How does the arrangement and movement of particles explain the properties of solids, liquids and gases?
Describe the arrangement, movement and energy of particles in solids, liquids and gases and use the particle model to explain their properties
A focused answer to the N(A) Chemistry outcome on the three states of matter. The arrangement, spacing, movement and energy of particles in solids, liquids and gases, and how the model explains shape, volume and compressibility.
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What this dot point is asking
The syllabus wants you to describe how the tiny particles that make up matter are arranged, how they move, and how much energy they have in each of the three states, solid, liquid, and gas, and then to use this particle model to explain everyday properties such as why a solid keeps its shape, why a liquid flows, and why a gas can be squeezed. The key idea is that all matter is made of particles, and the only difference between the states is how those particles are arranged and how much they move.
The answer
Particles in a solid
In a solid the particles are:
- Arrangement: packed very close together in a fixed, regular pattern.
- Movement: they cannot move around; they only vibrate about fixed positions.
- Energy: they have the least energy of the three states.
Because the particles are held in place by strong forces, a solid has a fixed shape and a fixed volume, and it cannot be compressed (squeezed smaller).
Particles in a liquid
In a liquid the particles are:
- Arrangement: still close together and touching, but not in a regular pattern.
- Movement: they can slide past one another and move around.
- Energy: more than in a solid.
Because the particles can move past each other, a liquid flows and takes the shape of its container, but because they stay close together the liquid has a fixed volume and cannot be compressed.
Particles in a gas
In a gas the particles are:
- Arrangement: far apart with large spaces between them.
- Movement: they move quickly in all directions and collide with the walls.
- Energy: the most of the three states.
Because the particles are far apart and free to move, a gas fills its container completely (no fixed shape or volume) and can be compressed, because there is empty space to push the particles into.
Examples in context
Example 1. Why gases are stored under pressure. A diver's air tank holds a large amount of gas in a small steel cylinder. This works because the gas particles are far apart, so squeezing them into the empty spaces packs a lot of gas into a small volume. The same idea explains why a bicycle pump can force air into a tyre.
Example 2. Why you can pour a liquid but not a solid block. Water pours from a jug because its particles slide past one another and flow, while a block of ice keeps its shape because its particles are locked in fixed positions. The single change from solid to liquid, the particles becoming free to move, explains the difference in behaviour.
Try this
Q1. State the arrangement and movement of the particles in a gas. [2 marks]
- Cue. The particles are far apart with large spaces between them, and they move quickly in all directions, colliding with the container walls.
Q2. Explain, using the particle model, why a solid cannot be compressed. [2 marks]
- Cue. The particles are already packed very close together with no spaces between them, so they cannot be pushed any closer, and the solid keeps its volume.
Q3. A gas is put into a sealed syringe and the plunger is pushed in. Explain what happens to the gas particles. [2 marks]
- Cue. The particles are pushed closer together into the empty spaces between them, so the gas is compressed into a smaller volume; the particles themselves do not change.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksUsing the particle model, explain (a) why a gas can be squeezed into a smaller volume but a liquid cannot, and (b) why a solid has a fixed shape.Show worked answer →
(a) In a gas the particles are far apart with large empty spaces between them, so they can be pushed closer together when squeezed. In a liquid the particles are already touching with almost no space between them, so a liquid cannot be compressed.
(b) In a solid the particles are held in fixed positions by strong forces and can only vibrate about those positions, so they cannot move past each other. This fixed arrangement gives the solid a fixed shape.
What markers reward: gas particles far apart with spaces to close up, liquid particles touching so it cannot be compressed, and solid particles in fixed positions held by strong forces.
Original3 marksA liquid takes the shape of its container but keeps the same volume. Explain both of these observations using the particle model.Show worked answer →
The particles in a liquid are close together but are not held in fixed positions, so they can slide and move past one another. Because they can move around, the liquid flows and takes the shape of its container. Because the particles stay close together and touching, the total volume does not change.
What markers reward: particles able to slide past each other so the liquid flows to the container shape, and particles staying close together so the volume is fixed.
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