AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to describe how, when substances change state…
Full specification wording
Changes of state: Students should be able to describe how, when substances change state (melt, freeze, boil, evaporate, condense or sublimate), mass is conserved.
A change of state rearranges particles; it does not create or destroy them.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A change of state rearranges particles; it does not create or destroy them. 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.
What earns the marks
- The number of particles being unchanged by a change of state.
- Stating that the mass is therefore the same before and after.
- Naming the change of state correctly in the direction given.
- Explaining an apparent loss of mass by gas escaping from an open container.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Changes of state: Students should be able to describe how, when substances change state (melt, freeze, boil, evaporate, condense or sublimate), mass is conserved. lesson, made from checked teaching notes for this specification point.
Have a go first
When a solid melts to form a liquid, what happens to the total mass of the substance?
Listen
A change of state rearranges particles; it does not create or destroy them. 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. A strong answer earns marks for these points. the number of particles being unchanged by a change of state. stating that the mass is therefore the same before and after. naming the change of state correctly in the direction given. explaining an apparent loss of mass by gas escaping from an open container. Exam tip: Answer conservation questions by counting particles, not by talking about weight.
See it
Key words
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conservation of mass
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changes of state
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particle count unchanged
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open and closed containers
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See every definition
- conservation of mass
- A change of state rearranges particles; it does not create or destroy them, so the mass is the same before and after.
- changes of state
- 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.
- particle count unchanged
- The same particles are present before and after a change of state, so the number of particles is unchanged.
- open and closed containers
- In an open container steam leaves the balance rather than vanishing, while in a sealed container the reading does not change at all.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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