Lesson focus: meltingWhen a solid warms up and turns into a liquid, like ice turning to water. and freezingWhen a liquid cools and turns into a solid, like water turning to ice. are changes of state caused by energy transfer.
Describe melting and freezing using particle movement and energy.
Build on earlier learning
States of matterRevisit how particle arrangement and movement distinguish a solid from a liquid.
You will be successful when you can…
- Describe how particles are arranged and move before, during and after melting.
- Explain why the temperature of a pure substance can remain constant while it melts.
- Interpret a heating curve using evidence from its axes and shape.
- Explain freezing as energy transfer to the surroundings and a change in particle arrangement.
Words that carry the science
- melting
- The physical change from solid to liquid.
- freezing
- The physical change from liquid to solid.
- melting point
- The temperature at which a pure substance changes between solid and liquid at a stated pressure.
- heating curve
- A graph showing how temperature changes as a substance is heated over time or as energy is supplied.
- attraction
- A force between particles that helps hold them close together.
Build the scientific idea step by step
01The substance stays the same
meltingWhen a solid warms up and turns into a liquid, like ice turning to water. and freezingWhen a liquid cools and turns into a solid, like water turning to ice. are physical changes. The substance keeps the same chemical identityWhat a substance actually is (which particles it is made of); this stays the same when it changes state.: solid water and liquid water are both H₂O. Its particles do not grow, shrink or turn into a different type of particle. What changes is their arrangementHow particles are placed or organised, such as in neat rows or jumbled up., movement and the effect of attractionA pulling force between particles that holds them close together. between them.
In a solidA state of matter that keeps its own shape and volume, because its particles are held in place., particles are held in a fixed arrangement and vibrate. In a liquidA state of matter that can flow and takes the shape of its container, but keeps the same volume., particles remain close together but can change neighbours and move past one another. A particle diagram is a modelA simple way of showing something that helps us explain and predict, like a drawing of particles as small balls.: the circles are enormously enlarged and do not show every detail of a real substance.
02Heating before the melting point
When energy is transferred to a solid below its melting pointThe temperature at which a solid melts, such as 0 °C for ice., its temperatureHow hot or cold something is, measured in degrees Celsius (°C). usually rises. In the particle model, the particles vibrate more vigorously about their fixed positions. Temperature is linked to the average kinetic energyHow much movement energy the particles have on average; the faster they move, the hotter the substance. of the particles, so a rise in temperature shows that their average motion has increased.
Heating does not guarantee immediate meltingWhen a solid warms up and turns into a liquid, like ice turning to water.. A pure substanceA substance with only one kind of element or compound in it, such as pure water. begins to melt when it reaches its melting pointThe temperature at which a solid melts, such as 0 °C for ice. at the surrounding pressure.
03What happens during melting
At the melting pointThe temperature at which a solid melts, such as 0 °C for ice., solid and liquid can be present together. Energy continues to be transferred into the substance, but it is used to change the arrangement of particles and overcome some attractions keeping them in fixed positions. The particles become able to move past one another.
While a pure substance is melting at constant pressure, its temperature can remain constant even though heating continues. Once all the solid has melted, further energy transfer makes the liquid particles move faster on average and the temperature rises again.
04Freezing reverses the change of arrangement
During freezingWhen a liquid cools and turns into a solid, like water turning to ice., energy is transferred from the liquid to the surroundings. At the freezing pointThe temperature at which a liquid turns into a solid, such as 0 °C for water., particles become fixed into a solid arrangement. For a pure substance at the same pressure, the freezingWhen a liquid cools and turns into a solid, like water turning to ice. point and melting point are the same temperature.
The particles continue to vibrate in the solid. They do not stop moving completely. Mass is conserved in a closed system because the number and identity of particles have not changed.
Heating curve of water
Ice is heated steadily. While it melts and while it boils, the temperature stays the same: the energy is breaking the forces between particles.
Controls
Read a flat section on a heating curve
A pure solid is heated at a steady rate. Its temperature rises from −10 °C to 0 °C, remains at 0 °C for four minutes, then rises again. Explain the three sections.
- Below 0 °C, transferred energy increases the average particle motion, so the solid warms.
- At 0 °C, solid and liquid are present together. Energy changes particle arrangement and overcomes some attractions, so the temperature remains constant while melting occurs.
- After all the solid has melted, further energy transfer increases average particle motion in the liquid, so its temperature rises.
How to make the reasoning visible: Quote the temperature pattern from the graph and connect each section to a change in particle motion or arrangement.
Analyse illustrative heating data
A learner heats a pure substance at a steady rate. These illustrative values are not measurements from a published experiment.
| Time / min | Temperature / °C |
|---|---|
| 0 | −10 |
| 2 | −5 |
| 4 | 0 |
| 6 | 0 |
| 8 | 0 |
| 10 | 10 |
| 12 | 25 |
Graph skills: SLAPUK and choosing a scaleHeating-curve data: practise scales that do not start at zero.
Between which times is the substance melting? Use the data to justify your answer.
Describe the particle arrangement at 2 minutes and at 10 minutes.
Explain why the graph alone does not tell you how much energy the heater transfers each second.
Use the illustrative data. What is the melting point of this substance? How can you tell?
Check your understanding
Answer from memory, then check against the suggested answer.
How are particles arranged in a solid?
How does particle movement differ in a liquid?
Does a particle itself melt?
What is the melting point?
What is freezing?
An ice–water mixture contains solid ice and liquid water at 0 °C. Explain how both states can be present at the same temperature.
Equal masses of pure substances A and B are heated with identical heaters. A has a flat section at 20 °C and B at 60 °C. What can you conclude, and what can you not conclude, from this evidence?
Why does the temperature stay the same while ice is melting, even though it is still being heated?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A pure solid is heated continuously. When it reaches its melting point, its temperature remains constant for several minutes. Explain why the temperature remains constant and describe what happens to the particles.
(4)Total for this question: 4 marks
Command word: For an explainA command word: say how or why something happens, giving reasons with words like "because" or "so". question, build a linked chain: continued energy transfer → changed attractions/arrangement → liquid particle movement. Naming only the states would not earn all four marks.