Solids, liquids & gases
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
States of Matter and Their Properties
Matter exists in three states: solid, liquid, and gas. The physical form a substance takes depends on the separation, arrangement, and motion of its particles.
- Solids have a fixed shape and a fixed volume
- Particles are very close together in a regular, repeating pattern
- Particles vibrate about fixed positions but cannot move from place to place
- Strong forces of attraction hold particles in position
- Liquids have no fixed shape but a fixed volume
- Particles are close together but arranged randomly, not in a regular pattern
- Particles can move around and slide past one another, so a liquid flows and takes the shape of its container
- Forces of attraction between particles are weaker than in a solid, but still significant
- Gases have no fixed shape and no fixed volume
- Particles are far apart with no regular arrangement
- Particles move rapidly and randomly in all directions, spreading to fill any container
- Forces of attraction between particles are very weak, almost negligible
Particle separation and arrangement - not particle size - is what determines whether a substance is a solid, liquid, or gas
Comparing the Three States
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular pattern, very close | Random, close together | Random, far apart |
| Motion | Vibrate about fixed positions | Move around each other | Move rapidly in all directions |
| Shape | Fixed | Takes the shape of its container | Fills its container |
| Volume | Fixed | Fixed | Not fixed - fills its container |
| Compressibility | Cannot be compressed | Very difficult to compress | Easily compressed |
| Density | High | High | Low |
| Forces between particles | Very strong | Strong | Very weak |
Solids and liquids cannot be compressed because their particles are already touching, with almost no free space between them. Gases can be compressed easily because there is a large amount of empty space between particles that can be reduced.
Changes of State
A change of state is a physical change - no new substance is formed, and the change can be reversed.
- Melting (solid → liquid): particles gain enough energy to partly overcome the forces holding them in fixed positions
- Freezing (liquid → solid): particles lose energy and forces of attraction lock them into fixed positions
- Boiling (liquid → gas): particles throughout the liquid gain enough energy to completely overcome the forces of attraction between them; this happens at a fixed temperature (the boiling point)
- Evaporation (liquid → gas): particles at the surface of a liquid escape into the gas state; this can happen at any temperature below the boiling point
- Condensation (gas → liquid): particles lose energy and forces of attraction pull them close enough together to form a liquid
Each named change of state connects two of the three states of matter, and involves energy absorbed or released
Explaining Changes of State Using Kinetic Particle Theory (Supplement)
When a pure substance is heated at a constant rate, a graph of its temperature against time has flat sections (plateaus) at the melting point and boiling point, where the temperature stays constant even though energy is still being supplied. During a plateau, the energy supplied is used to weaken or break the forces of attraction between particles, not to increase their kinetic energy - this is why the temperature does not rise until the change of state is complete.
Worked Example: Naming Changes of State
- Question: oxygen can exist in different states of matter. Name the change of state for: (a) solid to liquid, (b) gas to liquid. [2]
- Answer: (a) melting [1]; (b) condensing / condensation [1]
Effect of Temperature and Pressure on the Volume of a Gas (Supplement)
Gas particles are in constant, random motion and are constantly colliding with the walls of their container - the combined effect of billions of these collisions every second is what we measure as gas pressure.
- Effect of temperature: heating a gas gives its particles more kinetic energy, so they move faster
- If the volume is fixed (a sealed, rigid container), faster particles hit the walls more often and with greater force, so the pressure increases
- If the pressure is kept constant (e.g. a gas in a syringe with a freely-moving piston), the volume increases so the faster particles still collide with the walls no more often than before
- Effect of pressure: at a constant temperature, squeezing a gas into a smaller volume forces the same number of particles into a smaller space
- Particles have less distance to travel between collisions with the walls, so collisions become more frequent and the pressure increases
- Increasing the volume available to a gas has the opposite effect - collisions become less frequent, so the pressure decreases
Cooling a gas has the opposite effect to heating: particles slow down, so pressure falls (fixed volume) or volume falls (fixed pressure).
Explaining Properties Using Particle Theory
- Why can liquids flow but solids cannot?
- Liquid particles are not held in fixed positions, so they can slide past one another. Solid particles are locked in position by strong forces of attraction.
- Why do gases have a much lower density than solids or liquids?
- Gas particles are spaced far apart, so a given volume of gas contains far fewer particles than the same volume of solid or liquid.
Worked Example: Describing Particle Arrangement
- Question: oxygen is a gas at room temperature and pressure. Describe the arrangement and separation of the particles in gaseous oxygen. [2]
- Answer: the particles are arranged randomly, with no regular pattern [1]; the particles are far apart from one another, with mostly empty space between them [1]
Common Mistakes
- ❌ Thinking there are no forces at all between gas particles
- ✓ forces of attraction between gas particles are very weak, not zero - this is why gases can still be cooled and compressed into liquids
- ❌ Thinking particles in a solid are completely still
- ✓ solid particles are not still - they constantly vibrate about their fixed positions, they just cannot move from place to place
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