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Changes of State (AQA GCSE Physics: Combined Science)
Revision Note
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Changes of State
- When a substance changes state:
- The number of molecules in that substance does not change
- Therefore its mass does not change
- Unlike chemical changes, physical changes like changes of state are reversible
- In a solid:
- The molecules are very close together and arranged in a regular pattern
- The molecules vibrate about fixed positions
- In a liquid:
- The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
- The molecules are able to slide past each other
- In a gas:
- The molecules are widely separated - about 10 times further apart in each direction
- The molecules move about randomly at high speeds
- There are six changes of state that can occur between solids, liquids and gases:
- Melting - A solid turns into a liquid (e.g. ice to water) when energy is transferred to the system
- Boiling - A liquid turns into a gas (evaporating) when energy is transferred to the system
- Condensing - A gas turns into a liquid when energy is transferred away from the system
- Freezing - A liquid turns into a solid when energy is transferred away from the system
- Subliming - A solid turns into a gas when energy is transferred to the system
Diagram showing the arrangement and motion of different states of matter
In my experience of teaching this topic to GCSE students, there are a couple of common misconceptions that tend to occur.
- The molecules themselves melt and separate
- The only way the atoms in a molecule can separate is through a chemical reaction (and this requires much more energy than changing its state!)
- When a substance undergoes a chemical change (a chemical reaction) a new substance or substances are formed
- In a physical change, like changes of state, the substance doesn't change. Water is still water whether it is in the form of a solid (ice), a liquid (water) or a gas (water vapour)
- The mass of the substance changes
- The mass of the substance is determined by the mass of the individual particles and how many particles there are
- The number of particles in a substance does not change when it changes state
- Therefore, the mass of the substance does not change
- However, gas particles can escape if the container is open when a liquid changes into a gas, in which case the mass of the remaining liquid would decrease as it evaporates or boils
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