Year 7 · Science
Changes of state with the particle model: Year 7
What changes when a substance changes state?
A change of state is a physical change. The particles keep the same identity, while energy transfer changes their movement, spacing and the effect of attractions between them. The model is useful for explaining patterns, but the dots are not literal pictures of particles and their spacing is not drawn to scale.
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it happens | At the liquid surface | Throughout the liquid |
| Temperature | Can happen below the boiling point | Happens at the boiling point for the given pressure |
| What particles do | Some surface particles escape | Gas bubbles form throughout the liquid |
Keep practising: Use the Year 7 Science checklist
Understand the key idea
Melting, freezing, boiling and condensing are physical changes. The substance keeps the same particles, but their arrangement and movement change. During melting or boiling at constant pressure, a pure substance can absorb energy without its temperature rising.
Model answer: explain what happens when water boils
As water warms towards its boiling point, its particles move faster on average. Once pure water boils at constant pressure, its temperature stays constant: the energy supplied allows particles to overcome attractions and separate into the gas state. Bubbles of water vapour form throughout the liquid, not only at the surface. The bubbles contain water vapour, not air, and the particles remain water particles.
Common mix-up
Particles do not disappear or become a new substance during a change of state.
Try this first
- Name the starting and finishing states.
- Decide whether energy is transferred into or out of the substance.
- Describe how particle movement, spacing and attraction change.
Ask: What stays the same when ice melts, and what changes?
Year 7 learners practise explaining melting, freezing, boiling and condensing.
The activities include Compare Chemical and Physical Changes, Recognise Changes of State: Freezing and Condensing, Recognise Changes of State: Melting and Boiling, and related practice. The checked support below includes video, teaching, worksheet, lesson. Open a resource in its provider's website and choose the format that best helps the learner explain the idea independently.
Activities covered
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- Compare Chemical and Physical Changes — support from BBC Bitesize
- Recognise Changes of State: Freezing and Condensing — support from BBC Bitesize
- Recognise Changes of State: Melting and Boiling — support from Oak National Academy
- Understand Changes of State and Forces of Attraction — support from Oak National Academy
- Understand The Movement of Particles in Changes of State — support from FuseSchool
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Watch it explained
watch a video explanation
What is a chemical reaction?
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Changes of state
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Oak lesson video: Changes of state, energy and evaporation
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Melting Particle Model
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Changes of State
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Learn it properly
lessons and teaching pages
KS3 Chemical Reactions Lesson 1: Characteristics of Chemical Reactions Lesson Pack
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Melting: melting points and freezing points
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Boiling and condensing
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Practise it
questions and activities to try
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Print and finish
worksheets away from the screen
Oak worksheet: Characteristics Of Chemical Reactions
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Physical or Chemical Change Match and Draw
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Changing State Worksheet
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Temperature and Changing State Worksheet
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Changes of State Illustration Sheet
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