Changes of state and conservation of mass
Melting ice or boiling a kettle changes how particles behave — but not how many particles, or how much mass, there is.
Part of your national curriculum
- Chemistry: the particulate nature of matter: Understand changes of state in terms of the particle model, including conservation of mass
Lesson overview
What you'll learn in this lesson
Understand changes of state in terms of the particle model, including conservation of mass
Key learning points
- • Melting, freezing, evaporating, condensing
- • Changes of state are physical, not chemical
- • Conservation of mass
This lesson at a glance
- 24 minutes
- 14 parts to scroll through
- 3 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 14
Watch first
Changes of state and conservation of mass
An English teaching video on this topic from James and Chris Science Videos. Watch it, then scroll down for the lesson.
In a nutshell
Understand changes of state in terms of the particle model, including conservation of mass
Learning cycle
Part 2 of 14
Learning cycle 1 of 2
Part 1 · Melting, freezing, evaporating, condensing
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 14
Learn
Melting, freezing, evaporating, condensing
When a solid is heated, particles gain energy and vibrate more until they break free of fixed positions — melting. Heating further gives liquid particles enough energy to escape entirely as a gas — boiling or evaporating. Cooling reverses these changes: condensing and freezing.
Explore the idea
Part 4 of 14
Learn
Changes of state are physical, not chemical
No new substances are formed during a change of state — only the arrangement and movement of existing particles changes. Water becoming ice is still made of the same H2O molecules; the change can be reversed by heating.
Quick check
Part 5 of 14
Quick check
What happens to particles when a solid melts?
Reset break
Part 6 of 14
Pause
That's sitting 1 of 2 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 7 of 14
Quick check
Is melting a chemical or physical change?
Learning cycle
Part 8 of 14
Learning cycle 2 of 2
Part 2 · Conservation of mass
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 9 of 14
Learn
Conservation of mass
Because no particles are created or destroyed during a change of state, the total mass stays the same. If 100 g of ice melts completely, you get exactly 100 g of water (though its volume changes slightly).
Quick check
Part 10 of 14
Quick check
100 g of liquid water is frozen completely. What is the mass of the ice?
Challenge round
Part 11 of 14
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 12 of 14
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
When a solid is heated, particles gain energy and vibrate more until they break free of fixed positions — ____.
Cooling reverses these ____: condensing and freezing.
Mastery quiz
Part 13 of 14
Marked quiz
End of lesson quiz: Changes of state and conservation of mass
3 questions, marked with the reasoning shown. No timer.
1. What happens to particles when a solid melts?
2. Is melting a chemical or physical change?
3. 100 g of liquid water is frozen completely. What is the mass of the ice?
Lesson round-up
Part 14 of 14
Lesson round-up
Ready when you are
Quiz score
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Points this lesson
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Luna: 0 out of 3 on the practice checks. Only if you feel up to it — one more?
Ask LunaDo the work
Your written work for this lesson
This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.
Why the school greenhouse failed
A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.
The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.
The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.
Written for Good Learning Co.
Reading questions
- Name the variable the club changed the second time, and one variable they kept the same.
- Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
- Write a conclusion for the club's report, saying how far the evidence supports it.
Method and results
WritingWrite the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
WritingExplain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
DiagramDraw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
ReadingRead the lesson text again and write the key ideas in your own words.
Half a page of notes
Saved as you type.
Part 1 of 14 · Watch & discover
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Understand changes of state in terms of the particle model, including conservation of mass