It can change the state if the substance is at a melting point or boiling point.

A heating curve shows how temperature changes as energy is supplied to a pure substance.

Heating curve showing warming, melting, boiling and gas warming sections

During the flat parts of the graph, energy is still being transferred, but the temperature does not rise. The energy is used to change the arrangement of the particles, not to increase their average kinetic energy.

Example

Explaining a flat section on a heating curve

A solid is heated steadily. The temperature rises, then stays constant while the solid melts. Explain what is happening during the flat section.

Identify the flat section as a change of state: the substance is melting from solid to liquid.
Since the temperature is constant, the particles’ average kinetic energy is not increasing.
The supplied energy is used to overcome some of the attractions between particles, changing their arrangement from solid-like to liquid-like.

Common Mistake

Heat and temperature are not the same

Heating is an energy transfer. Temperature is a measure of how hot something is. Do not write that “heat increases” when you mean “temperature increases”.

Specific heat capacity

When a material is heated and its temperature changes, use specific heat capacity.

Definition

Specific heat capacity

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of the substance by 1 °C, without changing state. Its unit is J/kg °C.

OCR J249 supplies this relationship on the equation sheet, but you still need to know when to use it and how to rearrange it:

ΔE=mcΔθ\Delta E = m c \Delta \theta

where:

$ΔE\Delta E$ is the change in thermal energy or internal energy, in joules (J).
$m$ is mass, in kilograms (kg).
$c$ is specific heat capacity, in J/kg °C.
$Δθ\Delta \theta$ is change in temperature, in °C.

Example

Calculating energy to warm water

A 0.50 kg sample of water is heated from 20 °C to 80 °C. The specific heat capacity of water is 4200 J/kg °C. Calculate the energy transferred.

Find the temperature change: $Δθ=80−20=60∘C\Delta \theta = 80 - 20 = 60^{\circ}\text{C}$ .
Substitute into $ΔE=mcΔθ\Delta E = mc\Delta\theta$ : $ΔE=0.50×4200×60\Delta E = 0.50 \times 4200 \times 60$ .
Calculate the energy: $kJ\Delta E = 126000\,\text{J} = 126\,\text{kJ}$ .

Tip

Check the units

Mass must be in kilograms, not grams. If a question gives 250 g, convert it to 0.250 kg before using the equation.

Specific latent heat

When a material changes state but its temperature does not change, use specific latent heat.

Definition

Specific latent heat

The specific latent heat of a substance is the energy needed to change the state of 1 kg of the substance without changing its temperature. Its unit is J/kg.

There are two important types:

Specific latent heat of fusion: energy needed for melting or released during freezing.
Specific latent heat of vaporisation: energy needed for boiling or evaporation, or released during condensation.

OCR J249 supplies this relationship on the equation sheet:

ΔE=mL\Delta E = mL

where:

$ΔE\Delta E$ is the energy transferred, in joules (J).
$m$ is mass, in kilograms (kg).
$L$ is specific latent heat, in J/kg.

The word latent means “hidden”: energy is being transferred, but you do not see it as a temperature rise.

Example

Calculating energy to melt ice

A 0.20 kg block of ice at 0 °C melts into water at 0 °C. The specific latent heat of fusion of ice is 334000 J/kg. Calculate the energy needed.

The temperature is not changing, but the state is changing, so use $ΔE=mL\Delta E = mL$ .
Substitute the values: $ΔE=0.20×334000\Delta E = 0.20 \times 334000$ .
Calculate the energy: $kJ\Delta E = 66800\,\text{J} = 66.8\,\text{kJ}$ .

When both temperature and state change happen

Many questions involve more than one stage. For example, you might heat ice from below 0 °C to 0 °C, then melt it.

The method is to split the process into sections:

Use $ΔE=mcΔθ\Delta E = mc\Delta\theta$ for sections where temperature changes.
Use $ΔE=mL\Delta E = mL$ for sections where state changes.
Add the energy values together for the total.

Example

Heating then melting ice

A 0.10 kg piece of ice is heated from minus 10 °C to 0 °C, then completely melted. The specific heat capacity of ice is 2100 J/kg °C and the specific latent heat of fusion of ice is 334000 J/kg. Calculate the total energy needed.

Split the process into two stages: warming the solid ice from minus 10 °C to 0 °C, then melting the ice at 0 °C.
Calculate the energy for warming: $J\Delta E = mc\Delta\theta = 0.10 \times 2100 \times 10 = 2100\,\text{J}$ .
Calculate the energy for melting: $J\Delta E = mL = 0.10 \times 334000 = 33400\,\text{J}$ .
Add the two energy transfers: $J\Delta E_{\text{total}} = 2100 + 33400 = 35500\,\text{J}$ .

Common Mistake

Using the wrong equation

If the temperature changes, use specific heat capacity. If the state changes at constant temperature, use specific latent heat. If both happen, split the question into stages.

Exam technique

In the exam

Decide whether each part of the process is a temperature change, a state change, or both in separate stages.
Check units before substituting: mass in kg, energy in J, specific heat capacity in J/kg °C, and specific latent heat in J/kg.
Explain flat sections of heating or cooling curves using particles: temperature is constant because energy changes particle arrangement, not particle kinetic energy.

Self review

Check yourself

Why does the mass stay the same when water evaporates inside a sealed container?
What is the difference between specific heat capacity and specific latent heat?
On a heating curve, what is happening to the particles during a flat section?

Changes of state

What you'll learn

How the particle model explains solids, liquids, gases, and changes of state.
Why mass is conserved during melting, freezing, evaporation, condensation and sublimation.
The difference between temperature, internal energy, specific heat capacity and specific latent heat.
How to calculate energy changes using the two key OCR J249 equations.

Particles and the three states of matter

Matter is anything that has mass and takes up space. In GCSE Physics, we often model matter as being made from tiny particles.

Definition

Particle

A particle is a tiny part of a substance. Depending on the substance, particles could be atoms, molecules or ions — they are not always single atoms.

The particle model is a simplified way to explain how matter behaves:

In a solid, particles are close together in a regular arrangement and vibrate about fixed positions.
In a liquid, particles are close together but arranged randomly, and they can move past each other.
In a gas, particles are far apart and move randomly in all directions.

This diagram shows the changes of state and how the particle arrangement changes.

Particle model showing solids, liquids, gases and changes of state

Common Mistake

The gaps are not filled with air

In the particle model, the spaces between particles are usually just empty space. Do not say the gaps are filled with “air” unless the question is specifically about a mixture containing air.

The names of changes of state

A change of state is when a substance changes between solid, liquid and gas without becoming a new substance.

The main changes are:

Melting: solid to liquid.
Freezing: liquid to solid.
Evaporation: liquid to gas from the surface, at temperatures below boiling as well as at boiling point.
Boiling: liquid to gas throughout the liquid, at the boiling point.
Condensation: gas to liquid.
Sublimation: solid directly to gas.
Deposition: gas directly to solid, the reverse of sublimation.

Key Idea

Changes of state are physical changes

Changes of state are physical changes because no new substance is made. If the change is reversed, the material recovers its original properties.

Physical changes and chemical changes

A physical change changes the form, shape or state of a substance, but not what the substance is made of. Melting ice is physical because it is still water.

A chemical change produces one or more new substances. Burning a fuel is chemical because new substances such as carbon dioxide and water vapour are made.

State changes are physical changes because the particles themselves stay the same; it is their arrangement and movement that change.

Mass is conserved during changes of state

Mass is the amount of matter in an object or substance. During a change of state, particles are not created or destroyed, so the mass stays the same.

Definition

Conservation of mass

Conservation of mass means the total mass stays constant, as long as no particles enter or leave the system being measured.

A system is the object or collection of objects you are focusing on. A sealed container is a closed system because particles cannot escape.

Example

Mass in a closed and open container

A sealed flask contains 80.0 g of liquid water. It is heated until some water evaporates into steam inside the flask. An open beaker also contains 80.0 g of water and is heated.

For the sealed flask, choose the system as the whole flask and everything inside it. The liquid water and steam are both still inside the system.
Apply conservation of mass: no particles can escape, so the total mass of water in the flask remains 80.0 g.
For the open beaker, some water vapour can leave the beaker. The balance reading for the beaker and remaining water may decrease, but the missing mass is in the escaped vapour, not destroyed.

Heating, temperature and internal energy

When energy is transferred to a substance by heating, the energy stored inside the substance changes.

Definition

Internal energy

The internal energy of a material is the total energy stored by its particles due to their movement and their positions relative to each other.

Temperature tells you how hot something is. In the particle model, a higher temperature usually means the particles have a higher average kinetic energy, so they move or vibrate more vigorously.

Heating can do two different things:

It can raise the temperature if the substance stays in the same state.
It can change the state if the substance is at a melting point or boiling point.

A heating curve shows how temperature changes as energy is supplied to a pure substance.

Heating curve showing warming, melting, boiling and gas warming sections

Example

Explaining a flat section on a heating curve

A solid is heated steadily. The temperature rises, then stays constant while the solid melts. Explain what is happening during the flat section.

Identify the flat section as a change of state: the substance is melting from solid to liquid.
Since the temperature is constant, the particles’ average kinetic energy is not increasing.
The supplied energy is used to overcome some of the attractions between particles, changing their arrangement from solid-like to liquid-like.

Common Mistake

Heat and temperature are not the same

Heating is an energy transfer. Temperature is a measure of how hot something is. Do not write that “heat increases” when you mean “temperature increases”.

Specific heat capacity

When a material is heated and its temperature changes, use specific heat capacity.

Definition

Specific heat capacity

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of the substance by 1 °C, without changing state. Its unit is J/kg °C.

OCR J249 supplies this relationship on the equation sheet, but you still need to know when to use it and how to rearrange it:

ΔE=mcΔθ\Delta E = m c \Delta \theta

where:

$ΔE\Delta E$ is the change in thermal energy or internal energy, in joules (J).
$m$ is mass, in kilograms (kg).
$c$ is specific heat capacity, in J/kg °C.
$Δθ\Delta \theta$ is change in temperature, in °C.

Example

Calculating energy to warm water

A 0.50 kg sample of water is heated from 20 °C to 80 °C. The specific heat capacity of water is 4200 J/kg °C. Calculate the energy transferred.

Find the temperature change: $Δθ=80−20=60∘C\Delta \theta = 80 - 20 = 60^{\circ}\text{C}$ .
Substitute into $ΔE=mcΔθ\Delta E = mc\Delta\theta$ : $ΔE=0.50×4200×60\Delta E = 0.50 \times 4200 \times 60$ .
Calculate the energy: $kJ\Delta E = 126000\,\text{J} = 126\,\text{kJ}$ .

Tip

Check the units

Mass must be in kilograms, not grams. If a question gives 250 g, convert it to 0.250 kg before using the equation.

Specific latent heat

When a material changes state but its temperature does not change, use specific latent heat.

Definition

Specific latent heat

The specific latent heat of a substance is the energy needed to change the state of 1 kg of the substance without changing its temperature. Its unit is J/kg.

There are two important types:

Specific latent heat of fusion: energy needed for melting or released during freezing.
Specific latent heat of vaporisation: energy needed for boiling or evaporation, or released during condensation.

OCR J249 supplies this relationship on the equation sheet:

ΔE=mL\Delta E = mL

where:

$ΔE\Delta E$ is the energy transferred, in joules (J).
$m$ is mass, in kilograms (kg).
$L$ is specific latent heat, in J/kg.

The word latent means “hidden”: energy is being transferred, but you do not see it as a temperature rise.

Example

Calculating energy to melt ice

A 0.20 kg block of ice at 0 °C melts into water at 0 °C. The specific latent heat of fusion of ice is 334000 J/kg. Calculate the energy needed.

The temperature is not changing, but the state is changing, so use $ΔE=mL\Delta E = mL$ .
Substitute the values: $ΔE=0.20×334000\Delta E = 0.20 \times 334000$ .
Calculate the energy: $kJ\Delta E = 66800\,\text{J} = 66.8\,\text{kJ}$ .

When both temperature and state change happen

Many questions involve more than one stage. For example, you might heat ice from below 0 °C to 0 °C, then melt it.

The method is to split the process into sections:

Use $ΔE=mcΔθ\Delta E = mc\Delta\theta$ for sections where temperature changes.
Use $ΔE=mL\Delta E = mL$ for sections where state changes.
Add the energy values together for the total.

Example

Heating then melting ice

Split the process into two stages: warming the solid ice from minus 10 °C to 0 °C, then melting the ice at 0 °C.
Calculate the energy for warming: $J\Delta E = mc\Delta\theta = 0.10 \times 2100 \times 10 = 2100\,\text{J}$ .
Calculate the energy for melting: $J\Delta E = mL = 0.10 \times 334000 = 33400\,\text{J}$ .
Add the two energy transfers: $J\Delta E_{\text{total}} = 2100 + 33400 = 35500\,\text{J}$ .

Common Mistake

Using the wrong equation

If the temperature changes, use specific heat capacity. If the state changes at constant temperature, use specific latent heat. If both happen, split the question into stages.

Exam technique

In the exam

Decide whether each part of the process is a temperature change, a state change, or both in separate stages.
Check units before substituting: mass in kg, energy in J, specific heat capacity in J/kg °C, and specific latent heat in J/kg.
Explain flat sections of heating or cooling curves using particles: temperature is constant because energy changes particle arrangement, not particle kinetic energy.

Self review

Check yourself

Why does the mass stay the same when water evaporates inside a sealed container?
What is the difference between specific heat capacity and specific latent heat?
On a heating curve, what is happening to the particles during a flat section?

Changes of state

What you'll learn

Particles and the three states of matter

The names of changes of state

Physical changes and chemical changes

Mass is conserved during changes of state

Heating, temperature and internal energy

Specific heat capacity

Specific latent heat

When both temperature and state change happen

Recap questions

You've reached the end

Changes of state

What you'll learn

Particles and the three states of matter

The names of changes of state

Physical changes and chemical changes

Mass is conserved during changes of state

Heating, temperature and internal energy

Specific heat capacity

Specific latent heat

When both temperature and state change happen

Recap questions

You've reached the end

Changes of state

What you'll learn

Particles and the three states of matter

The names of changes of state

Physical changes and chemical changes

Mass is conserved during changes of state

Heating, temperature and internal energy

Specific heat capacity

Specific latent heat

When both temperature and state change happen

Recap questions

You've reached the end

Changes of state Revision Guide

Changes of state

What you'll learn

Particles and the three states of matter

The names of changes of state

Physical changes and chemical changes

Mass is conserved during changes of state

Heating, temperature and internal energy

Specific heat capacity

Specific latent heat

When both temperature and state change happen

Recap questions

You've reached the end

Changes of state Revision Guide