AQA GCSE Chemistry Foundation (8462), Foundation tier · Bonding, structure and the properties of matter › Bonding, structure and properties
Practise The three states of matter. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Solids, liquids and gases, the changes of state between them, and why different substances melt and boil at different temperatures. You need to predict states from melting and boiling point data, explain changes of state in terms of energy and forces.
Key facts
Solid: close, regular, vibrate about fixed positions
Liquid: close, random, move around each other
Gas: far apart, random, move quickly in all directions
Melting and freezing at the melting point; boiling and condensing at the boiling point
Stronger forces between particles → more energy needed → higher melting and boiling points
Below mp: solid; between mp and bp: liquid; above bp: gas
Notes
Solids, liquids and gases
diagram
In the simple particle model, each particle (atom, ion or molecule) is drawn as a small solid sphere.
Solid: particles packed closely in a regular arrangement; they vibrate about fixed positions.
Liquid: particles close together but randomly arranged; they move around each other.
Gas: particles far apart and randomly arranged; they move quickly in all directions.
Solid: regular, touching. Liquid: random, touching. Gas: far apart.
Changes of state, energy and forces
Melting (solid → liquid) and freezing (liquid → solid) happen at the melting point. Boiling (liquid → gas) and condensing (gas → liquid) happen at the boiling point.
To melt or boil a substance, energy must be transferred to it to overcome the forces between its particles. When it freezes or condenses, energy is transferred to the surroundings.
The stronger the forces between the particles, the more energy is needed, so the higher the melting and boiling points.
The strength of the forces depends on the type of bonding and structure, e.g. strong ionic bonds in sodium chloride, but only weak forces between the molecules in oxygen.
To predict the state at a temperature: below the melting point it is a solid; between the melting and boiling points a liquid; above the boiling point a gas.
Atoms and bulk properties
Melting point, colour, hardness and conductivity are bulk properties: they belong to a large collection of particles, not to a single atom.
For example, a single copper atom is not orange-brown, and a single water molecule does not have a boiling point.
How to answer each type of question
Predict the state of a substance from data
1 to 3 marksGrade 5
Compare the temperature with the melting point first, then the boiling point.
Below the melting point → solid; between the two → liquid; above the boiling point → gas.
Take care with negative numbers: −183 °C is colder than −7 °C.
Example. The table gives data for three substances.
Substance
Melting point in °C
Boiling point in °C
A
−218
−183
B
−7
59
C
801
1413
Give the state of each substance at 25 °C.Show the model answerHide the model answer
A: gas (1) B: liquid (1) C: solid (1)
Describe what happens to the particles during a change of state
2 to 3 marksGrade 5
Say that energy is transferred to (or from) the particles.
Say what happens to the forces between the particles.
Describe the new arrangement and movement of the particles.
Example. Describe what happens to the particles when a solid is heated until it melts.
Show the model answerHide the model answer
Energy is transferred to the particles, so they vibrate more (1). At the melting point they have enough energy to overcome some of the forces holding them in fixed positions (1). The particles can then move around each other in a random arrangement (1).
Don’t lose marks
Getting negative temperatures the wrong way round: −183 °C is lower than −7 °C.
Saying the particles in a solid don't move. They vibrate about fixed positions.
Saying the particles themselves expand or melt. The particles don't change; their energy, movement and arrangement do.
Saying that boiling always breaks bonds. For small molecules only the weak forces between the molecules are overcome.
Writing 'it needs heat' without linking the energy to overcoming the forces between particles.
More tips
Memory tricks
Number line trick: mark the melting point and boiling point on a line, then place the temperature. Left of the mp = solid, between = liquid, right of the bp = gas.
Stronger forces, higher temperatures: the strength of the forces between particles decides the melting and boiling points.
Melting and freezing happen at the same temperature, in opposite directions. So do boiling and condensing.
Exam technique
In data questions, quote the numbers: 'at 25 °C, B is above its melting point (−7 °C) but below its boiling point (59 °C), so it is a liquid'.
Always say what is overcome: 'the forces between the particles', or name the actual bonds or forces.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the changes of stateMelting and freezing happen at the melting point; boiling and condensing happen at the boiling point.
Grade 4
Describe particles in solids, liquids and gasesRegular and vibrating in solids, close but moving around in liquids, far apart and moving quickly in gases.
Grade 5
Predict states from melting and boiling pointsBelow the melting point: solid; between the melting and boiling points: liquid; above the boiling point: gas.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Substances can exist as solids, liquids or gases. Name the change of state when a liquid turns into a gas.
boiling / evaporating
Describe the arrangement and movement of the particles in a liquid.
Close together in a random arrangement; they move around each other.
Complete the sentence. Melting and freezing take place at the same temperature, called the .......... .
melting point
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Substances can exist as solids, liquids or gases.
(a) Name the change of state when a liquid turns into a gas.[1]
(b) At which temperature does a liquid freeze? Tick (✓) one box.[1]
At its melting point
At 0 °C for all substances
At 100 °C for all substances
At room temperature
(c) Which statement describes the particles in a solid? Tick (✓) one box.[1]
Far apart and moving randomly at high speed
Close together and moving around each other
Close together in a regular arrangement, vibrating about fixed positions
Far apart in a regular arrangement
(d) Name the change of state when a gas turns into a liquid.[1]
Show the answer and mark scheme
(a)Answer: boiling / evaporating
boiling / evaporating
(b)Answer: At its melting point
(c)Answer: Close together in a regular arrangement, vibrating about fixed positions
(d)Answer: condensing
condensing / condensation
Question 2Medium7 marks
Particle theory can be used to explain changes of state.
(a) Describe the arrangement and movement of the particles in a liquid.[2]
(b) Describe what happens to the particles when a solid melts.[3]
(c) Sodium chloride melts at 801 °C. Ice melts at 0 °C. Explain this difference in terms of the forces between particles.[2]
Show the answer and mark scheme
(a)Answer: Close together in a random arrangement; they move around each other.
close together / touching in a random arrangement
(particles) move around / slide past each other
(b)Answer: The particles gain energy and vibrate more; some of the forces between them are overcome; they can then move around each other.
particles gain (kinetic) energy / vibrate more
(some of the) forces between the particles are overcome
the particles can move around / past each other
(c)Answer: The forces between the ions in sodium chloride are much stronger than the forces between water molecules in ice, so much more energy is needed to overcome them.
the forces between ions in sodium chloride are much stronger than the forces between water molecules