Lesson focus: The properties of solids, liquids and gases emerge from particleA tiny piece of matter, such as an atom or molecule. arrangement, movement, energy and attraction.
Use the particle modelA simple way of showing something that helps us explain and predict, like a drawing of particles as small balls. to explain state properties and predict changes caused by heating or cooling.
Build on earlier learning
States of matterYear 7 introduced particle diagrams for states. This lesson strengthens the model through energy and force explanations.
You will be successful when you can…
- Compare all three states.
- Link properties to particle behaviour.
- Separate particle motion from bulk motion.
- Evaluate the limits of diagrams.
Words that carry the science
- state of matter
- A physical form described here as solid, liquid or gas.
- particle
- A model unit representing an atom, molecule or ion in a substance.
- internal energy
- Energy associated with particle motion and interactions in a system.
- compressible
- Able to decrease appreciably in volume under pressure.
- density
- Mass per unit volume.
- model
- A simplified representation used to explain and predict observations.
Arrangement and movement explain state properties
01Arrangement and movement explain state properties
In a solidA state of matter that keeps its own shape and volume, because its particles are held in place., particleA tiny piece of matter, such as an atom or molecule. occupy closely packed positions and vibrate. In a liquidA state of matter that can flow and takes the shape of its container, but keeps the same volume., particles remain close but continually change neighbours. In a gasA state of matter that spreads out to fill any container, because its particles are far apart and move quickly., particles are widely separated and move rapidly in random directions.
Solids retain shape because particles cannot move past one another easily. Liquids flow but keep nearly fixed volume. Gases fill their container because freely moving particles spread through the available space.
02Spacing explains compression and density
There is little empty space between particles in most solids and liquids, so they are difficult to compress. Gases contain much more empty space and compress readily.
densityThe mass per unit volume of a material (density = mass ÷ volume), measured in g/cm³ or kg/m³. depends on particle mass and spacing. The modelA simple way of showing something that helps us explain and predict, like a drawing of particles as small balls. does not imply that every solid is denser than every liquid; substance identity and structure also matter.
03A useful model is deliberately incomplete
Particle circles are not literal photographs and are not usually to scale. Empty-looking space in a gas is not filled with “air” unless air particles are part of the sample.
A stronger explanation names arrangement, movement, energy and attractions, then links these microscopic ideas to an observed macroscopic property.
Particles in solids, liquids and gases
Change the temperature of some water and watch its particles. The particles themselves do not change, only how they are arranged and how they move.
Controls
Explain gas compression
A sealed syringe of air can be compressed, but a water-filled syringe barely compresses.
- Air is a gas whose particles have large spaces between them.
- Pushing the plunger reduces those spaces.
- Water particles are already close together.
- There is little empty space to remove, so its volume changes very little.
How to make the reasoning visible: Compression changes particle spacing, not particle size.
Interrogate the particle model
Match each observation to the particle evidence that best explains it.
| Observation | Arrangement | Movement | Explanation |
|---|---|---|---|
| metal block keeps shape | close, ordered | vibrate in positions | positions resist rearrangement |
| water flows | close, disordered | change neighbours | takes container shape |
| perfume fills room | widely spaced | rapid and random | spreads through space |
| air compresses | widely spaced | random | spaces become smaller |
Why is a liquid not easily compressed even though it flows?
Why can particle diagrams mislead if drawn without labels?
Check your understanding
Answer from memory, then check against the suggested answer.
How do particles move in a solid?
Why are gases compressible?
Do particles expand on heating?
Explain why a gas fills a container but a liquid forms a surface.
Why is glass at room temperature modelled as a solid despite lacking a regular crystal pattern?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Bromine is a liquid at room temperature. A few drops of bromine evaporate and the brown vapour fills a gas jar.
Explain, using the particle model, why bromine is a liquid.
(3)Explain why the vapour fills the gas jar.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., link each visible property to a particular feature of the particle model.