What are the three states of matter, and what happens to a substance when it is heated or cooled?
Describe the three states of matter in terms of particle arrangement, and explain the changes of state when a substance is heated or cooled
A simple, scaffolded answer to the N(T) Science point on the three states of matter. Particle arrangement in solids, liquids and gases, and the changes of state caused by heating and cooling.
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What this dot point is asking
This dot point wants you to describe the three states of matter - solid, liquid and gas - using the idea of tiny particles, and to explain what happens to those particles when a substance is heated or cooled. The big idea is simple: everything is made of particles, and how those particles are arranged and how fast they move decides whether something is a solid, a liquid or a gas. Heating gives particles more energy; cooling takes energy away. That is what causes a substance to change from one state to another.
The answer
The three states of matter
Everything around you is made of tiny particles that are far too small to see. The way these particles are packed together gives us the three states of matter.
A solid has its particles packed closely together in a fixed, regular pattern. The particles can only vibrate on the spot; they cannot move around. This is why a solid keeps its own shape and does not flow. An ice cube, a coin and a wooden ruler are all solids.
A liquid has its particles still close together, but they are no longer in a fixed pattern. They can slide past one another. This is why a liquid can flow and takes the shape of its container, while still keeping the same volume. Water, milk and cooking oil are liquids.
A gas has its particles far apart with large spaces between them. The particles move quickly in all directions. This is why a gas spreads out to fill any container completely. The air around you, steam and the gas used in a stove are all gases.
How particle movement changes
The key idea is energy. The hotter a substance is, the more energy its particles have, and the more they move.
In a solid the particles only vibrate gently. In a liquid they have more energy and can slide around. In a gas they have the most energy and move fastest of all. So as you heat a substance, you give its particles more energy and they move more freely.
Changes of state when heating
When you heat a substance, it can change state. There are two heating changes to know.
Melting is the change from a solid to a liquid. When a solid is heated, its particles gain enough energy to break out of their fixed pattern, so the solid turns into a liquid. Ice melting into water is the everyday example.
Boiling (a fast kind of evaporation) is the change from a liquid to a gas. When a liquid is heated enough, its particles gain enough energy to escape from the liquid and spread out as a gas. Water boiling into steam is the everyday example.
Changes of state when cooling
When you cool a substance, the changes go the other way because the particles lose energy.
Condensation is the change from a gas to a liquid. When a gas is cooled, its particles lose energy, move closer together and form a liquid. Water droplets forming on a cold window or a cold drink can is condensation.
Freezing (also called solidifying) is the change from a liquid to a solid. When a liquid is cooled enough, its particles lose so much energy that they settle into a fixed pattern, forming a solid. Water freezing into ice in a freezer is the everyday example.
Examples in context
Example 1. Steam from a hot shower. When you take a hot shower, hot water turns some liquid water into water vapour (a gas) by evaporation. When that gas hits the cooler bathroom mirror, it loses energy and turns back into tiny liquid droplets. That is condensation, and it is why the mirror fogs up.
Example 2. A melting ice lolly. On a hot day an ice lolly turns from a solid into a sticky liquid. The warm air gives the particles in the frozen lolly more energy, so they break out of their fixed pattern and the lolly melts. Put it back in the freezer and the cooling makes the liquid freeze into a solid again.
Try this
Cue. Name the change of state when water vapour in the air forms droplets on a cold window. Think about a gas being cooled into a liquid: this is condensation.
Cue. Describe the difference between the particles in a solid and the particles in a gas. Solid particles are closely packed in a fixed pattern and only vibrate; gas particles are far apart and move quickly in all directions.
Cue. Explain, using particles, why a metal spoon stays the same shape but water poured from a jug takes the shape of the cup. In the solid spoon the particles are fixed in place, so the shape is kept; in the liquid water the particles can slide past one another, so it takes the shape of its container.
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 marksIce is taken out of a freezer and left on a table in a warm room. (a) Name the change of state that happens to the ice. (b) Describe what happens to the particles as the ice changes. (c) Name the change of state if the resulting liquid is later left to boil in a kettle.Show worked answer →
(a) The ice melts. This is the change from a solid to a liquid.
(b) As the ice is warmed, the particles gain energy and vibrate more. They break away from their fixed positions, so they can now slide past one another. The fixed, regular arrangement of the solid becomes the close but moving arrangement of the liquid.
(c) When the liquid water boils in the kettle it turns into a gas (water vapour). This change of state is boiling (evaporation into steam).
What markers reward: naming the correct change of state (melting, then boiling), and linking the change to particles gaining energy and moving more freely. Always use the words "particles" and "energy".
Original3 marksA student says, 'A gas has no mass because you cannot see it.' (a) State whether the student is correct. (b) Describe the arrangement and movement of the particles in a gas. (c) Give one everyday example that shows a gas takes up space.Show worked answer →
(a) The student is not correct. A gas is still made of particles, so it does have mass, even though we usually cannot see it.
(b) In a gas the particles are far apart, with large spaces between them. They move quickly in all directions and spread out to fill any container they are in.
(c) Any sensible example, such as: blowing up a balloon makes it bigger because the gas fills it, or the smell of food spreads across a room because gas particles move and spread out.
What markers reward: correctly rejecting the wrong idea, describing gas particles as far apart and fast-moving, and giving one clear everyday example of a gas filling space.
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