Changes of State and Heating and Cooling Curves

When you heat ice it melts to water, and when you heat that water enough it boils to steam. These are changes of state — and although they 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; only their arrangement and energy change. This lesson, part of Topic C1 of OCR Gateway Science A, 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.

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The Changes of State

There are six named changes of state, in 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, which moves 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 becoming a liquid — this is sublimation.

Exam Tip: Learn both directions of each pair. A common slip is to know "melting" but forget that the reverse is freezing, or to know "boiling" but not its reverse, condensing. State the direction (solid → liquid, etc.) to be sure.

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Boiling and Evaporating

Both boiling and evaporating turn a liquid into a gas, but they are slightly different:

• Evaporation happens only at the surface of a liquid, and can occur at any temperature (a puddle dries up on a cool day because the fastest surface particles escape).
• Boiling happens throughout the liquid at a fixed temperature, the boiling point, where bubbles of gas form inside the liquid and rise to the surface.

In both cases the particles that leave the liquid are gaining enough energy to escape the forces of attraction holding them in the liquid.

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Changes of State Are Physical and Reversible

A change of state is a physical change, not a chemical one. This means:

• No new substance is made — melted ice is still water, just in a different state.
• The particles themselves are unchanged; only their arrangement and energy change.
• The change is reversible — cooling the water turns it straight back into ice.

This is quite different from 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 $\text{H}_2\text{O}$H2​O.

Exam Tip: If asked why melting or boiling is a physical change, the marking points are: no new substance is formed, the particles are unchanged, and the change is reversible. The formula stays the same.

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Energy and the Forces Between Particles

To melt or boil a substance you must supply energy, and this energy does a specific job: it weakens or overcomes the forces of attraction between the particles so they can move further apart.

• Melting: energy is supplied to partly overcome the strong forces holding the solid lattice together, so the particles can break free of their fixed positions and move around — the solid becomes a liquid.
• Boiling: more energy is supplied to completely overcome the forces between the liquid particles, so they can separate fully and move apart as a gas.

This explains a key fact: during a change of state, the temperature stays constant, even though energy is still being supplied. The reason is that the incoming energy is being used to break the forces of attraction between particles, not to make the particles move faster. Temperature is a measure of the average kinetic energy (the speed of movement) 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".

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Heating Curves

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. It has a very characteristic shape: 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:

1. Solid heating up — the temperature rises; the particles vibrate faster.
2. First plateau (melting) — the temperature is constant at the melting point while the solid melts; energy overcomes the forces holding the lattice.
3. Liquid heating up — the temperature rises again; the liquid particles move faster.
4. Second plateau (boiling) — the temperature is constant at the boiling point while the liquid boils; energy fully separates the particles.
5. Gas heating up — the temperature rises once more.

The height of the first plateau is the melting point; the height of the second plateau is the boiling point. The plateaus are flat because, as explained above, the energy is overcoming forces between particles rather than raising the temperature.

Exam Tip: On a heating curve, the flat sections are the changes of state. The lower plateau gives the melting point and the upper plateau gives the boiling point — read across to the temperature axis to find their values.

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Cooling Curves