The Three States of Matter
This topic explains the properties of solids, liquids, and gases by modelling them as collections of tiny particles. Understanding how these particles are arranged and how they move is fundamental to explaining physical properties like density, shape, and changes of state.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- In a solid, particles are tightly packed in a fixed, regular pattern (a lattice). They possess the least energy and can only vibrate about their fixed positions.
- In a liquid, particles are still closely packed but are arranged randomly. They have enough energy to slide past one another, allowing the liquid to flow.
- In a gas, particles are far apart with no regular arrangement. They have the most energy, moving rapidly and randomly in all directions, with negligible forces between them.
- The energy of the particles is kinetic energy. The higher the temperature, the greater the average kinetic energy of the particles in a substance.
- The forces of attraction between particles are strongest in solids, weaker in liquids, and almost non-existent in gases.
Definitions
- Kinetic Particle Theory
- The model used to describe matter as being composed of a large number of tiny particles (atoms, molecules, or ions) which are in constant, random motion.
- State of Matter
- The distinct physical form of a substance, primarily solid, liquid, or gas, which is determined by the arrangement, energy, and forces between its constituent particles.
Worked example
Compare and contrast the arrangement and motion of particles in a liquid with those in a gas. Use your answer to explain why gases can be easily compressed but liquids are considered virtually incompressible.
- 1
Step 1:
Describe the particles in a liquid.
State that they are closely packed but randomly arranged, and are able to slide past each other.
- 2
Step 2:
Describe the particles in a gas.
State that they are very far apart, randomly arranged, and move rapidly in all directions.
- 3
Step 3:
Compare the arrangement.
The key difference is the large intermolecular distance in gases compared to the small distance in liquids.
- 4
Step 4:
Relate this to compressibility.
Explain that because gas particles are far apart, the large empty spaces between them can be easily reduced by applying pressure.
In contrast, liquid particles are already touching, so there is very little empty space to reduce, making them incompressible.
Answer: Particles in both liquids and gases are randomly arranged and can move freely. However, liquid particles are packed closely together, whereas gas particles are very far apart. Because of the large empty spaces between gas particles, gases can be easily compressed. Liquids are virtually incompressible because their particles are already in close contact with little free space between them.
Common mistakes
- ×Forgetting that particles in a solid are not static; they are constantly vibrating in their fixed positions.
- ×Assuming particles themselves change size or shape with temperature. It is the spacing and energy of the particles that change, not the particles themselves.
- ×Describing liquids as having weak forces. The forces are weaker than in solids, but still strong enough to keep the particles close together.
No-calculator tips
- ✓This is a descriptive topic. Focus on using precise keywords: 'vibrate', 'fixed positions', 'slide past', 'far apart', 'random motion'.
- ✓Use analogies to remember the states: solids are like a packed audience sitting in fixed seats, liquids are a crowd mingling on a dance floor, and gases are a few people running around a large empty stadium.
- ✓When asked to explain a property (e.g., density, compressibility), always link it back to the spacing and/or forces between the particles.