Changes of state and heating curves
Changes of state: melting, boiling, freezing, condensation and sublimation, plus interpreting heating/cooling curves using particle theory.
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State-change questions are particle questions: what happens to particle motion, spacing, and energy.
1. Definition
A change of state is when a substance changes between solid, liquid, and gas by absorbing or releasing heat energy.
- Melting point: temperature where a pure solid melts at a stated pressure.
- Boiling point: temperature where a liquid boils (pure substance at a stated pressure).
- Freezing point: temperature where a pure liquid freezes at a stated pressure.
2. Key Ideas
- Melting, boiling, and sublimation absorb heat energy.
- Freezing and condensation release heat energy.
- During a change of state, temperature can stay constant because energy is used to overcome forces of attraction, not to increase kinetic energy.
- Boiling happens throughout the liquid (bubbles); evaporation happens only at the surface and can occur at any temperature.
- Pure substances have sharp melting/boiling points; impure substances show a range.
3. Detailed Explanations
- Heating usually increases kinetic energy, but during a state change the energy is used to overcome forces of attraction. - Flat line on a heating/cooling curve = temperature constant during melting/boiling/condensing/freezing. - Endothermic state changes: melting, boiling, sublimation. Exothermic: freezing, condensation.
A. Summary of State Changes
| Change | Name | Heat energy |
|---|---|---|
| Solid → liquid | Melting | Absorbed |
| Liquid → gas | Boiling / Evaporation | Absorbed |
| Gas → liquid | Condensation | Released |
| Liquid → solid | Freezing | Released |
| Solid → gas | Sublimation | Absorbed |
Swipe or scroll sideways to inspect the complete overview.
B. Melting and Freezing (Particle Explanation)
Melting is solid → liquid at the melting point (pure substance).
- Heat energy is absorbed.
- Before melting, the temperature rises and particles gain kinetic energy: they vibrate more rapidly on average.
- During melting at fixed pressure, the temperature and average kinetic energy stay constant. Supplied energy overcomes attractions holding particles in fixed positions.
- Particles can now move/slide past each other (liquid).
Freezing is liquid → solid at the freezing point.
- Cooling before freezing reduces temperature and average kinetic energy.
- During freezing at fixed pressure, the pure substance releases energy while its temperature stays constant.
- Attractions hold particles in fixed positions. They still vibrate; they do not stop moving.
For a pure substance, melting point = freezing point. For an impure substance, melting/freezing happens over a range of temperatures.
C. Boiling, Evaporation, and Condensation
Boiling is liquid → gas at the boiling point (pure substance at a stated pressure).
- Heat energy is absorbed.
- Before boiling, heating raises the temperature and the particles move faster on average.
- During boiling at fixed pressure, temperature and average kinetic energy stay constant. Supplied energy overcomes attractions and separates particles into the gas state.
Evaporation is liquid → gas at the surface and can occur below the boiling point. Some surface particles have enough energy to overcome attractions and escape even though the average kinetic energy is lower than at the boiling point.
| Feature | Boiling | Evaporation |
|---|---|---|
| Temperature | Occurs only at a fixed boiling point | Occurs at any temperature |
| Location | Throughout the liquid (bubbles form) | Surface only |
Higher temperature, larger surface area, moving air (wind), and lower humidity increase evaporation.
Condensation is gas → liquid. Energy is released as particles move closer together. A pure substance condensing at fixed pressure stays at constant temperature during the change; cooling the gas before condensation is a separate stage.
D. Sublimation (and Deposition)
Sublimation is solid → gas without becoming liquid.
Examples: iodine, ammonium chloride, naphthalene, dry ice (solid CO₂).
The reverse change, gas → solid, is called deposition.
E. Heating Curves
A heating curve shows how temperature changes as a substance is heated.
The labelled graph below makes the axes, units and constant-temperature plateaux explicit:
Heating Curve (Pure Substance)
Example heating curve showing temperature rising and flat sections during melting and boiling for a pure substance.
Scroll across the graph to read all labels.
View figure data
| Time (min) | Heating curve |
|---|---|
| 0 | 20 |
| 2 | 60 |
| 4 | 80 |
| 6 | 80 |
| 8 | 120 |
| 10 | 120 |
| 12 | 140 |
During melting/boiling, temperature stays constant because energy is used to overcome forces of attraction, not to increase kinetic energy.
Heat ice to steam, or cool steam back to ice, and match each part of the curve to what the particles are doing.
10 g of water heated at 200 W. After 0 s it is at −20 °C and is solid: particles vibrate about fixed positions.
- Temperature
- −20 °C
- State
- solid
- Energy transferred
- 0.0 kJ
- Mean speed of NH₃
- — m/s
- Mean speed of HCl
- — m/s
- Ring formed after
- — s
- Ring from NH₃ end
- — cm
Try this
0 of 4 doneHeat the ice and watch the thermometer while it melts. (not done yet)
The temperature stays at 0 °C until all the ice has melted. The energy overcomes the attractions holding particles in fixed positions; it does not make them move faster.
Boil all the water, then do it again with a different heater power. (not done yet)
The stronger heater shortens the flat parts but the water still boils at 100 °C. A pure substance has a fixed boiling point.
Release ammonia and hydrogen chloride and wait for the white ring. (not done yet)
The ring of NH₄Cl forms nearer the HCl end. NH₃ (Mr 17) is lighter than HCl (Mr 36.5), so its particles move faster and it diffuses faster.
Form the ring at 25 °C and again at 200 °C. (not done yet)
At the higher temperature the particles move faster, so both gases diffuse faster and the ring forms sooner, at about the same place.
Hot apparatus and steam can burn. Use tongs/heatproof gloves if instructed and keep test tubes pointed away from people.
4. Common Mistakes
- Saying “temperature increases during melting”. Wrong for pure substances: temperature is constant during melting/boiling.
- Forgetting the keyword surface only for evaporation.
- Writing “boiling happens at any temperature”. Wrong: boiling happens at a fixed temperature (pure substance at a given pressure).
- Writing “particles expand”. Wrong: spacing increases, not particle size.
- Ignoring impurity: pure substances have sharp points; impure substances show a range.
5. Exam Tips
- State the observation (e.g., “temperature stays constant at 80°C”). 2) Explain using particles + forces of attraction.
If a melting/boiling point is a range, conclude “impure”. See Purity of Substances.
6. Worked Examples
Modelled example 1
Naming the Change
Problem
Name the change of state when water vapour forms liquid water on a cold window.
Study the worked solution
Identify the initial and final states
Method
Classify water vapour as a gas and the droplets as a liquid.
Reason
State-change names are determined by the direction between the initial and final states.
Working
Gas → liquid.Name the change
Working
Gas → liquid is condensation.
Guided practice 2
Evaporation vs Boiling
Problem
Choose the process before explaining
Hints
Hint 1: use the stated temperature
The water is below its boiling point under the stated conditions.
Hint 2: locate the change
One liquid-to-gas process occurs only at the surface and can happen at any temperature.
View solution step by step
Use the temperature evidence
Method
Reject boiling under the stated conditions.Reason
Boiling occurs at the boiling point throughout the liquid, whereas this puddle is below that temperature.
Working
No boiling bubbles are required for the puddle to lose water.
Identify evaporation
Method
Name evaporation and state its distinguishing feature.
Reason
Evaporation occurs at the surface and can occur at any temperature.
Working
The puddle disappears by evaporation.
Common misconception 3
Heating Curve Plateau
Learner response
Track where the supplied energy goes
View solution step by step
Locate the first error
Method
Reject the inference that a flat line means no energy is supplied.
Reason
A heating curve can remain flat while the heater continues transferring energy during a state change.
Working
The graph shows constant temperature, not zero energy transfer.
Identify the energy pathway
Method
State that the supplied energy overcomes forces of attraction between particles.
Reason
The substance is changing state, so particles separate or rearrange rather than gaining average kinetic energy.
Working
Energy changes the particle arrangement during melting or boiling.
Explain the plateau
Working
Average kinetic energy and temperature remain constant until the change of state is complete.
Examiner practice 4
Impurity Interpretation
Examination question
State the conclusion and decisive evidence
View solution step by step
Read the melting behaviour
1 markMethod
Identify a melting range from 112°C to 118°C.Reason
The sample does not melt at one sharp temperature.Working
Melting range: 6°C.Deduce purity
1 markWorking
The solid is impure because a pure substance has a sharp melting point.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the evidence and purity conclusion separately.
Challenge 5
Pressure and Boiling Point
Changed-condition transfer
Water boils at a lower temperature on a mountain than at sea level. Explain why.
Follow the external-pressure change
Hints
Hint 1: compare atmospheric pressure
A mountain has lower atmospheric pressure than sea level.
Hint 2: apply the pressure rule
Lower external pressure lowers the temperature at which boiling occurs.
View solution step by step
Identify the changed condition
Method
State that atmospheric pressure is lower on the mountain.
Reason
Altitude changes the external pressure acting on the water.
Working
Mountain: lower atmospheric pressure than sea level.
Infer the boiling point
Method
Conclude that water boils at a lower temperature.Reason
At lower external pressure, the condition for bubbles to form throughout the liquid is reached at a lower temperature.
Working
Lower pressure → lower boiling point.
7. Mind Stretchers
Mind stretcher 1: Sweating and CoolingExtension
Question: Explain why sweating cools your skin using evaporation.
Show Answer
Sweat evaporates from the skin surface. Evaporation absorbs heat energy from the skin, so the skin loses energy and feels cooler.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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