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KS3 Science · Year 7Chapter 4 · States of matter and the particle model
Year 7 · Chapter 4

States of matter and the particle model

By the end of this chapter you will be able to

  • Describe the arrangement and movement of particles in a solid, a liquid and a gas
  • Explain the shape, volume and compressibility of each state using the particle model
  • Name the changes of state, and say whether energy is taken in or given out
  • Explain that mass is conserved in a change of state
  • Read a heating curve, and explain why the temperature stays flat while a substance melts

You should already know

  • Solids, liquids and gases, from primary school science (Year 4 Science)
  • Reading tables and line graphs (Chapter 1 of this book)

Where this leads

  • Diffusion and density, which use the particle model further (Chapter 5)

National Curriculum for England, Key Stage 3 science, Year 7. Content: C1 solids, liquids and gases are described by the particle model: the arrangement, movement and energy of particles, C1 changes of state are physical changes; the mass is conserved in a change of state.

Exercise P  Do you remember?

1
FName the three states of matter. Write one of them: solid, liquid or gas.
2
FAt what temperature in degrees Celsius does pure water freeze? Write the number.
3
FWhat is the name of the change when a liquid turns into a gas? Write the word.
4
FWhat is 3 times 20? Write the number.
5
FWhich is heavier: 1 kilogram or 500 grams? Write the words.
4.1

Particles in solids, liquids and gases

Year 7
C1: describe solids, liquids and gases using the particle model, including arrangement and movement of particles
Video lesson: Particles in solids, liquids and gasesComing soon
Story. Ali drops an ice cube into a glass of water. He notices the ice first keeps its shape, then slowly becomes water that spreads out. “Why does the ice keep its shape?” he asks. His teacher says, “Think about how the particles are arranged and how they move.”

All materials are made of tiny particles. The particle model uses this idea to explain the three states of matter.

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

solidliquidgas
Particles in a solid (regular and close), a liquid (close and random), and a gas (far apart).

The table compares the three states.

StateShapeVolumeArrangementMovement
solidfixed shapefixed volumeclose together, in a regular patternvibrate about fixed positions
liquidtakes the shape of its containerfixed volumeclose together, but randommove around and slide past each other
gastakes the shape of its containerfills the containerfar apart, and randommove quickly in all directions

Key ideas

  • Solid: particles close together, in a regular pattern, vibrating about fixed positions.
  • Liquid: particles close together but random, and they can move past each other.
  • Gas: particles far apart, random, and moving quickly in all directions.
  • The particle model explains the properties of each state.
Worked example 4.1.1
In which state are the particles far apart and moving quickly in all directions? Write the word.

Method. Match the description to a state.

  1. Read the description. Particles far apart and moving quickly in all directions describe a gas.
    gas
  2. Check. A gas has particles that are far apart and random.
    far apart
  3. Write the answer.
    gas

Check. The table gives gas particles as far apart. ✓

Worked example 4.1.2
In a solid, how are the particles arranged? Write the words.

Method. Read the table.

  1. Read the table. The table says the solid particles are close together in a regular pattern.
    regular pattern
  2. Write the answer. So the particles are close together in a regular pattern.
    regular pattern
  3. Write the answer.
    close together in a regular pattern

Check. Solid particles vibrate about fixed positions. ✓

Worked example 4.1.3
In a liquid, are the particles arranged in a regular pattern? Write yes or no.

Method. Compare with the solid.

  1. Read the table. Liquid particles are close together but arranged randomly.
    randomly
  2. Decide. So they are not in a regular pattern, and the answer is no.
    no
  3. Write the answer.
    no

Check. Liquid particles can slide past each other. ✓

Watch out

Watch out: particles are not the same as the material. The particles themselves are not solid, liquid or gas; the arrangement and movement of the particles define the state.

Exam tip

Exam tip. Describe a state using both the arrangement and the movement of the particles.

Try it (with your teacher, or a grown-up at home). With a grown-up, put an ice cube on a plate at room temperature and look at it every few minutes. Describe what you see. Then say what the particles of the ice are doing, using the words arrangement and movement.

Exercise 4A  Particles in solids, liquids and gases

1
FIn which state are the particles close together, in a regular pattern? Write the word.
2
FIn which state are the particles close together but arranged randomly? Write the word.
3
FIn which state are the particles far apart? Write the word.
4
FIn a solid, do the particles vibrate about fixed positions? Write yes or no.
5
FIn a gas, are the particles arranged randomly? Write yes or no.
6
FWhich state has particles that move around and slide past each other? Write the word.
7
FWhich state has particles that are far apart and move quickly in all directions? Write the word.
8
PSPut the three states in order from the most closely packed particles to the least. Write the three words in order.
9
PSBen says the particles in a solid are moving about freely. Is Ben right? Write yes or no.
10
PSWorking scientifically: a pupil draws a solid with the particles far apart. Is the drawing correct? Write yes or no.
11
RExplain why a gas has particles that are far apart. Write one sentence.
12
RChallenge: explain why the particles in a solid vibrate but do not move from place to place. Write one sentence.
4.2

Shape, volume and compressibility

Year 7
C1: use the particle model to explain the shape, volume and compressibility of solids, liquids and gases
Video lesson: Shape, volume and compressibilityComing soon
Story. Mo squeezes a syringe full of air, then tries to squeeze a syringe full of water. “The air goes smaller, but the water hardly changes,” he says. His teacher says, “The particle model explains why. Think about the space between the particles.”

A solid keeps its shape and its volume. A liquid takes the shape of its container, but keeps its volume. A gas takes the shape of its container, and fills it, so its volume is the volume of the container.

Gases can be compressed (squashed) easily, because there is a lot of space between the particles. Liquids and solids are hardly compressed, because their particles are already close together.

Key ideas

  • A solid has a fixed shape and a fixed volume.
  • A liquid has a fixed volume, but takes the shape of its container.
  • A gas fills its container, and can be compressed easily.
  • Liquids and solids are hard to compress because their particles are close together.
Worked example 4.2.1
Why can a gas be compressed easily? Write the reason in a few words.

Method. Use the spaces between the particles.

  1. Think about the spaces. In a gas, the particles are far apart, with a lot of space between them.
    far apart
  2. Decide. So the particles can be pushed closer together when the gas is squashed.
    pushed closer
  3. Write the answer.
    the particles are far apart

Check. Liquids have little space between particles. ✓

Worked example 4.2.2
A liquid is poured into a tall glass, then into a wide dish. Does its volume change? Write yes or no.

Method. Use the properties of a liquid.

  1. Think about the shape. The liquid takes the shape of its container, so its shape changes.
    shape changes
  2. Think about the volume. Its volume stays the same, so the answer is no.
    no
  3. Write the answer.
    no

Check. A liquid keeps its volume but not its shape. ✓

Worked example 4.2.3
Why does a solid keep its shape? Write the reason in one sentence.

Method. Think about the particles.

  1. Think about the arrangement. Solid particles are held in fixed positions in a regular pattern.
    fixed positions
  2. Decide. So the solid keeps its shape.
    keeps its shape
  3. Write the answer.
    Its particles are held in fixed positions.

Check. A solid cannot flow like a liquid. ✓

Watch out

Watch out: a liquid does not have a fixed shape, but it does have a fixed volume. A gas has neither a fixed shape nor a fixed volume.

Exam tip

Exam tip. Explain a property with the particles: say how close they are, and whether they can move past each other.

Try it (with your teacher, or a grown-up at home). With a grown-up, pour a small amount of water into a tall thin glass and then into a wide, shallow dish. Note the shape and the depth. Explain why the shape changes but the volume does not.

Exercise 4B  Shape, volume and compressibility

1
FWhich state has a fixed shape and a fixed volume? Write the word.
2
FWhich state has a fixed volume but takes the shape of its container? Write the word.
3
FWhich state fills its container completely? Write the word.
4
FCan a gas be compressed easily? Write yes or no.
5
FCan a solid be compressed easily? Write yes or no.
6
FWhich two states are hardly compressed? Write the two words.
7
PSA liquid is poured from a tall glass into a wide dish. Does its shape change? Write yes or no.
8
PSA liquid is poured from a tall glass into a wide dish. Does its volume change? Write yes or no.
9
PSA gas is put into a sealed syringe, and the plunger is pushed in. What happens to the gas? Write one word: smaller or bigger.
10
PSRuby says a solid can be squashed into a smaller volume with a syringe, as easily as a gas. Is Ruby right? Write yes or no.
11
PSWorking scientifically: a pupil says a liquid changes its volume when it is poured into a wider dish. Explain why this claim is not supported. Write one sentence.
12
RExplain why liquids are hard to compress. Write one sentence.
13
RChallenge: use the particle model to explain why a gas takes the shape of its container, but a solid does not. Write one sentence.
4.3

Changes of state

Year 7
C1: changes of state are physical changes; name the changes and say whether energy is taken in or given out
Video lesson: Changes of stateComing soon
Story. Sam leaves a bottle of water in the freezer and it becomes solid ice. Later, the ice melts on the kitchen table. “Is the water a different substance now?” he asks. His teacher says, “No. This is a physical change, so the water is still water.”

A change of state happens when a substance changes from one state to another. It is a physical change: no new substance is made, so the change can be reversed.

Melting is solid to liquid, and freezing is liquid to solid. Evaporation and boiling are liquid to gas. Condensation is gas to liquid. Sublimation is solid straight to gas.

Energy is taken in when a substance melts, evaporates or boils. Energy is given out when it freezes or condenses.

solidliquidgasmeltingfreezingevaporationcondensationsublimation: solid straight to gas (and back)
The changes of state between solid, liquid and gas.

The table gives each change, what happens, and whether energy is taken in or given out.

Change of stateWhat happensEnergy
meltingsolid to liquidenergy is taken in
freezingliquid to solidenergy is given out
evaporationliquid to gas, at the surface at any temperatureenergy is taken in
boilingliquid to gas, throughout the liquid at the boiling pointenergy is taken in
condensationgas to liquidenergy is given out
sublimationsolid to gas, without becoming a liquidenergy is taken in

Key ideas

  • Melting: solid to liquid; energy is taken in.
  • Freezing: liquid to solid; energy is given out.
  • Evaporation and boiling: liquid to gas; energy is taken in.
  • Condensation: gas to liquid; energy is given out.
Worked example 4.3.1
What is the name of the change when a solid turns into a liquid? Write the word.

Method. Find the change in the table.

  1. Read the table. The change from solid to liquid is melting.
    melting
  2. Check the energy. Melting takes in energy.
    energy taken in
  3. Write the answer.
    melting

Check. Freezing is the opposite change. ✓

Worked example 4.3.2
When water vapour turns into liquid water on a cold window, what is the change called? Write the word.

Method. Find the change from gas to liquid.

  1. Read the table. The change from gas to liquid is condensation.
    condensation
  2. Check the energy. Energy is given out during condensation.
    energy given out
  3. Write the answer.
    condensation

Check. Condensation forms the drops on a cold window. ✓

Worked example 4.3.3
In which change is energy given out, and the substance becomes a solid? Write the word.

Method. Find the change to a solid.

  1. Read the table. The change from liquid to solid is freezing.
    freezing
  2. Check the energy. Energy is given out when a liquid freezes.
    given out
  3. Write the answer.
    freezing

Check. Ice forms when water freezes. ✓

Watch out

Watch out: evaporation can happen at any temperature, from the surface only. Boiling happens throughout the liquid, at its boiling point.

Exam tip

Exam tip. For a change of state, say the start state and the end state, then say whether energy is taken in or given out.

Try it (with your teacher, or a grown-up at home). With a grown-up, leave a small dish of water in a warm room, and another dish of water in the fridge. Each day, look at the level. Name the change that happens in each dish, and whether energy is taken in or given out.

Exercise 4C  Changes of state

1
FWhat is the change from solid to liquid called? Write the word.
2
FWhat is the change from liquid to solid called? Write the word.
3
FWhat is the change from gas to liquid called? Write the word.
4
FWhat is the change from solid straight to gas called? Write the word.
5
FIs melting an energy-taking change? Write yes or no.
6
FIs freezing an energy-giving change? Write yes or no.
7
FIs a change of state a chemical change? Write yes or no.
8
PSWhich change makes a liquid turn into a gas at any temperature, from the surface only? Write the word.
9
PSWhich change happens throughout a liquid, at its boiling point? Write the word.
10
PSBen says that when wet clothes dry on a line, the water changes into a solid. Is Ben right? Write yes or no.
11
PSWorking scientifically: a pupil says melting and freezing are the same change. Is the pupil right? Write yes or no.
12
RExplain why a change of state is called a physical change. Write one sentence.
13
RChallenge: explain why energy is given out when water vapour condenses on a cold window. Write one sentence.
4.4

Mass is conserved in a change of state

Year 7
C1: a change of state is a physical change, so the mass of the substance is the same before and after
Video lesson: Mass is conserved in a change of stateComing soon
Story. Ruby freezes 50 grams of water in a sealed bag, then weighs it. “Has the ice lost any weight?” she asks. Her teacher says, “Weigh it again once it has melted. The particles are the same, only arranged differently.”

In a change of state, the particles are not created or destroyed. They are only arranged differently, and they move differently. So the mass is conserved: the mass before the change equals the mass after the change, if no substance escapes.

The table gives the masses of the same water before and after melting, in a sealed bag.

SampleMass (grams)
ice cubes in a sealed bag, before melting50
the same water, after melting50

Key ideas

  • Mass is conserved in a change of state.
  • The particles are rearranged, not created or destroyed.
  • The mass before equals the mass after, when nothing escapes.
  • The volume of a substance can change in a change of state, even when its mass does not.
Worked example 4.4.1
In the table, what is the mass of the ice before melting? Write the number of grams.

Method. Read the table.

  1. Read the table. The ice before melting has a mass of 50 grams.
    50 grams
  2. Write the answer. So the mass before melting is 50 grams.
    50
  3. Write the answer.
    50 g

Check. The mass after melting is also 50 grams. ✓

Worked example 4.4.2
The water in the sealed bag has a mass of 120 grams before it freezes. What is its mass after it freezes? Write the number of grams.

Method. Use conservation of mass.

  1. Think. In a sealed bag, no particles escape, so the mass does not change.
    no particles escape
  2. Decide. So the mass after freezing is the same as before.
    same
  3. Write the answer.
    120 g

Check. Conservation of mass applies to every change of state. ✓

Worked example 4.4.3
When ice melts, does its mass go up, down, or stay the same? Write the word.

Method. Think about the particles.

  1. Think about the particles. The particles are only rearranged, not added or removed.
    rearranged
  2. Decide. So the mass stays the same.
    stays the same
  3. Write the answer.
    stays the same

Check. Volume can change, but mass stays the same. ✓

Watch out

Watch out: ice is less dense than water, so it takes up more space. The volume changes, but the mass does not.

Exam tip

Exam tip. In a sealed container, the mass is the same before and after a change of state. If a gas escapes, the mass is less.

Try it (with your teacher, or a grown-up at home). With a grown-up, put 50 grams of ice in a sealed plastic bag and weigh it on kitchen scales. Let it melt, and weigh it again. Write down both masses and say whether the mass was conserved.

Exercise 4D  Mass is conserved in a change of state

1
FThe ice has a mass of 50 grams before it melts. What is its mass after melting? Write the number of grams.
2
FIs mass conserved when water freezes in a sealed bag? Write yes or no.
3
FDoes the mass of a substance change when it melts? Write yes or no.
4
FWhat is conserved in a change of state? Write one word.
5
FA sealed bag of water has a mass of 120 grams. It freezes. What is its mass now? Write the number of grams.
6
PSA cup of water has a mass of 300 grams. Some water evaporates into the air, and the cup is then weighed as 280 grams. How many grams have escaped? Write the number.
7
PSIn question 6, the water vapour escaped into the room. Is the mass of the water conserved in the room as a whole? Write yes or no.
8
PSA melting ice cube has a mass of 30 grams. The water formed weighs 30 grams. Is this consistent with mass being conserved? Write yes or no.
9
PSBen says the mass of the ice goes down when it melts, because the ice is less dense. Is Ben right? Write yes or no.
10
PSWorking scientifically: a pupil weighs a sealed bag of ice before and after melting. Name one control variable. Accept any one of: the same bag, the same scales, the same amount of ice.
11
RExplain why the mass of the water in an open dish can seem to fall as it evaporates. Write one sentence.
12
RChallenge: explain why the volume of water can change when it freezes, but its mass stays the same. Write one sentence.
4.5

Heating curves and changes of state

Year 7
C1: read a heating curve; the temperature stays the same during a change of state, while energy is used to change the arrangement of particles
Video lesson: Heating curves and changes of stateComing soon
Story. Priya heats a block of ice and records its temperature every two minutes. She plots the results on a graph. “The temperature stopped rising for a while,” she says. “Why did it go flat, when the heater was still on?” Her teacher says, “Look at what the energy is doing while the ice melts.”

A heating curve is a line graph of temperature against time. While a pure substance melts, the temperature stays flat: the energy being supplied is used to break the particles away from their fixed positions, not to raise the temperature. Once it has melted, the temperature rises again.

For pure water, the melting point is 0 degrees Celsius and the boiling point is 100 degrees Celsius, at normal pressure.

0246810-1001020temperature (degrees C)time (minutes)melting: flat at 0 degrees C
Heating curve for ice: the temperature rises, stays flat at 0 degrees while melting, then rises again.

The table gives the temperature every two minutes.

Time (minutes)Temperature (degrees C)
0-10
20
40
60
810
1020

Key ideas

  • A heating curve is a line graph of temperature against time.
  • The temperature stays flat while a pure substance melts or boils.
  • The energy during a change of state changes the arrangement of particles, not their temperature.
  • Pure water melts at 0 degrees Celsius and boils at 100 degrees Celsius.
Worked example 4.5.1
From the table, what is the temperature at 4 minutes? Write the number of degrees Celsius.

Method. Read the table at 4 minutes.

  1. Read the table. At 4 minutes, the temperature is recorded as 0 degrees Celsius.
    0
  2. Write the answer. So the temperature is 0 degrees Celsius.
    0 degrees C
  3. Write the answer.
    0 degrees C

Check. The temperature is flat between 2 and 6 minutes. ✓

Worked example 4.5.2
For how many minutes is the temperature flat at 0 degrees Celsius? Write the number of minutes.

Method. Find the start and end of the flat part.

  1. Find the start. The flat part starts at 2 minutes.
    2
  2. Find the end. The flat part ends at 6 minutes.
    6
  3. Subtract. The length of the flat part is 6 minus 2.
    6 - 2 = 4
  4. Write the answer.
    4 minutes

Check. The melting takes 4 minutes on this curve. ✓

Worked example 4.5.3
What is the temperature at 10 minutes? Write the number of degrees Celsius.

Method. Read the table at 10 minutes.

  1. Read the table. At 10 minutes the temperature is 20 degrees Celsius.
    20
  2. Write the answer. So the temperature is 20 degrees Celsius.
    20 degrees C
  3. Write the answer.
    20 degrees C

Check. Once melted, the temperature rises again. ✓

Watch out

Watch out: a flat part of a heating curve does not mean the heater is off. The energy is still going in, and it is used to change the state.

Exam tip

Exam tip. Read the flat part of a heating curve as the change of state, and give the temperature at which the change happens.

Try it (with your teacher, or a grown-up at home). With a grown-up, record the temperature of ice in a beaker of water every minute, using a thermometer, while the ice melts in a warm room. Look for a time when the temperature stays flat near 0 degrees Celsius. Do not heat the beaker.

Exercise 4E  Heating curves and changes of state

1
FWhat is the temperature of the flat part of the heating curve for pure ice? Write the number of degrees Celsius.
2
FAt what time does the temperature first reach 0 degrees Celsius? Write the number of minutes.
3
FWhat is the temperature at 0 minutes? Write the number of degrees Celsius.
4
FIs the temperature flat between 2 and 6 minutes? Write yes or no.
5
FWhat is the temperature at 8 minutes? Write the number of degrees Celsius.
6
FWhat is the melting point of pure water, in degrees Celsius? Write the number.
7
FWhat is the boiling point of pure water, in degrees Celsius? Write the number.
8
PSHow many degrees does the temperature rise between 6 and 10 minutes? Write the number of degrees.
9
PSRuby says the temperature stopped rising because the heater was switched off. Is Ruby right? Write yes or no.
10
PSHow many minutes did the melting take, from the table? Write the number of minutes.
11
PSWorking scientifically: a pupil says the heating curve shows the melting point of pure water is 0 degrees Celsius. Is this supported by the table? Write yes or no.
12
RExplain why the temperature stays flat while the ice melts, even though energy is still going in. Write one sentence.
13
RChallenge: explain why a heating curve for a mixture of salt and water would not stay flat at one temperature while it boils. Write one sentence.

Chapter summary

SkillMethod
Particle modelSolid: close, regular, vibrating. Liquid: close, random, sliding. Gas: far apart, random, fast.
PropertiesSolid: fixed shape and volume. Liquid: fixed volume. Gas: fills its container and is easily compressed.
Changes of stateMelting, freezing, evaporation, boiling, condensation and sublimation. They are physical changes.
EnergyMelting, evaporation, boiling and sublimation take in energy. Freezing and condensation give energy out.
MassMass is conserved in a change of state.
Heating curvesThe temperature is flat while a pure substance changes state; pure water melts at 0 degrees and boils at 100 degrees.

Can you…?

  • I can describe the arrangement and movement of particles in a solid, a liquid and a gas
  • I can explain the shape, volume and compressibility of each state
  • I can name the changes of state, and say whether energy is taken in or given out
  • I can explain why mass is conserved in a change of state
  • I can read a heating curve, and explain the flat part

Key terms

particle model
a model that explains the states of matter using moving particles
solid
a state with a fixed shape and volume, and particles in fixed positions
liquid
a state with a fixed volume, which takes the shape of its container
gas
a state with no fixed shape or volume, which fills its container
melting
a change from solid to liquid, taking in energy
freezing
a change from liquid to solid, giving out energy
evaporation
a change from liquid to gas at the surface, at any temperature
boiling
a change from liquid to gas throughout the liquid, at the boiling point
condensation
a change from gas to liquid, giving out energy
sublimation
a change from solid straight to gas
heating curve
a line graph of temperature against time as a substance is heated
compressed
squashed into a smaller volume

Exercise 4Z  Check-up

1
Which state has particles that are close together in a regular pattern? Write the word.
2
Which state has particles that are far apart and move quickly in all directions? Write the word.
3
Which change turns a solid into a liquid? Write the word.
4
Which change turns a gas into a liquid? Write the word.
5
Does a gas take the shape of its container? Write yes or no.
6
Are solids and liquids easy to compress? Write yes or no.
7
A sealed bag of ice has a mass of 80 grams. After it melts, what is its mass? Write the number of grams.
8
A heating curve for ice is flat at 0 degrees Celsius. What is the melting point? Write the number of degrees Celsius.
9
Name the change in which energy is given out and a liquid turns into a solid. Write the word.
10
Explain why a gas can be compressed easily. Write one sentence.

Total 15 marks. Allow about 15 minutes.

Exercise 4X  Practice test questions

1
FWhich state has particles that slide past each other and have a fixed volume? Write the word.[1 mark]
2
RExplain why a solid keeps its shape. Write one sentence.[2 marks]
3
FName the change in which a solid turns straight into a gas. Write the word.[1 mark]
4
FA sealed bag has 75 grams of ice. It melts. What is the mass of the water? Write the number of grams.[1 mark]
5
PSFrom a heating curve, the temperature is 0 degrees Celsius from 2 to 6 minutes. For how many minutes is it flat? Write the number of minutes.[2 marks]
6
FWhich change takes in energy: freezing or melting? Write the word.[1 mark]
7
RExplain why a gas is hard to describe with a fixed volume. Write one sentence.[2 marks]
8
PSBen says that water evaporating from a dish is a chemical change. Is Ben right? Write yes or no.[2 marks]
9
FWrite the boiling point of pure water in degrees Celsius. Write the number.[1 mark]
10
PSA cup of water loses 25 grams by evaporation into the air. The cup weighs 200 grams after. What was its mass before? Write the number of grams.[2 marks]
← Chapter 3: Specialised cells, tissues and organsChapter 5: Diffusion and density →

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