Changes of state — AQA GCSE Combined Science
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Changes of state explained
In a change of state, particles gain or lose energy but no particles are created or destroyed, so total mass stays the same.
Read the full explanation
Melting and freezing swap solid and liquid; boiling and evaporating swap liquid and gas; condensing swaps gas to liquid; sublimation goes straight from solid to gas. For example, 18 g of ice melts to give 18 g of water, and 18 g of water boils to give 18 g of steam. In a sealed container the mass before and after is identical. In an open container steam may escape, so the mass of what remains falls, but the mass of the whole system is still conserved.
Changes of state are physical changes which differ from chemical changes because the material recovers its original properties if the change is reversed.
A change of state is physical: the particles keep their identity, only their arrangement and energy change. If the change is reversed, the material gets back its original properties. Steam condensed to water can be frozen to ice. The ice melts at 0 °C, and the resulting water boils at 100 °C, having the same density and appearance as before. In a chemical change, new substances form with different properties. For example, burning magnesium gives white magnesium oxide, which cannot be turned back into shiny magnesium by simply cooling. Comparing properties before and after, and testing whether reversing restores them, distinguishes physical from chemical change.
Your focus
- Name and describe the six changes of state and identify the direction of each.
- Apply conservation of mass to a numerical example of a change of state.
- Explain apparent mass loss in an open system using the particle model.
Show all 6 objectives
- Distinguish physical changes from chemical changes using properties before and after.
- Describe how reversing a change of state restores the original material and its properties.
- Justify a classification of a given change as physical or chemical using evidence.
Changes of state exam tips
Marking Points
- State that during any change of state the number of particles is unchanged, so total mass is unchanged.
- Name the six changes: melting, freezing, boiling, evaporating, condensing and sublimating, and give the direction of each.
- Use a numerical example, such as 18 g ice → 18 g water → 18 g steam, to show mass before equals mass after.
- Explain that in an open system escaped gas can make the measured mass appear to decrease, but the total mass of all products is still conserved.
- Link conservation of mass to the particle model: arrangement and energy change, but particle count and identity stay the same.
- Define a physical change as one where no new substance forms and the particles keep their identity.
- State that reversing a change of state restores the original properties, such as melting point, boiling point, density and appearance.
- Contrast with a chemical change, where new substances with different properties form and simple reversal does not recover the original material.
- Use a concrete example, such as ice → water → steam → water → ice, and note that the final ice behaves exactly like the starting ice.
- Explain that particle arrangement and energy change during a change of state, but the particles themselves are unchanged.
Examiner Tips
- 💡Always name the starting and finishing state, for example 'liquid → gas', before explaining conservation of mass.
- 💡If a question mentions an open container, comment on gas escaping as well as on conservation of mass.
- 💡Use the phrase 'no particles are created or destroyed' to show the reason mass is conserved, not just the observation.
- 💡When asked to compare, give one feature of a physical change and one feature of a chemical change, then link each to an example.
- 💡Use the word 'reversible' only when you can state the reverse process and the property recovered.
- 💡If a question gives data, compare a numerical property before and after, such as melting point or density, to support your answer.
Common Mistakes
- Thinking mass is lost when a liquid evaporates in an open dish. Correction: the vapour has escaped to the surroundings; the total mass of liquid plus vapour is unchanged.
- Believing boiling destroys particles or creates new ones. Correction: particles separate and move faster, but each particle still exists, so mass is conserved.
- Confusing sublimation with melting. Correction: sublimation is solid → gas directly, with no liquid stage, for example dry ice (solid carbon dioxide) forming carbon dioxide gas.
- Calling melting or boiling a chemical change because it looks dramatic. Correction: no new substance forms, so it is physical.
- Claiming that reversing a chemical change is impossible. Correction: some chemical changes are reversible. While many require chemical reagents to reverse, some, like the decomposition of ammonium chloride, reverse simply by cooling.
- Saying the material 'recovers its original properties' without naming any property. Correction: give a specific property, such as melting point, density or colour.