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When ice melts to water, or water boils away to steam, the substance changes from one state to another — but it is still the same substance, made of the same particles. These changes of state are physical changes: they can be reversed, and no new substance is made. That single idea has an important consequence — because no particles are created or destroyed, the mass stays the same throughout. This lesson, part of Topic P1 (Matter) of OCR Gateway Combined Science A, names the changes of state and their directions, explains them with the particle model, shows why they are physical changes in which mass is conserved, and previews the heating curve you will meet in detail in the next two lessons.
By the end of this lesson you should be able to name the changes of state and their directions, explain them using the particle model, explain why a change of state is a physical change in which mass is conserved, and interpret a simple heating curve.
This lesson is AO1 for naming the changes of state and the conservation-of-mass principle, AO2 for applying the particle model to explain each change, and AO3 for interpreting a heating curve — reading off melting and boiling points and explaining why the plateaus are flat.
There are six changes of state to learn, each with a name and a direction between two of the three states. The diagram below shows them as a cycle between solid, liquid and gas.
graph LR
S[Solid] -->|melting| L[Liquid]
L -->|freezing| S
L -->|boiling / evaporating| G[Gas]
G -->|condensing| L
S -->|sublimation| G
G -->|deposition| S
The changes are:
| Change of state | Direction | What happens |
|---|---|---|
| Melting | solid → liquid | a solid is heated until it turns to liquid (at its melting point) |
| Freezing | liquid → solid | a liquid is cooled until it turns to solid (at its freezing point) |
| Boiling / evaporating | liquid → gas | a liquid turns to gas (boiling happens throughout at the boiling point; evaporation happens at the surface, below boiling point) |
| Condensing | gas → liquid | a gas is cooled until it turns to liquid |
| Sublimation | solid → gas | a solid turns straight to gas without melting (e.g. solid carbon dioxide) |
| Deposition | gas → solid | a gas turns straight to solid without becoming liquid (e.g. frost forming) |
Melting and freezing happen at the same temperature for a given substance — the melting point (for ice/water this is 0°C). Boiling and condensing happen at the boiling point (for water this is 100°C at normal atmospheric pressure).
Exam Tip: Learn the changes as three pairs of opposites: melting/freezing, boiling/condensing, sublimation/deposition. Knowing that freezing is just melting in reverse, at the same temperature, saves you learning two separate facts.
Each change of state is a rearrangement of the same particles, driven by adding or removing energy:
Notice that in every case it is the arrangement, spacing and motion of the particles that change — not the particles themselves. The number of particles is the same before and after.
Exam Tip: When you explain a change of state, describe what happens to the particles' spacing and motion: melting frees the particles to move; boiling separates them completely; cooling brings them back together. This is the particle-level explanation examiners reward.
It is worth being clear about the difference between boiling and evaporating, since both take a liquid to a gas:
In both cases the fastest-moving particles (at the surface in evaporation, or throughout in boiling) gain enough energy to break away from the liquid and become gas.
Changes of state are not just laboratory curiosities — they happen constantly all around us, and understanding them through the particle model explains a surprising range of everyday events. Being able to link a familiar example to what the particles are doing is exactly the kind of application examiners reward.
The water cycle is one enormous, continuous change of state on a planetary scale. Energy from the Sun causes water in oceans, rivers and puddles to evaporate: the fastest-moving surface particles gain enough energy to escape the liquid and rise as invisible water vapour. High in the cooler air, that vapour condenses back into tiny droplets as its particles lose energy, slow down and come close together again — forming the clouds you see. When the droplets grow large and heavy enough they fall as rain, and the whole cycle begins again. Freezing and melting also play their part when water falls as snow or hail and later thaws. Every stage is the same water particles simply being rearranged as they gain or lose energy, which is why the total amount of water on Earth stays essentially constant.
Cooling by evaporation is another everyday consequence worth understanding. When you step out of a swimming pool you feel cold, even on a warm day, because water is evaporating from your skin. Evaporation removes the fastest-moving particles from the liquid first, since those are the ones with enough energy to escape. The particles left behind are, on average, moving more slowly — and slower particles mean a lower temperature. So evaporation cools whatever is left behind, which is exactly why sweating cools you down and why a wet cloth wrapped round a bottle keeps a drink cool. Refrigerators use the same principle on purpose, evaporating a special fluid inside the cold compartment to carry heat away.
Condensation appears whenever warm, moist air meets a cold surface: water vapour in the air touches the cold surface, its particles lose energy, slow down and gather together as a liquid. This is the mist you see on a bathroom mirror after a hot shower, the droplets that form on the outside of a cold glass of lemonade on a summer day, and the dew that settles on grass overnight as the ground cools. In each case the invisible water vapour in the air is condensing back into visible liquid water on the cold surface.
Because all of these processes rearrange the same particles rather than making new substances, they are all physical changes — and they can all run in either direction if the energy flow is reversed. Recognising the change of state, naming its direction, and describing what the particles do are the three moves that turn a familiar observation into a full mark-scheme answer.
Exam Tip: When a question gives a real-world example (dew, a drying puddle, mist on a mirror, sweat cooling the skin), name the change of state, its direction, and what the particles do (gaining or losing energy, moving apart or together). Evaporation cooling a surface is a favourite: the fastest particles leave, so the average energy — and the temperature — of what remains falls.
A change of state is a physical change, not a chemical one. This means:
This contrasts with a chemical change (such as burning), where new substances are made and the change usually cannot be simply reversed.
Exam Tip: The key phrase is "no new substance is made" and "the change is reversible". A change of state rearranges the same particles; it does not turn them into anything new.
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