Particle Model of Matter
This topic explains the distinct properties of solids, liquids, and gases by modelling them as collections of particles. Understanding this model allows you to link the microscopic behaviour of particles (their motion, spacing, and forces) to the macroscopic characteristics we observe.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Solids consist of particles in a fixed, tightly packed arrangement (often a lattice), held by strong forces. Particles can only vibrate about their fixed positions, giving solids a definite shape and volume.
- Liquids have particles that are closely packed but arranged randomly. The forces are weaker than in solids, allowing particles to slide past one another, which means liquids have a fixed volume but take the shape of their container.
- Gases are composed of particles that are far apart with negligible forces between them (except during collisions). They move randomly and at high speeds, causing gases to fill any container and be easily compressed.
- Both liquids and gases are classified as 'fluids' because their particles are able to move from place to place, allowing them to flow.
- Density is significantly lower in the gaseous state compared to the liquid or solid state of the same substance. This is because the particles in a gas are spread out over a much larger volume for the same mass.
› Why does this happen?
Why does heating change a substance's state?
It's all about energy. The particles in a substance are always moving, so they have kinetic energy. When you heat a substance, you increase the energy of its particles. This usually makes them move or vibrate faster, which we measure as a rise in temperature. However, something different happens during a change of state.
Melting (Solid → Liquid): In a solid, particles vibrate in fixed positions. As you heat the solid, they vibrate more and more vigorously. At the melting point, the energy being added is no longer used to increase the particles' kinetic energy. Instead, it's used to overcome the strong forces holding them in place. This is why the temperature stays constant while a substance melts. Once enough energy has been supplied, particles break free from their fixed positions and can slide past one another, forming a liquid.
Boiling (Liquid → Gas): As you continue to heat the liquid, its particles move faster. At the boiling point, the added energy is once again used to overcome forces between particles, not to increase their kinetic energy and temperature. The energy allows particles to break away from each other completely. They escape from the liquid and move far apart at high speeds, becoming a gas. This also explains why gases are so much less dense: the same number of particles (the same mass) takes up a much larger volume.
Formulae
ρ = m / V To relate a substance's mass (m), volume (V), and density (ρ). Use this conceptually to understand why gases are much less dense than solids or liquids for the same mass of substance.
Definitions
- Particle Model
- A scientific model that describes all matter as being made of tiny, constantly moving particles (atoms, ions, or molecules). The energy, spacing, and forces between these particles determine the state of matter.
- Fluid
- A substance that can flow and does not have a fixed shape. This term applies to both liquids and gases.
Worked example
A sealed, flexible balloon filled with a fixed amount of gas is cooled down significantly. Its volume is observed to decrease. Using the particle model, explain why the balloon shrinks.
- 1
The particle model states that gas particles are in constant, random motion.
The temperature of a gas is related to the average kinetic energy of its particles.
- 2
Cooling the gas reduces the average kinetic energy of the particles.
This means they move more slowly.
- 3
Slower-moving particles collide with the inner walls of the balloon less frequently and with less force.
- 4
The constant external atmospheric pressure is now greater than the pressure exerted by the gas particles inside the balloon.
- 5
This net inward force compresses the balloon, reducing its volume until the internal and external pressures re-balance.
The balloon therefore shrinks.
Answer: Cooling the gas reduces the kinetic energy and speed of its particles. This leads to fewer and less forceful collisions with the balloon's inner surface, lowering the internal pressure. The greater external atmospheric pressure then compresses the balloon until the pressures equalise.
Common mistakes
- ×Confusing macroscopic properties with particle properties. For example, stating 'gas particles can be compressed' is incorrect. It is the large empty space between the particles that allows a gas to be compressed.
- ×Incorrectly describing particle motion in solids. Particles in a solid are not stationary; they are constantly vibrating about fixed positions.
- ×Mixing up 'no forces' with 'weak forces'. Gas particles have negligible forces between them *except during collisions*. Liquid particles have weaker forces than solids, but these forces are still significant and hold the liquid together.
No-calculator tips
- ✓For density comparisons, think in orders of magnitude. For a given substance, the density of the gas is typically about 1/1000th the density of the liquid or solid, because the volume is about 1000 times greater.
- ✓When explaining pressure, always link it back to the rate and force of particle collisions with a surface. More frequent or more energetic collisions mean higher pressure.
- ✓Conceptualise compressibility in terms of empty space. Solids and liquids have very little empty space between particles, so they are virtually incompressible. Gases have vast empty spaces, making them highly compressible.