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Changes In States Of Matter

Changes of state: melting, boiling, freezing, condensation and sublimation, plus interpreting heating/cooling curves using particle theory.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the solid, liquid and gaseous states of matter and explain their interconversion in terms of the kinetic particle theory and of the energy changes involved.

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 solid melts (pure substance).
  • Boiling point: temperature where a liquid boils (pure substance at a stated pressure).
  • Freezing point: temperature where a liquid freezes (pure substance).

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

Quick Recall (energy + particles)
  • 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

ChangeNameHeat energy
Solid → liquidMeltingAbsorbed
Liquid → gasBoiling / EvaporationAbsorbed
Gas → liquidCondensationReleased
Liquid → solidFreezingReleased
Solid → gasSublimationAbsorbed
Changes between states of matterChanges of state: melting, boiling/evaporation and sublimation absorb energy; freezing, condensation and deposition release energy.solidliquidgasmeltingfreezingboiling / evaporationcondensationsublimationdepositionBlue arrows: energy absorbedGrey arrows: energy released
Changes of state: melting, boiling/evaporation and sublimation absorb energy; freezing, condensation and deposition release energy.

B. Melting and Freezing (Particle Explanation)

Melting is solid → liquid at the melting point (pure substance).

  1. Heat energy is absorbed.
  2. Particles gain kinetic energy and vibrate more.
  3. At the melting point, energy is used to overcome forces of attraction holding particles in fixed positions.
  4. Particles can now move/slide past each other (liquid).

Freezing is liquid → solid at the freezing point.

  1. Heat energy is released.
  2. Particles lose kinetic energy and move more slowly.
  3. Forces of attraction pull particles into fixed positions (solid).
Pure substance rule

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).

  1. Heat energy is absorbed.
  2. Particles gain kinetic energy and move faster.
  3. At the boiling point, particles have enough energy to overcome attractions and escape as gas.

Evaporation is liquid → gas at the surface and can happen at any temperature.

FeatureBoilingEvaporation
TemperatureOccurs only at a fixed boiling pointOccurs at any temperature
LocationThroughout the liquid (bubbles form)Surface only
RateFasterSlower
Factors that increase evaporation rate

Higher temperature, larger surface area, moving air (wind), and lower humidity increase evaporation.

Condensation is gas → liquid. Heat energy is released and particles move closer together.

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.

Example heating curve showing temperature rising and flat sections during melting and boiling for a pure substance.Example heating curve showing temperature rising and flat sections during melting and boiling for a pure substance.
Flat sections show melting/boiling where temperature stays constant because energy is used to overcome forces of attraction.
Open full-size graph
View figure data
Values for Heating Curve (Pure Substance)
Time (min)Heating curve
020
260
480
680
8120
10120
12140
Why flat lines happen

During melting/boiling, temperature stays constant because energy is used to overcome forces of attraction, not to increase kinetic energy.

Safety: heating liquids

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

2-line answers score
  1. State the observation (e.g., “temperature stays constant at 80°C”). 2) Explain using particles + forces of attraction.
Link to purity when points are not sharp

If a melting/boiling point is a range, conclude “impure”. See Purity of Substances.

6. Worked Examples

Modelled example 1

Naming the Change

Core

Problem

Name the change of state when water vapour forms liquid water on a cold window.
Study the worked solution

  1. 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.

  2. Name the change

    Working

    Gas → liquid is condensation.

Guided practice 2

Evaporation vs Boiling

About 4 min

Problem

A puddle disappears on a warm day even though the water never reaches 100°C. Decide whether this is boiling or evaporation and give one reason.

Choose the process before explaining

Process

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

  1. 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.

  2. 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

Find and correct the mistake

Learner response

A heating curve has a flat section at 80°C. A student says, “The temperature is constant because the heater must have been switched off.” Locate the first error and give the particle-energy explanation.

Track where the supplied energy goes

Energy use during the plateau

View solution step by step

  1. 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.

  2. 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.

  3. Explain the plateau

    Working

    Average kinetic energy and temperature remain constant until the change of state is complete.

Examiner practice 4

Impurity Interpretation

2 marks

Examination question

A solid starts melting at 112°C and finishes melting at 118°C. Deduce whether it is pure and explain. [2 marks]

State the conclusion and decisive evidence

View solution step by step

  1. Read the melting behaviour

    1 mark

    Method

    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.

  2. Deduce purity

    1 mark

    Working

    The solid is impure because a pure substance has a sharp melting point.

Challenge 5

Pressure and Boiling Point

Minimal support

Changed-condition transfer

Water boils at a lower temperature on a mountain than at sea level. Explain why.

Follow the external-pressure change

Atmospheric pressure on the mountain
Resulting boiling point

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

  1. 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.

  2. 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.

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz