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Mass is conserved during a change of state: when substances melt, freeze, boil or evaporate, the total mass stays the same. Free GCSE Physics notes.
Changes of State: Mass Conservation and Reversibility is part of Changes of state and the particle model in GCSE AQA Higher Physics.
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Mass is conserved during a change of state: when substances melt, freeze, boil or evaporate, the total mass stays the same. Free GCSE Physics notes.
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Use this lesson to stay inside Changes of state and the particle model while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from Changes of State: Mass Conservation and Reversibility before testing yourself from memory.
You boil 100g of water in a kettle. Steam pours out and seems to vanish into thin air. Has the water disappeared? Has mass been lost?
This puzzle reveals a fundamental principle of science. When substances change state, whether melting, freezing, boiling, evaporating, condensing or sublimating, the total mass stays the same. The particles do not disappear and no new substance is made during a physical change. This is why a careful mass measurement before and after a state change gives the same result if all of the substance is kept in the system.
Picture 100g of ice cubes in a sealed container. You heat them until they melt completely into liquid water. What's the mass now?
The answer is still exactly 100g. Here's why: the water molecules ($H_{2}O$) haven't changed at all. They're still the same molecules with the same mass. They've just gained energy and can move around more freely.
This applies to all state changes. Whether you're melting chocolate, freezing orange juice, or boiling alcohol, the total mass stays constant. The particles don't disappear - they just rearrange.
State changes are physical changes, not chemical changes. There's a simple test to tell them apart: reversibility.
| Physical Change | Chemical Change |
|---|---|
| No new substances formed | New substances formed |
| Can be reversed | Usually cannot be reversed |
| Original properties recovered | Original properties lost |
| Example: Ice → Water → Ice | Example: Wood → Ash + Smoke |
When you melt butter and then cool it down, you get butter back with all its original properties. The molecules haven't changed - they've just been rearranged by energy changes.
Compare this to burning paper. You can't 'unburn' paper to get the original back. New substances (ash, carbon dioxide, water vapour) have formed with completely different properties.
Worked example: Mass Conservation in Evaporation
A student heats 250 g of water in an open beaker until 50 g has evaporated. Calculate the mass of water remaining in the beaker.
Answer: 200 g of water remains in the beaker
Notice how the 50 g of water did not vanish. If the steam escapes from an open container the measured mass of the remaining liquid falls, but the total mass of water plus water vapour is unchanged. In a sealed system, you would still measure the full original mass after boiling and condensation.
Examiners love to test whether you truly understand conservation of mass. Here are the most common mistakes and how to avoid them.
Mark-winner: When describing state changes, always mention that 'the particles remain the same type' and 'no new substances are formed'. These exact phrases appear in mark schemes.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A physical change is a change where no new substances are formed and the original properties can be recovered by reversing the change. The particles remain the same type but change their arrangement or energy, as in melting, freezing or boiling.
Conservation of mass is the principle that mass cannot be created or destroyed in any physical or chemical process, so the total mass before equals the total mass after. During a change of state, the particles do not disappear; they just rearrange.
A reversible change is a change that can be undone to restore the original material with its original properties, with no new substances formed. Melting butter and then cooling it gives butter back with all its original properties.
In a physical change no new substances are formed, the change can be reversed and the original properties are recovered, like ice to water to ice. In a chemical change new substances are formed and it usually cannot be reversed, like wood burning to ash and smoke.
Burning paper is a chemical change, because new substances such as ash, carbon dioxide and water vapour form with completely different properties. It cannot be reversed to get the original paper back, unlike a physical change such as melting ice.
Mass is conserved during a change of state: when substances melt, freeze, boil or evaporate, the total mass stays the same. The particles do not disappear and no new substance is made, so 100g of ice melts into 100g of water.
A change of state is a physical change because no new substances are formed and the change can be reversed to recover the original properties. The particles remain the same type and only change their arrangement or energy.
When water evaporates, its mass does not vanish; it becomes water vapour. If 50 g of 250 g of water evaporates from an open beaker, 200 g remains, but 200 g of liquid water plus 50 g of water vapour still makes 250 g in total.
The changes of state are melting, freezing, boiling, evaporating, condensing and sublimating. In each one the total mass stays the same, because the particles do not disappear and no new substance is made.
Melting butter is a physical change, because cooling it down gives butter back with all its original properties. The molecules have not changed; they have just been rearranged by energy changes.
If 50 g of water evaporates from 250 g in an open beaker, 200 g of water remains. The evaporated water has not disappeared: 200 g of liquid water plus 50 g of water vapour still makes 250 g in total.
The measured mass goes down when water boils in an open container because the steam escapes, so less liquid is left. The total mass of water plus water vapour is unchanged; in a sealed system the full original mass would still be measured.
100 g of ice in a sealed container still has a mass of exactly 100 g after it melts. The water molecules have not changed; they are the same molecules with the same mass, but they have gained energy and can move around more freely.
A physical change can be told from a chemical change by the reversibility test. A physical change forms no new substances, can be reversed and recovers the original properties, while a chemical change forms new substances and usually cannot be reversed.
A change of state should be described by stating that the particles remain the same type and that no new substances are formed. It is a physical change, the total mass stays the same, and the change can be reversed.