Changes of state — AQA GCSE Physics
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Changes of state explained
A change of state rearranges particles; it does not create or destroy them.
Read the full explanation
The same particles are present before and after, so the total mass is unchanged: 1 kg of ice melts to exactly 1 kg of water, and boiling that water produces exactly 1 kg of steam. Learn the six names with their directions, so melting is solid to liquid, freezing is liquid to solid, boiling and evaporating are liquid to gas, condensing is gas to liquid, and sublimating is solid straight to gas. Mass appearing to fall when a beaker of water is heated does not break the rule, because the steam has left the balance rather than vanished. In a sealed container, such as liquid nitrogen boiling in a closed flask, the reading does not change at all.
Your focus
- Name the change of state for each of the six transitions between solid, liquid and gas.
- Explain why the mass of a substance is unchanged when it melts or boils.
- Account for a falling balance reading when water is boiled in an open container.
Changes of state exam tips
Marking Points
- one mark for the number of particles being unchanged by a change of state
- one mark for stating that the mass is therefore the same before and after
- one mark for naming the change of state correctly in the direction given
- one mark for explaining an apparent loss of mass by gas escaping from an open container
Examiner Tips
- 💡Answer conservation questions by counting particles, not by talking about weight.
- 💡Check whether the container in the question is open or sealed before commenting on the balance reading.
- 💡Use the exact state-change verb from the question so the direction is unambiguous.
Common Mistakes
- saying mass is lost when water boils away because steam is lighter
- swapping condensing with evaporating, or melting with freezing
- claiming sublimation is a chemical change because no liquid ever appears
- saying the particles get smaller when a solid melts