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The particle model is a simplified way of thinking about matter. It explains the properties of solids, liquids, and gases in terms of the arrangement, movement, and energy of particles. This lesson covers the model in detail, including how it accounts for the different behaviours of the three states of matter.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed pattern (lattice) | Close together but irregularly arranged | Far apart, randomly arranged |
| Movement | Vibrate about fixed positions | Move around each other (flow) | Move rapidly in all directions |
| Spacing | Very close | Close (slightly more than solid) | Very far apart |
| Forces | Strong forces hold particles in place | Weaker forces — particles can slide past | Very weak forces — particles move independently |
| Shape | Fixed | Takes the shape of the container | Fills the container |
| Volume | Fixed | Fixed | Fills the container |
| Compressible? | No | Almost no | Yes (lots of space between particles) |
| Density | High | Medium–High | Low |
flowchart LR
subgraph Solid
S1["● ● ●"]
S2["● ● ●"]
S3["● ● ●"]
end
subgraph Liquid
L1["● ●"]
L2[" ● ● ●"]
L3["● ●"]
end
subgraph Gas
G1["● ●"]
G2[" ●"]
G3["● ●"]
end
Solid -->|"Melting"| Liquid
Liquid -->|"Boiling"| Gas
style Solid fill:#3b82f6,color:#fff,stroke:#2563eb
style Liquid fill:#10b981,color:#fff,stroke:#059669
style Gas fill:#f59e0b,color:#000,stroke:#d97706
Particles are held in fixed positions by strong forces. They can only vibrate, not move apart, so the solid maintains its shape.
Particles can slide past each other because the forces between them are weaker than in solids. This allows liquids to take the shape of their container.
There are large gaps between gas particles. When pressure is applied, the particles are pushed closer together, reducing the volume.
Gas particles move rapidly and randomly. They collide with the walls of the container, exerting a force on the walls. This force per unit area is pressure.
The internal energy of a substance is the total kinetic energy and potential energy of all the particles in the substance.
| Energy Type | What It Describes |
|---|---|
| Kinetic energy | Energy due to the movement (vibration or motion) of particles |
| Potential energy | Energy due to the position of particles relative to each other (related to the forces between them) |
When you heat a substance:
Temperature is a measure of the average kinetic energy of the particles. If two objects are at the same temperature, their particles have the same average kinetic energy — even if they feel different (e.g. a metal rail feels colder than a wooden fence at the same temperature because metal conducts heat away from your hand faster).
The simple particle model has limitations:
| Assumption | Reality |
|---|---|
| Particles are solid spheres | Atoms have internal structure (nucleus, electrons) |
| All particles in a substance are identical | Mixtures contain different particle types |
| Forces between particles are simple | Intermolecular forces vary in type and strength |
| Particles have no size compared to gaps (in gases) | Particles do have a finite size |
Despite these limitations, the model is extremely useful for explaining everyday phenomena at GCSE level.
A drop of ink spreads through water without stirring. Explain using the particle model.
Answer: The ink particles and water particles are in constant random motion. As they move, the ink particles collide with water particles and gradually spread out from a region of high concentration to a region of low concentration. This process is called diffusion. It happens faster at higher temperatures because particles have more kinetic energy and move faster.
In a gas, particles are far apart with large spaces between them. This means there is less mass per unit volume, so the density is lower. In a liquid, particles are close together — more mass is packed into the same volume, giving a higher density.
Heating gives particles more kinetic energy. They move faster, colliding with the walls of the container more frequently and with greater force. Since pressure = force ÷ area, both factors increase the pressure.
| Mistake | Correction |
|---|---|
| Saying particles "expand" when heated | Particles do not expand — they move faster and the gaps between them increase |
| Saying there are no forces in gases | There are very weak forces between gas particles (they are just much weaker than in solids/liquids) |
| Confusing particle movement in solids | Solid particles vibrate in fixed positions — they do NOT move from place to place |
| Saying temperature = total energy | Temperature = average kinetic energy; total internal energy includes potential energy too |
| Drawing gas particles touching each other | Gas particles should be drawn far apart with empty space between them |
| Saying "particles melt" | Particles do not change — the substance melts; particles gain energy and overcome forces |
When a gas in a flexible container (e.g. a balloon) is heated, the balloon expands. Explain using the particle model.
Answer: Heating gives the gas particles more kinetic energy, so they move faster. The faster-moving particles exert a greater force when they collide with the walls of the balloon. This pushes the balloon walls outward, increasing the volume. In a rigid container, the volume cannot change, so instead the pressure increases.
In 1827, Robert Brown observed pollen grains jiggling randomly in water under a microscope. This was later explained by Einstein: the pollen grains are being constantly bombarded by invisible water particles from all directions.
| Observation | Explanation |
|---|---|
| Pollen grains jiggle randomly | Unequal bombardment by water molecules from different directions |
| Movement never stops | Water molecules are in constant random motion |
| Smaller particles jiggle more | They have less mass, so each collision has a bigger effect |
| Higher temperature → more jiggling | Water molecules have more kinetic energy at higher temperatures |
Smoke cell experiment: Smoke particles viewed under a microscope show the same random jiggling — evidence that air molecules are constantly colliding with them.
| Observation | Key Particles Concept |
|---|---|
| Perfume spreads across a room | Diffusion — particles move randomly from high to low concentration |
| A sealed balloon deflates slowly over days | Gas particles are small enough to slowly diffuse through tiny gaps in the rubber |
| Solids cannot be poured | Particles are held in fixed positions by strong forces |
| You can walk through air but not through a wall | Air particles are far apart (low density); wall particles are locked tightly together |