Changes of state — AQA GCSE Physics
Test yourself on Changes of state with AQA GCSE practice questions.
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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.
Changes of state are physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.
In a physical change the substance itself is unchanged, since only the arrangement and energy of its particles differ, so reversing the change returns the original material with its original properties. Freeze the water you melted from ice and you get ice again, with the same melting point and the same density; no new substance has been made and no bonds within the molecules have been broken. A chemical change is different, because burning magnesium produces magnesium oxide, a new substance with its own properties, and cooling it does not give the magnesium back. The test to apply in an exam is whether the original properties return on reversal. Since no new substance forms, mass is conserved in a change of state as well.
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.
Show all 6 objectives
- Explain why melting is classed as a physical change rather than a chemical one.
- Name a property that must return when a change of state is reversed and say how it could be measured.
- Sort a list of everyday changes into physical and chemical, giving a reason for each.
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
- one mark for a change of state being a physical change in which no new substance is formed
- one mark for the original properties returning when the change is reversed
- one mark for a chemical change producing a new substance that is not recovered on reversal
- one mark for the particles themselves being unchanged, with only their arrangement and energy differing
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.
- 💡Justify 'physical' by naming a property that returns, such as the melting point or the density.
- 💡Say explicitly that no new substance is formed, because that phrase carries a mark.
- 💡Keep the particle-level statement separate from the observation, since examiners want both.
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
- calling boiling a chemical change because bubbles are produced
- saying a physical change is reversible simply because it is easy to undo
- claiming the molecules are broken apart into atoms when a substance boils
- offering a chemical change such as rusting as an example of a change of state