The particle model
What you'll learn
- How the particle model represents solids, liquids and gases.
- How changes of state happen without making a new substance.
- How to tell physical changes from chemical changes.
- Why the simple “particles as hard spheres” model is useful but limited — this bit is Higher Tier only.
Why chemists use models
Chemistry deals with particles that are far too small to see directly. An atom is typically about 1×10−101 \times 10^{-10}1×10−10 m across, so chemists use models to help explain what we observe.
Particle model
The particle model is a simplified way of representing matter as tiny particles. It describes how the particles are arranged, how they move, and how strongly they attract each other.
A model is not the same as reality. It is a useful tool: it helps you explain observations, but it does not include every detail.
Particle
A particle means a tiny piece of matter. At GCSE this could mean an atom, a molecule, or an ion, depending on the substance.
States of matter
The three main states of matter are solid, liquid and gas. A state of matter is the physical form a substance is in.
The same substance can exist in different states. For example, water can be solid ice, liquid water, or steam/water vapour.
The particle model explains the states of matter using three ideas:
- Arrangement — how close together and ordered the particles are.
- Movement — how the particles move.
- Forces of attraction — how strongly particles pull on each other.
Solids
In a solid, particles are very close together in a fixed, regular arrangement. They cannot move from place to place, but they do vibrate about fixed positions.
This explains why solids usually:
- have a fixed shape
- have a fixed volume
- are difficult to compress
Solids
Solid particles are close together, strongly attracted to each other, and only vibrate around fixed positions.
Liquids
In a liquid, particles are still close together, but they are arranged randomly. They can move past each other.
This explains why liquids:
- have a fixed volume
- take the shape of their container
- are difficult to compress
- can flow
Liquids
Liquid particles are close together but can move around each other, so a liquid flows but keeps roughly the same volume.
Gases
In a gas, particles are far apart, randomly arranged, and move quickly in all directions.
This explains why gases:
- have no fixed shape
- have no fixed volume
- spread out to fill their container
- are easy to compress
A very important GCSE idea is that there is mostly empty space between gas particles.
Particles do not expand
When a substance is heated, the particles themselves do not get bigger. They move more, and in gases they may become further apart.
Explaining why a gas can be compressed
A gas in a syringe is pushed into a smaller volume. Explain this using the particle model.
- Compare the spacing of gas particles with the spacing of particles in solids and liquids: gas particles are much further apart.
- Apply the idea of empty space: because there is lots of empty space between gas particles, the particles can be pushed closer together.
- Link this to the observation: the gas volume decreases because the spaces between particles get smaller, not because the particles shrink.
Changes of state
A change of state happens when a substance changes between solid, liquid and gas.
Change of state
A change of state is a physical change where a substance changes state, such as solid to liquid or liquid to gas, without forming a new substance.
The main changes of state are:
- melting: solid → liquid
- freezing: liquid → solid
- boiling: liquid → gas, throughout the liquid at its boiling point
- evaporating: liquid → gas, from the surface of the liquid
- condensing: gas → liquid
- subliming: solid → gas, without becoming liquid first
- depositing: gas → solid, without becoming liquid first
Energy and changes of state
When a substance is heated, energy is transferred to its particles. The particles move more or vibrate more strongly.
During melting and boiling, energy is used to overcome some forces of attraction between particles. The particles separate more or become able to move more freely.
During freezing and condensing, energy is transferred from the particles to the surroundings. The particles move less and become more strongly held in position.
State changes are reversible
Changes of state are usually reversible. Ice can melt to form water, and water can freeze to form ice again. The substance is still water.
Explaining melting using particles
Ice is heated and melts to form liquid water. Explain what happens to the particles.
- Identify the starting state: in solid ice, the particles are close together and vibrate about fixed positions.
- Apply the energy transfer: heating transfers energy to the particles, so they vibrate more strongly.
- Explain the change in arrangement: some forces of attraction are overcome, so particles can move past each other.
- Link to the final state: the substance becomes a liquid because the particles are still close together but are no longer fixed in place.
Use particle language
For state changes, aim to mention energy, movement, arrangement, and forces of attraction. These are the ideas examiners are usually looking for.
Physical changes and chemical changes
Not all changes are the same. Chemistry makes a big distinction between physical changes and chemical changes.
Physical change
A physical change changes the form, state, or appearance of a substance, but no new substance is made.
Melting, freezing, evaporating, condensing and dissolving are physical changes. The particles may be arranged differently, but the particles themselves are not changed into new particles.
For example:
- Ice melting: water particles are still water particles.
- Steam condensing: water particles are still water particles.
- Iodine subliming: iodine particles are still iodine particles.
Chemical change
A chemical change is a change where new substances are made. The particles are rearranged into different combinations.
Burning, rusting and reacting acids with metals are chemical changes. The original particles are no longer just arranged differently — atoms have been rearranged to make new substances.
For example, magnesium burning in oxygen forms magnesium oxide:
2Mg(s) + O₂(g) → 2MgO(s)
The product, magnesium oxide, has different properties from magnesium and oxygen.
Distinguishing physical and chemical changes
A candle is lit. Some wax melts near the flame, and some wax vapour burns. Decide which change is physical and which is chemical.
- Consider the melting wax: solid wax becomes liquid wax, but the wax substance is still wax, so this is a physical change.
- Consider the burning wax vapour: it reacts with oxygen and forms new substances such as carbon dioxide and water, so this is a chemical change.
- Compare reversibility: liquid wax can cool and solidify again, but the burnt wax cannot simply be turned back into wax by cooling.
Reversible does not always mean easy
Many physical changes are reversible, but that does not mean they reverse instantly or easily in the classroom. The key GCSE point is whether a new substance has been made.
Elements, atoms and compounds
The simple particle model often shows particles as identical circles. That is helpful for thinking about states of matter, but real substances are made from different types of particles.
Element
An element is a substance made from only one type of atom.
Examples include oxygen, carbon, iron and copper. Each element has its own type of atom.
Compound
A compound is a substance made when atoms of two or more different elements are chemically joined together.
Water is a compound because it contains hydrogen atoms and oxygen atoms chemically joined together. Carbon dioxide is a compound because it contains carbon atoms and oxygen atoms chemically joined together.
The particle model can show whether particles are close together or far apart, but it does not fully explain why copper conducts electricity, why oxygen is a gas, or why salt dissolves in water. For that, you need to know what the particles are and how they are bonded.
Limitations of the simple particle model
This section is Higher Tier only.
At GCSE, particles are often drawn as small solid spheres, a bit like tiny bowling balls. This is useful because it makes states of matter easy to picture. However, it is not a perfect model.
All models have limits
The particle model is good for explaining states of matter and changes of state, but it is too simple to explain all chemical properties.
Limitation 1: particles are not really hard balls
Atoms and molecules are not tiny solid bowling balls. They have internal structure, such as electrons, and their outer regions are not hard surfaces.
Limitation 2: particles can attract each other
If particles were truly like inelastic bowling balls, they would simply collide and not stick together. But real particles have forces of attraction between them. These attractions help explain why substances can be solids or liquids.
Inelastic
An inelastic object does not bounce back perfectly after a collision; some energy is transferred to the surroundings, often as heat or sound.
Limitation 3: the model does not show bonds
The simple particle model does not show how atoms are chemically joined in compounds. It also does not show the difference between atoms, molecules and ions unless extra labels or diagrams are added.
Limitation 4: drawings can be misleading
Particle diagrams are two-dimensional pictures of three-dimensional substances. A flat diagram can make particles look more ordered or more separated than they really are.
Evaluating a particle model
A diagram shows gas particles as identical hard circles moving inside a box. Explain one strength and one limitation of this model.
- Identify the useful feature: the particles are spread far apart, which helps explain why gases can be compressed and fill a container.
- Identify what the model leaves out: the circles do not show the actual structure of atoms or molecules, such as electrons or chemical bonds.
- Judge the model carefully: it is useful for explaining gas behaviour, but it is too simple to explain why different gases have different chemical properties.
In the exam
- For state questions, describe arrangement, movement and forces of attraction between particles.
- For change-of-state questions, say whether energy is transferred to or from the particles, and remember no new substance is made.
- For Higher Tier model questions, give both sides: explain what the model helps with, then state a clear limitation.
Check yourself
- Why can gases be compressed much more easily than solids or liquids?
- What is the difference between melting and burning in terms of particles?
- Why is the “particles as tiny bowling balls” model useful but not fully accurate?
