Year 5 Science: Changes of state and mixing
Part of the Year 5 Science reversible and irreversible changes chapter. Chapter overview · whole chapter on one page
Changes of state and mixing
A change of state happens when a material changes between solid, liquid and gas. Melting, freezing, evaporating and condensing are changes of state.
When a solid melts, it becomes a liquid. When the liquid cools, it can freeze back into the solid. In both directions the material is still the same kind of material, so the change is reversible.
Mixing is also reversible. A mixture of sand and pebbles can be separated again, by sieving. The parts of the mixture are still the same materials after they are mixed.
Key ideas
- Melting changes a solid into a liquid.
- Freezing changes a liquid back into a solid.
- A reversible change of state can be undone, and the material returns to its start.
- Mixing is reversible, because the parts can be separated again.
Method. Think about the change of state.
- Think about the change. When the liquid cools, it can freeze back into the solid.freeze back
- Decide. So chocolate can change back, and the answer is yes.yes
- Write the answer. yes
Check. Melting and cooling are reversible changes of state. ✓
Method. Think about evaporation.
- Think about the change. Water turns into water vapour, which is a gas, by evaporation.water vapour
- Name the change. The change of state is evaporation.evaporation
- Write the answer. evaporation
Check. Evaporation can be reversed by condensing the vapour. ✓
Method. Think about mixing.
- Think about the mixture. The sand and pebbles are still the same materials after mixing.same materials
- Decide. So they can be separated again, by sieving. The answer is yes.yes
- Write the answer. yes
Check. Mixing is a reversible change. ✓
Watch out
Watch out: a change that is reversible is not always easy to reverse. Melting chocolate needs warmth, and cooling it needs a fridge. The change can still be undone.
Try it (with your teacher). Put a few small pieces of chocolate in a dish, and watch them melt in a warm place (with your teacher). Then let the dish cool, and see whether the chocolate becomes hard again. Write one sentence about whether the change is reversible.
Exam tip
Grown-up tip. Ask: “What happened when the chocolate melted? What happened when it cooled? Did it change back?” Keep the heating very gentle, with a grown-up.
Exercise 7B Changes of state and mixing
Method. Think about whether the chocolate can change back.
- Think about cooling. When the melted chocolate cools, it can become hard again.hard again
- Decide. So melting chocolate is reversible.yes
- Write the answer. yes
Check. Melting and cooling are reversible changes of state. ✓
Method. Read section 7.2.
- Read the description. When a liquid cools and becomes a solid, it is freezing.liquid to a solid
- Name the change. So the change is freezing.freezing
- Write the answer. freezing
Check. Freezing is a change from liquid to solid. ✓
Method. Read section 7.2.
- Read the description. When a solid melts, it becomes a liquid.becomes a liquid
- Fill the gap. So the missing word is liquid.liquid
- Write the answer. liquid
Check. Melting is the change from solid to liquid. ✓
Method. Read section 7.2.
- Read the description. Water turns into water vapour, a gas, by evaporation.water vapour
- Name the change. So the change is evaporation.evaporation
- Write the answer. evaporation
Check. Evaporation is a change of state. ✓
Method. Read section 7.2.
- Read the description. Mixing is reversible, because the parts can be separated again.reversible
- Decide. So the statement is false.false
- Write the answer. false
Check. Sand and pebbles can be separated by sieving. ✓
Method. Read section 7.2.
- Read the description. The parts of a mixture can be separated again.separated again
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. A sieve separates the mixture. ✓
Method. Think about the chocolate.
- Think about the chocolate. Melted chocolate can cool and become hard again, the same material.same material
- Decide. So Mo is not right.no
- Write the answer. no
Check. Melting is a reversible change of state. ✓
Method. Read the arrows in the picture.
- Read the arrows. Both arrows are labelled, so the change can go both ways.both ways
- Decide. So the change from water to ice is reversible.yes
- Write the answer. yes
Check. Freezing water can melt again. ✓
Method. Think about what makes a test fair.
- Think about the test. Only the thing you change should be different between tests.only one thing
- Write one thing. The size of the ice cube, the dish and the room must stay the same.stay the same
- Write the answer. Accept any one of: the same size of ice cube, the same dish, the same room temperature.
Check. A fair test keeps everything else the same. ✓
| Trial | Minutes for the ice to melt |
|---|---|
| Trial 1 | 12 |
| Trial 2 | 14 |
| Trial 3 | 13 |
Method. Add the three trials and divide by three.
- Add the trials. The three trials add to 39 minutes.12 + 14 + 13 = 39
- Divide by three. Divide 39 by 3 to get the mean.39 divided by 3 is 13
- Write the answer. 13
Check. Check: 13 + 13 + 13 = 39. ✓
Method. Think about what melting does.
- Think about the water. The water is still water, the same material as the ice.same material
- Write the reason. So cooling it again turns it back into ice.back into ice
- Write the answer. Accept any sentence that says the water is still the same material, so it can freeze back into ice.
Check. A change of state can be reversed. ✓
Method. Think about the materials.
- Think about the materials. The sand and pebbles do not change into new materials.do not change
- Write the reason. So they can be separated again, for example by sieving.separated again
- Write the answer. Accept any sentence that says the sand and pebbles are still the same materials, so they can be separated again.
Check. A reversible change keeps the same materials. ✓