📏 Fixed shape
A steel ruler keeps its shape when you pick it up. Its particles are held in fixed positions in a regular pattern. They can only vibrate, so they cannot move to new places and the shape cannot change.
Change of State » Particles in Solids, Liquids and Gases
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 5.10P
- How particles are arranged in solids, liquids and gases
- How the particles move in each state
- How to draw the three particle diagrams for the exam
- How the particle model explains shape, flowing, squashing and density
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseThe particle model
This lesson is examined in Paper 2 only.
Everything around you is made of tiny particles that are far too small to see. A steel spoon, the tea in a cup and the air in the room are all made of particles. What makes them so different is how those particles are arranged and how they move.
The exam asks you to describe exactly those two things for each state, so learn them as two separate lists: arrangement and motion.
Key terms:
- Particle model: a way of picturing all matter as made of tiny particles, used to explain how solids, liquids and gases behave.
- States of matter: the three forms a substance can be in: solid, liquid and gas.
- Arrangement: how the particles are placed. This covers how close together they are and whether they are in a pattern.
- Compress: to squash something into a smaller volume.
In the model we draw each particle as a small circle. The particles themselves do not change from one state to another. Particles of water are the same in ice, in liquid water and in steam. Only their arrangement and motion are different.
Arrangement and motion in each state
🔩 Solid
Arrangement: the particles are close together, touching their neighbours, in a regular pattern of neat rows.
Motion: the particles vibrate about fixed positions. They jiggle on the spot but cannot move from place to place.
💧 Liquid
Arrangement: the particles are still close together, mostly touching, but the arrangement is irregular. There is no pattern.
Motion: the particles move around each other. They slide past their neighbours and keep changing places.
💨 Gas
Arrangement: the particles are far apart, with large spaces between them, and there is no pattern.
Motion: the particles move quickly and randomly. Each one travels in a straight line until it hits another particle or a wall, then heads off in a new direction.
Notice that the particles are moving in all three states. Even in a solid they are never still.
The forces between the particles explain the differences. In a solid, strong forces of attraction hold each particle in its place. In a liquid, the forces still keep the particles close together, but the particles are free to move past each other. In a gas, the particles are so far apart that the forces between them are very weak, so they fly around freely.
The three states side by side
| Solid | Liquid | Gas | |
|---|---|---|---|
| How close are the particles? | Close together | Close together | Far apart |
| Is there a pattern? | Yes, a regular pattern | No, irregular | No, random |
| How do the particles move? | Vibrate about fixed positions | Move around each other | Move quickly and randomly in all directions |
| Shape | Fixed | Takes the shape of the container it is in | Spreads out to fill the whole container |
| Can it flow? | No | Yes | Yes |
| Can it be squashed? | No | No | Yes, easily |
| Density | High | High | Very low |
How to draw the particle diagrams
You may be asked to draw or complete a particle diagram. Start each one with a square box and use small circles, all the same size, for the particles.
- Solid: draw the circles in neat rows, each one touching its neighbours, with every row lined up with the row below. Fill the bottom of the box with at least three full rows.
- Liquid: draw about the same number of circles, still touching their neighbours, but jumbled up with no rows. They should fill the bottom of the box. Leave only tiny gaps, never a gap big enough to fit another particle.
- Gas: draw only a few circles, perhaps five or six, spread across the whole box. Put them far apart in random places, not in a line or a pattern.
To show motion, you can add tiny curved lines on each side of a solid particle to show vibration, and a short arrow on each gas particle, with the arrows pointing in different directions.
Using the model to explain what we see
🥛 Flowing
Milk pours from a carton and takes the shape of the glass. Its particles can move around each other, so the liquid flows. Gas particles are free to move too, so gases also flow. A gas spreads out until it fills its whole container.
💉 Squashing
Seal the end of a syringe full of air and you can push the plunger in easily. There is a lot of empty space between gas particles, so they can be pushed closer together. Fill the syringe with water and the plunger hardly moves. The particles in a liquid or a solid are already touching, so there is no space to push them into.
⚖️ Density
In solids and liquids the particles are packed close together, so there is a lot of mass in each cubic centimetre and the density is high. In a gas the particles are far apart, so the same volume holds far fewer particles. There is much less mass in each cubic centimetre, so the density is very low.
You met density values in the lesson called Density. Water has a density of 1000 kg/m3, but air is only a little over 1 kg/m3. The particle model tells you why: the particles in air are much further apart.
Worked example
Question: A gas cylinder can hold a very large amount of gas. Explain, using ideas about particles, why a gas can be compressed but a solid cannot.
Step 1: describe the spacing in the gas. The particles in a gas are far apart, with large spaces between them.
Step 2: say what squashing does. When the gas is squashed, the particles are pushed closer together into the empty space, so the gas takes up a smaller volume.
Step 3: compare with the solid. The particles in a solid are already close together and touching. There is no space for them to move into, so the solid cannot be compressed.
A good 'explain' answer always links what the particles are doing to what we see.
Common mistakes
- Drawing liquid particles spread out. Liquid particles are still close together and mostly touching. A liquid drawn with big gaps looks like a gas and loses the mark.
- Saying solid particles do not move. They vibrate about fixed positions. Always use the word 'vibrate'.
- Mixing up arrangement and motion. If the question asks for both, give at least one point about each.
- Drawing particles of different sizes. The particles of a substance are the same size in all three states.
- Drawing gas particles in a pattern. They must be far apart and in random places.
Exam-style question
A student has an iron nail, some cooking oil in a beaker and some air in a sealed syringe.
(a) Describe the arrangement and motion of the particles in the iron nail. [3 marks]
(b) The student draws a particle diagram for the cooking oil. She draws six circles spread far apart in a box. State what is wrong with her diagram. [1 mark]
(c) The student can push the plunger of the syringe in, even though the end is sealed. Explain why, using ideas about particles. [2 marks]
(d) Explain why the air has a much lower density than the iron. [2 marks]
Model answer
(a) The particles are close together (1) in a regular pattern (1). They vibrate about fixed positions (1).
(b) The particles in a liquid should be close together, touching each other, not far apart (1).
(c) The particles in a gas are far apart, with a lot of space between them (1), so they can be pushed closer together into a smaller volume (1).
(d) The particles in the air are much further apart than the particles in the iron (1), so there is much less mass in the same volume (1).
Exam tip
Part (a) asks for arrangement and motion, so give both. Write one sentence about how the particles are placed and one about how they move. An answer with three points about arrangement and nothing about motion cannot score full marks.