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evaporation
Evaporation occurs at a liquid surface; boiling is another vaporisation process.
Class 7 · English Medium · Pearson Edexcel · General science
Matter, materials and chemical change
Distinguish physical state changes from chemical reactions using evidence.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
A substance’s physical state depends on particle arrangement and motion. Melting changes solid to liquid without changing chemical identity. A chemical reaction rearranges atoms into new substances. A colour change or bubbles can be useful evidence, but must be interpreted alongside alternatives such as dissolving gas.
Worked example
Ice melting is a physical change. Magnesium reacting with oxygen forms magnesium oxide, a new substance. These examples are for discussion; do not attempt burning metals at home.
- Identify the starting and resulting substances.
- Describe the particle change.
- Compare a reaction explanation with other possibilities.
Stretch example
For 18 g water, using molar mass 18 g/mol gives 1 mol of water molecules. Each molecule contains two hydrogen atoms and one oxygen atom; a mole counts entities, not grams.
Your written practice
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liquid
A liquid takes the container’s shape.
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no
Both ice and liquid water are H2O.
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The sugar can remain chemically unchanged while dispersed in water.
Distinguish disappearance from new substance formation.
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Atoms rearrange but none enter or leave the closed system, so total mass stays constant.
State the closed-system condition.
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2
n = mass/molar mass = 36/18.
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Gas can leave the flask, so the measured system loses matter even though total mass including escaped gas is conserved.
Define the measurement boundary.
Particle diagrams
Draw labelled particle models for solid, liquid and gas of the same substance.
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- Keep particle identity the same.
- Show arrangement and spacing.
- Explain one limit of the drawing.
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