Changes of state (AQA GCSE Physics Combined Science): Revision Notes
Changes of state
What are the three states of matter?
All substances can exist in three different states: solid, liquid, and gas. The difference between these states is how the particles are arranged and how they move.
Solid
- Particles are packed very close together in a regular pattern
- Particles can only vibrate in fixed positions
- They cannot move around freely
- This gives solids a fixed shape and volume
Liquid
- Particles are close together but not in a regular pattern
- Particles can slide past each other and move around randomly
- They have more energy than solid particles
- Liquids have a fixed volume but take the shape of their container
The key difference between solids and liquids is that liquid particles have enough energy to move past each other, while solid particles are locked in position and can only vibrate.
Gas
- Particles are far apart and move around very quickly
- Particles have lots of kinetic energy and move in all directions
- They move around all the time at high speeds
- Gases fill any container they're put in
How do substances change state?
Changes of state happen when substances gain or lose energy. These are physical changes, not chemical changes.
Physical changes are completely reversible - you can always get back to the original state by adding or removing energy. This is different from chemical changes, which create new substances and cannot be easily reversed.
Key facts about state changes:
- Physical changes can be reversed (unlike chemical changes)
- The mass stays exactly the same before and after the change
- When a substance changes state, the temperature stays constant during the change
- Energy is either transferred to the substance or transferred away from it
The state change cycle
There are six main processes that change states:
From solid to liquid:
- Melting - happens when a solid is heated and gains thermal energy
From liquid to solid:
- Freezing - happens when a liquid is cooled and loses thermal energy
From liquid to gas:
- Evaporation - happens when a liquid gains thermal energy
From gas to liquid:
- Condensation - happens when a gas loses thermal energy
From solid directly to gas:
- Sublimation - when a solid turns straight into a gas (like dry ice)
From gas directly to solid:
- Desublimation - when a gas turns straight into a solid
Sublimation and desublimation are less common than the other state changes because they skip the liquid phase entirely. You might see sublimation when dry ice (solid carbon dioxide) turns directly into gas, or when ice cubes seem to "disappear" in the freezer over time.
Important points about temperature and energy
When you heat a liquid, its temperature will rise until it reaches its boiling point. Then the temperature stays constant while it changes into a gas.
Worked Example: Water Boiling
When water boils, it stays at exactly 100°C while it turns into steam. The energy goes into changing the state, not increasing the temperature.
This means if you measure the temperature of boiling water, it will remain at 100°C throughout the entire boiling process, even though you're still adding heat energy.
The same thing happens in reverse when substances cool down and change state.
Simple example
Real-Life Example: Ice Cubes on the Kitchen Bench
Think about what happens to ice cubes left on a kitchen bench:
- The ice (solid) gains thermal energy from the warm air
- It melts into water (liquid) at 0°C
- If left longer, the water will evaporate into water vapour (gas)
- The mass of water stays the same throughout these changes
This demonstrates all the key principles of state changes in a simple, everyday situation.
Key Points to Remember:
- Three states: solid (particles vibrate in place), liquid (particles slide past each other), gas (particles move quickly everywhere)
- Physical changes can be reversed - chemical changes cannot
- Mass never changes during state changes
- Temperature stays constant while the substance is actually changing state
- Energy transfers cause all state changes - gaining energy moves towards gas, losing energy moves towards solid