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Heat ice and it melts to water; heat that water hard enough and it boils to steam. These are changes of state, and although they can look dramatic, no new substance is ever made. A change of state is a physical change: the particles are exactly the same before and after, and only their arrangement and energy alter. This lesson, part of Topic C1 of your OCR Gateway Combined Science course, names all the changes of state in both directions, explains why they are reversible, and shows how to read heating and cooling curves to find melting and boiling points and predict the state of a substance at any temperature.
By the end of this lesson you should be able to name the changes of state in both directions, explain why they are physical and reversible, describe what happens to the particles and the energy during a change of state, interpret heating and cooling curves, and predict the state of a substance from its melting and boiling points.
This lesson develops AO1 (recalling the named changes of state) and AO2 (interpreting heating and cooling curves to read melting and boiling points), extending to AO3 when you predict a substance's state at a given temperature and analyse why the temperature holds constant during a change of state.
There are six named changes of state, forming three reversible pairs. Each one moves a substance between two of the three states.
| Change | From → To | Direction |
|---|---|---|
| Melting | solid → liquid | gaining energy |
| Freezing (solidifying) | liquid → solid | losing energy |
| Boiling / Evaporating | liquid → gas | gaining energy |
| Condensing | gas → liquid | losing energy |
| Sublimation | solid → gas (directly) | gaining energy |
| Deposition | gas → solid (directly) | losing energy |
Heating a substance adds energy, moving it "up" the states (solid → liquid → gas). Cooling removes energy, moving it "down" (gas → liquid → solid). A few substances — such as solid carbon dioxide ("dry ice") and iodine — can change directly between solid and gas without passing through the liquid state; this is sublimation.
Exam Tip: Learn both directions of every pair. A common slip is to remember "melting" but forget its reverse is freezing, or to remember "boiling" but not condensing. Stating the direction (solid → liquid, and so on) keeps you safe.
Both boiling and evaporating turn a liquid into a gas, but they differ slightly:
In both cases, the particles that leave the liquid are gaining enough energy to escape the forces of attraction holding them in place.
There is a neat everyday consequence of this. Evaporation actually cools the liquid left behind, because it is the fastest, highest-energy particles that escape first, leaving the slower, lower-energy ones behind — and a lower average particle energy means a lower temperature. This is why sweating cools you down: as sweat evaporates from your skin, the most energetic water particles carry energy away, and your skin is left cooler. The same idea explains why a wet swimmer feels cold in a breeze. These observations are all evidence that a change of state is bound up with the energy of the particles, not just their arrangement.
It is easy to think of changes of state as something that happens only in the lab, but they are all around you, and connecting the theory to familiar examples makes it much easier to recall in an exam.
In every one of these cases the substance keeps the same chemical formula — the water in a puddle, in a cloud and in frost is all H2O — which is exactly what tells you that a change of state is a physical change and not a chemical one.
A change of state is a physical change, not a chemical one. This means:
This is quite unlike a chemical change such as burning, where new substances are made and the change usually cannot be reversed. You can tell a change of state is physical because the substance keeps the same chemical formula throughout: ice, water and steam are all H2O.
Exam Tip: If asked why melting or boiling is a physical change, the marking points are that no new substance is formed, the particles are unchanged, and the change is reversible. The formula stays the same throughout.
To melt or boil a substance you must supply energy, and that energy does a specific job: it weakens or overcomes the forces of attraction between the particles so they can move further apart.
This explains a key fact: during a change of state, the temperature stays constant, even though energy is still being supplied. The incoming energy is being used to break the forces of attraction between particles, not to make the particles move faster. Temperature measures the average kinetic energy — the average speed — of the particles, and while the forces are being overcome that average speed does not change. Only once the change of state is complete does further heating make the particles move faster and the temperature rise again.
Exam Tip: The phrase examiners reward is that, at the melting or boiling point, "the energy is used to overcome the forces of attraction between particles, not to raise the temperature, so the temperature stays constant".
A heating curve is a graph of temperature (y-axis) against time or energy supplied (x-axis) as a substance is heated steadily from solid to gas. Its shape is very distinctive: two sloping sections where the temperature rises, separated by two flat sections (plateaus) where the temperature stays constant during melting and boiling.
Reading the curve from left to right:
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