Changes of State

A heating curve with two flat sections appears somewhere in Papers 1-4 nearly every series, and the question “explain why the temperature stays constant” defeats more candidates than any other part of topic 1. The answer is two lines long once you know it. Naming the six changes of state correctly is Core; explaining the energy changes behind them in particle terms is where Paper 4 marks sit.

The six changes of state (Core)

Change	Name	Energy
Solid → liquid	Melting	Taken in
Liquid → gas	Boiling / evaporation	Taken in
Gas → liquid	Condensation	Given out
Liquid → solid	Freezing / solidifying	Given out
Solid → gas	Sublimation	Taken in
Gas → solid	Deposition (reverse sublimation)	Given out

Melting happens at the melting point; boiling at the boiling point; for a pure substance these are sharp, fixed temperatures (and an impure sample melts over a range and at a lower temperature, a fact reused in purity questions in topic 12). Each change is physical, not chemical: no new substance forms, and the change is reversed by reversing the heating.

In particle terms, melting means the particles gain enough energy to overcome the forces holding them in fixed positions, so the regular arrangement breaks down. Boiling means particles gain enough energy to overcome the forces of attraction completely and escape as a gas. Condensing and freezing are the same statements run backwards, with energy released as forces re-form. The underlying model is kinetic particle theory, and the same vocabulary scores in both subtopics.

Boiling versus evaporation (Core)

Two differences earn the marks. Boiling occurs at a single fixed temperature; evaporation occurs at any temperature below the boiling point. Boiling occurs throughout the liquid (bubbles form in the body of the liquid); evaporation occurs only at the surface. Add, if asked, that evaporation cools the remaining liquid because the highest-energy particles leave first.

Heating and cooling curves (Core, explained at Supplement depth)

A heating curve plots temperature against time as a substance is heated steadily. Sloped sections are one state warming up. Flat sections are a change of state: the first plateau is melting, the second is boiling. Reading values off the curve is Core. The melting point is the temperature of the first plateau.

The explain mark: during a plateau, the energy supplied is used to overcome the forces of attraction between particles rather than to increase their kinetic energy, so the temperature does not rise. During the plateau both states are present at once: solid and liquid coexist at the melting point. Cooling curves mirror this: flat sections occur where forces re-form and energy is released. State changes also obey the data-reading rule from solids, liquids and gases: compare the temperature with the melting and boiling points to fix the state.

Worked exam question

Solid ethanamide is heated steadily from 60 °C to 240 °C. Its melting point is 82 °C and its boiling point is 222 °C. (a) Sketch the heating curve, labelling the axes. (2) (b) Explain, in terms of particles, why the temperature stays constant at 82 °C even though heating continues. (2)

Model answer: (a) Temperature (°C) on the y-axis, time on the x-axis (1); a rising line with flat sections at 82 °C and at 222 °C, rising in between and after (1). (b) The energy supplied is used to overcome/weaken the forces of attraction between particles (1), so it does not increase the kinetic energy / speed of the particles, and the temperature stays constant (1).

Mark-by-mark: in (a) the two plateaus must sit at the stated temperatures. A plateau drawn at roughly 100 °C suggests copying a water curve from memory. In (b), mark 1 is energy used against forces between particles; mark 2 connects that to no increase in kinetic energy. “The heat is used for melting” restates the observation and scores zero.

The mistakes that cost marks

1. Writing “the bonds break” without saying which. For simple substances it is the forces of attraction between particles (intermolecular forces) that are overcome, not covalent bonds within molecules. Water boiling does not split H2O.
2. Plateau explained as “the thermometer stops working” or “heating pauses”. Energy input continues; it is redirected into separating particles.
3. Mixing up evaporation and boiling: claiming water “boils at room temperature, slowly”. Evaporation below the boiling point is surface-only.
4. Forgetting energy direction: condensation and freezing release energy. Cooling-curve plateaus are where energy is given out, which is why they are also flat.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”The heat breaks the solid apart"	"Energy is used to overcome the forces of attraction between particles"
"Boiling is just fast evaporation"	"Boiling occurs at a fixed temperature throughout the liquid; evaporation occurs at the surface below the boiling point"
"The graph goes flat because it’s melting"	"Temperature is constant because energy supplied overcomes forces between particles, not increasing their kinetic energy"
"The water disappears"	"The liquid evaporates / particles escape from the surface as a gas”

The Malaysia note

Heating curves reward calm graph-reading, and Malaysian students sitting the Oct/Nov series after a packed Form 4-equivalent year tend to lose these marks to rush rather than ignorance, with plateaus drawn at the wrong temperatures and axes unlabelled. Ten minutes of deliberate practice fixes it. If your child’s topic 1 answers look right but keep scoring 1 out of 2, our free 1-hour trial will diagnose whether it is content or exam technique. With this topic, it is usually technique.

Test yourself

Try these three from memory first. The answers stay hidden until you click.

Q1 (1 mark). Solid carbon dioxide (“dry ice”) turns directly into a gas at room temperature. Name this change of state.

Show answer

• Sublimation [1]

Q2 (2 marks). State two differences between boiling and evaporation.

Show answer

• Boiling occurs at one fixed temperature (the boiling point); evaporation occurs below the boiling point, over a range of temperatures [1] • Boiling occurs throughout the liquid (bubbles form in the body of the liquid); evaporation occurs only at the surface [1]

Q3 (3 marks). Liquid candle wax is left to cool. Its temperature falls steadily, stays constant at 52 °C for a few minutes, then falls again. Explain, in terms of particles and energy, why the temperature stays constant at 52 °C.

Show answer

• The wax is freezing/solidifying at 52 °C, so both liquid and solid are present [1] • Forces of attraction form between the particles as they settle into fixed positions, and energy is released [1] • The energy released compensates for the energy lost to the surroundings, so the temperature does not fall during the change of state [1]