45-minute lesson
Year 4 Science: Melting and freezing
Lesson objective
- Describe the difference between solids and liquids.
- Explain that heating can melt a solid and cooling can freeze a liquid.
- Compare how quickly different solids melt in the same conditions.
Success criteria:
- I can sort materials into solids and liquids.
- I can explain what melting and freezing mean.
- I can predict and test which solid will melt first.
Materials
- Ice cubes
- Small equal-sized pieces of butter and chocolate
- Three small identical plates or patty-cake cases
- A sunny windowsill or a tray of warm (not hot) water
- A clock or timer
- An ice-cube tray and some fruit juice
Introduction
5 minIntroduce today's words:
- solid: A material that keeps its own shape, like ice or a block of wood.
- liquid: A material that flows and takes the shape of its container, like water or juice.
- melt: To change from a solid to a liquid when heated.
- freeze: To change from a liquid to a solid when cooled.
- state: Whether a material is a solid, liquid or gas.
Ask your child what they already know about melting and freezing.
Explanation
9 minMaterials can be solid or liquid (or gas, which we will learn about later). A solid keeps its shape. A liquid flows and takes the shape of the container it is in.
When a solid is heated enough, it melts and becomes a liquid. Ice melts into water, and chocolate melts in a warm hand.
When a liquid is cooled enough, it freezes and becomes a solid. Water in the freezer turns into ice.
Different materials melt at different temperatures. Ice melts at 0 °C. Butter and chocolate melt at warmer temperatures, which is why they stay solid in the fridge and go soft on a hot summer day.
Melting and freezing can be reversed. Melted chocolate can be cooled and set solid again. It is still chocolate, just in a different shape.
In a fair test we keep everything the same except one thing. To compare melting, we use same-sized pieces, the same plates and the same warm spot.
Worked examples
7 minWhich melts first?
An ice cube, a piece of butter and a piece of chocolate (all the same size) are put on plates in a sunny window. Which melts first?
Step 1: Predict and write down your guess.
Step 2: Put all three plates in the same sunny spot at the same time.
Step 3: Check every five minutes and record what you see.
Step 4: On a warm day the ice usually turns to liquid first because it melts at the lowest temperature (0 °C).
Answer: Usually the ice melts first; butter and chocolate soften more slowly.
Juice ice blocks
How can we turn juice into a solid?
Step 1: Pour juice into an ice-cube tray.
Step 2: Put the tray in the freezer for several hours.
Step 3: The cold makes the liquid juice freeze.
Answer: Cool it in the freezer so it freezes into a solid.
Guided practice (do together)
9 min1. Which of these is a liquid at room temperature?
(a) A brick (b) Milk (c) A spoon (d) An ice cube
2. What makes ice melt?
(a) Heating it (b) Cooling it (c) Shaking it in the dark (d) Putting it in the freezer
3. Why does chocolate melt in a car on a hot summer day?
(a) The car is moving (b) It is too dark (c) It gets warm enough to melt (d) It gets too cold
4. Melted chocolate is put in the fridge. What happens?
(a) It turns into water (b) It boils (c) It disappears (d) It becomes solid again
Independent practice
11 min5. In a fair test of melting, what should be different?
(a) The size of each piece (b) Only the type of material (c) The place each plate is left (d) The starting time
6. At what temperature in degrees Celsius does ice melt? (number only)
7. Water changing into ice is called f_______.
8. Is an ice cube a solid or a liquid?
Questions to check understanding
- Can you sort materials into solids and liquids?
- Can you explain what melting and freezing mean?
- Can you predict and test which solid will melt first?
- What was the trickiest part today?
Answer guide
1. Milk — Milk flows and takes the shape of its container, so it is a liquid.
2. Heating it — Heating makes solids melt.
3. It gets warm enough to melt — A hot car warms the chocolate above its melting temperature.
4. It becomes solid again — Cooling makes the liquid chocolate set solid again.
5. Only the type of material — Only one thing should change: here, the material being tested.
6. 0 — Ice melts (and water freezes) at 0 °C.
7. freezing — A liquid turning into a solid when cooled is freezing.
8. solid — Ice keeps its shape, so it is a solid.
Review
4 minRecap the success criteria together. Watch for these common misconceptions:
- Some children think melting makes a material lighter or 'disappear'. A melted ice cube makes the same amount of water.
- Children may think only ice melts. Many solids melt, including butter, chocolate, wax and metals (at very high temperatures).
- Some think sand is a liquid because it can be poured. Each sand grain is a tiny solid.
Extension activities
- Make fruit-juice ice blocks and time how long they take to freeze.
- Hold an ice cube competition: who can keep an ice cube solid the longest using a cloth, foil or a cup?
- List five solids and five liquids in your kitchen.
Suggested follow-up
- Revisit melting and freezing tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
Tips for parents
- Cut the butter and chocolate into pieces of the same size so the test is fair.
- Take photos every five minutes so your child can line them up and compare.
- Safety: use only sunlight or warm tap water for heating; no stovetops, candles or boiling water for this activity.