Changes of state (AQA GCSE Physics): Revision Notes
Changes of state
The three states of matter
Matter exists in three main states. Each state has different particle arrangements and movements.
Solid
- Particles are tightly packed in a regular pattern
- Particles vibrate but cannot move freely
- This is why solids keep their shape
Liquid
- Particles are close together but not in a fixed pattern
- Particles can move past each other and move around randomly
- This is why liquids can flow and take the shape of their container
Gas
- Particles are spread far apart
- Particles move around very quickly and have lots of space between them
- This happens because gas particles have lots of kinetic energy
The key difference between states is how much freedom the particles have to move. In solids, particles are restricted to vibrating in place. In liquids, they can slide past each other. In gases, they move completely freely with lots of space between them.
What happens when substances change state
When substances change from one state to another, this is called a change of state. These changes depend on whether thermal energy is added to or taken away from the substance.
Key facts about state changes:
- State changes are physical changes, not chemical changes
- This means they can be reversed
- The substance stays the same - only its state changes
- Mass is conserved - the mass before the change equals the mass after the change
For example, when 500kg of solid ice melts, it becomes 500kg of liquid water. The mass stays exactly the same.
Temperature during state changes
Understanding how temperature behaves during state changes is crucial for predicting what will happen to substances.
When heating or cooling without changing state:
- The temperature of the substance will change
- For example, heating liquid water from 20°C to 80°C
During actual state changes:
- The temperature stays constant
- For example, water boils at exactly 100°C
- All the thermal energy goes into changing the state, not increasing temperature
Critical Concept: During a state change, all thermal energy goes into breaking or forming bonds between particles, NOT into changing temperature. This is why water stays at exactly 100°C while boiling, even though you're still adding heat.
The state change processes
There are six main processes that change the state of matter. These processes can be grouped based on whether thermal energy is being added or removed:
Heating processes (thermal energy is transferred to the system):
- Melting: solid → liquid
- Evaporation: liquid → gas
- Sublimation: solid → gas (skips liquid stage)
Cooling processes (thermal energy is transferred from the system):
- Freezing: liquid → solid
- Condensation: gas → liquid
- Desublimation: gas → solid (skips liquid stage)
These processes form a cycle. When you heat matter, it moves from solid → liquid → gas. When you cool matter, it moves from gas → liquid → solid.
Worked example
Worked Example: Steam Cooling Process
When steam cools down:
- 2.0kg of steam at 100°C will turn into 2.0kg of water at 100°C
- The water will then cool from 100°C to room temperature
- If put in a freezer, it will cool further and turn to ice at 0°C
- The ice will then cool to around -18°C
Notice how the mass stays the same throughout (2.0kg) but the temperature and state change.
Remember!
Key Points to Remember:
- Matter has three states: solid, liquid, and gas, each with different particle arrangements
- State changes are physical changes that can be reversed
- Mass is always conserved during state changes
- Temperature stays constant during the actual moment of state change
- Six main processes: melting, freezing, evaporation, condensation, sublimation, and desublimation