The three states of matter: arrangement, movement and energy
Everything in this unit is examined on both papers, except the two sections that say they are Paper 2 only.
The unit starts with the idea that the whole course rests on: every substance is made of tiny particles, and how those particles are arranged, how they move and how much energy they have decides whether the substance is a solid, a liquid or a gas.
Particles and the forces between them
The particles may be atoms, molecules or ions.
Whatever they are, two things compete.
The particles attract each other, which pulls them together.
They also have energy of movement (kinetic energy), which shakes them apart.
When the attractions win, the particles stay packed together.
When the energy of movement wins, the particles escape from each other and fly about.
Heating a substance gives its particles more energy.
So the same substance can be a solid when it is cold, a liquid when it is warmer and a gas when it is hotter still.
Water is the everyday example: ice, liquid water and steam are all made of the same water molecules.
The diagram below shows the three states as particle pictures.
Each circle is one particle.
The three states side by side
Solid
Liquid
Gas
Arrangement
close together, in a regular arrangement (a lattice)
close together, in a random arrangement
far apart, in a random arrangement
Movement
vibrate about fixed positions
move around each other (slide past each other)
move quickly and randomly in all directions
Energy of the particles
lowest
higher
highest
Forces between particles
strongest
weaker
very weak
Why each state behaves as it does
- A solid keeps its shape. Its particles are held in fixed positions by strong attractions. They vibrate, but they cannot move from place to place, so the solid cannot flow.
- A liquid flows and takes the shape of its container. The particles are still touching, so a liquid has a fixed volume and is very hard to squash. But they can move around each other, so the liquid flows to fill the bottom of its container.
- A gas spreads out to fill any container and can be compressed. The particles are far apart, with empty space between them. Pushing them closer together is easy, which is why the air in a bicycle pump can be squashed but the water in a syringe cannot.
The spacing also explains density.
In a solid or a liquid the particles are packed closely, so a small volume holds many particles.
In a gas the same number of particles takes up roughly a thousand times more space, so gases have much lower densities.
Drawing particle diagrams
You may be asked to complete a box to show a state.
Examiners look for the same features every time:
- Solid: particles touching, in neat rows, filling the space evenly.
- Liquid: most particles touching their neighbours, no regular pattern, gathered at the bottom of the box with a few small gaps.
- Gas: exactly the number of particles asked for, drawn the same size as the others, far apart, not touching, spread over the whole box and not in a pattern.
- Solid: particles close together in a regular arrangement, vibrating about fixed positions.
- Liquid: particles close together in a random arrangement, moving around each other.
- Gas: particles far apart in a random arrangement, moving quickly and randomly in all directions.
- Kinetic energy: the energy a particle has because it is moving; the particles of a gas have the most.
Describe the arrangement and the movement of the particles in a block of copper at room temperature.
(2)
1. Arrangement: the particles are close together (touching) in a regular arrangement (1).
2. Movement: they vibrate about fixed positions (1).
Answer: two separate points, one for arrangement and one for movement.
"The particles do not move" scores nothing for movement: particles in a solid always vibrate.
A box shows six particles of a gas, drawn far apart.
Complete the box by adding three more particles of the gas.
(1)
1. Draw exactly three circles, the same size as the ones already printed.
2. Place them far apart from each other and from the printed particles, with none touching.
3. Spread them over the whole box, including the empty corners, and avoid rows or a grid.
Answer: one mark for three particles placed randomly, far apart and not touching.
Drawing two or four, or lining them up, loses the mark.
Cooking oil poured into a bottle takes the shape of the bottle.
A cube of butter put in the same bottle keeps its shape.
Explain this difference in terms of particles.
(2)
1. In the liquid oil the particles can move around each other (slide past each other), so the oil flows to fill the bottom of the bottle (1).
2. In the solid butter the particles only vibrate about fixed positions, so the butter cannot flow (1).
Answer: one linked point for each substance.
"Liquids flow" alone repeats the question; the mark needs what the particles do.
In a "complete the diagram" item, count before you draw: the mark is lost for one particle too many or too few.
Gas particles must be far apart, not touching, the same size as the printed ones, and scattered over the whole box with no pattern.
- Drawing gas particles in rows. Gas particles move randomly in all directions, so a regular pattern is wrong. Scatter them.
- Saying the particles in a solid do not move. They vibrate about fixed positions. What they cannot do is move from place to place.
- Drawing liquid particles far apart. Liquid particles are almost as close as solid particles; only the arrangement is random. A liquid is not a "half-way gas".
- Saying particles get bigger when a substance is heated. The particles stay the same size. They move faster and, in a gas, spread further apart.
Check your understanding
1. At room temperature, the particles of a substance are close together in a random arrangement and move around each other. Which substance could it be? (1)
- A copper
- B ethanol
- C oxygen
- D sodium chloride
Total [1]
2. An empty box represents a small volume of carbon dioxide gas. Draw five particles in the box to show the arrangement of the particles in the gas. (1)
Total [1]
3. 1.0 g of liquid water has a volume of about 1.0 cm³. When it boils, the same 1.0 g of steam at 100 °C has a volume of about 1700 cm³.
(a) State what happens to the number of water molecules when the water boils. (1)
(b) Explain, in terms of particles, why the steam takes up a much larger volume than the liquid water. (2)
(c) Give the state of matter in which the particles have the most energy. (1)
Total [4]
4. A sealed gas jar holds a little liquid bromine. After a few minutes the liquid is still at the bottom of the jar, but brown bromine gas fills the whole space above it. Explain, in terms of particles, why the liquid stays at the bottom but the gas fills the jar. (2)
Total [2]
5. Ethanol can be a liquid or a gas. Describe the differences between the particles in liquid ethanol and the particles in ethanol gas. Refer to arrangement, movement and energy in your answer. (4)
Total [4]
Answers
1. B (1). The particles described are those of a liquid, and ethanol is a liquid at room temperature. A is incorrect as copper is a solid, with particles in a regular arrangement. C is incorrect as oxygen is a gas, with particles far apart. D is incorrect as sodium chloride is a solid.
2. Five particles of equal size, far apart, not touching, randomly spread over the box (1). More or fewer than five, touching particles or a pattern scores 0.
3. (a) it stays the same (no molecules are made or lost) (1)
(b) In the liquid the molecules are close together (touching) (1). In the steam they are far apart, with large spaces between them, so the same number of molecules fills a much larger volume (1).
(c) gas (1)
4. In the liquid the particles are close together (attracted to each other) and only move around each other, so the liquid stays together at the bottom (1). In the gas the particles are far apart and move quickly and randomly in all directions, so they spread out to fill the jar (1).
5. Any four from (1 each):
- in the liquid the particles are close together (touching); in the gas they are far apart
- in the liquid the particles move around each other (slide past each other); in the gas they move quickly and randomly in all directions
- the particles in the gas have more (kinetic) energy than those in the liquid
- the forces of attraction between the particles are much weaker (overcome) in the gas
- the liquid has a fixed volume; the gas spreads out to fill its container
Allow a correct comparison stated either way round.