Year 5 science · Key Stage 2 · Properties and changes of materials
Year 5 materials: dissolving, separating and reversible changes
In Year 5, children sort everyday materials by their properties, find out how solids dissolve to make solutions, and use that knowledge to separate mixtures by filtering, sieving and evaporating. They also learn the difference between changes that can be reversed, such as dissolving and melting, and changes that make new materials, such as burning and the fizzing of vinegar on bicarbonate of soda.
Dissolving, separating and changing materials, explained
The first strand is comparing and grouping materials by their properties: hardness, solubility (whether something dissolves), transparency, electrical and thermal conductivity, and whether they respond to magnets. Children build on Year 3 work on magnets and Year 4 work on electricity. They are expected to give reasons for why a material is used for a job, based on evidence from comparative and fair tests. A metal pan has a plastic or wooden handle because metal conducts heat well and the handle must not, and a window is glass because it is hard and transparent. A typical class test is comparing fabrics: wrap the same amount of warm water in different materials, change only the material and see which keeps the water warm longest. Children aren’t expected to measure conductivity in numbers, only to compare.
Dissolving is the next idea. When sugar or salt is stirred into water, the solid breaks up into pieces too small to see and spreads through the liquid. The result is a solution, and the solid is said to be soluble. Sand or flour does not dissolve, so it is insoluble. A solution can be clear and still have a colour, so “transparent” and “colourless” are not the same thing. Because the dissolved solid is still there, it can be recovered. If the water is left to evaporate, the solid is left behind.
Children use what they know about solids, liquids and gases to decide how to separate a mixture. Sieving separates solid pieces of different sizes, such as gravel from sand. Filtering, using filter paper in a funnel, separates an insoluble solid from a liquid. Evaporating recovers a dissolved solid from a solution. The key skill is choosing the method by thinking about what each ingredient does, for instance whether it dissolves, how big the pieces are and whether the liquid needs to be kept.
Finally, children sort changes into two kinds. Dissolving, mixing and changes of state (melting, freezing, evaporating, condensing) are reversible: the material can be got back. Some changes make a new material and are not usually reversible. Burning is the main example, along with rusting and the reaction between an acid such as vinegar and bicarbonate of soda, which makes a gas. Children also meet the difference between melting and dissolving: melting is a solid turning to a liquid when it is heated, while dissolving needs a liquid for the solid to spread through. Many schools also tell the stories of chemists who created useful new materials. Which ones is the school’s choice, for example Spencer Silver, whose weak glue ended up on sticky notes, or Ruth Benerito, who worked on wrinkle-resistant cotton. Any burning is done by the teacher with great care.
Words your child will hear
- Soluble and insoluble
- Soluble materials dissolve in a liquid, such as sugar in water. Insoluble ones do not, such as sand.
- Solution
- The mixture made when a solid dissolves completely in a liquid, such as salty water.
- Filtering
- Pouring a mixture through filter paper so that an insoluble solid is caught and the liquid passes through.
- Evaporating
- A liquid turning into a gas, which can leave a dissolved solid behind.
- Conductor and insulator
- A conductor lets heat or electricity pass through easily. An insulator does not.
- Reversible change
- A change that can be undone, such as melting chocolate and letting it set again.
- Irreversible change
- A change that makes a new material and cannot easily be undone, such as burning.
What the National Curriculum says
In Year 5, pupils should be taught to:
- Compare and group together everyday materials on the basis of their properties, including their hardness, solubility, transparency, conductivity (electrical and thermal), and response to magnets
- Know that some materials will dissolve in liquid to form a solution, and describe how to recover a substance from a solution
- Use knowledge of solids, liquids and gases to decide how mixtures might be separated, including through filtering, sieving and evaporating
- Give reasons, based on evidence from comparative and fair tests, for the particular uses of everyday materials, including metals, wood and plastic
- Demonstrate that dissolving, mixing and changes of state are reversible changes
- Explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible, including changes associated with burning and the action of acid on bicarbonate of soda
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example
A mixture contains sand, salt and water. How can you get the sand and the salt back?
- Think about each ingredient. Salt dissolves in water, so it is soluble and has spread through the water as a solution. Sand does not dissolve, so it is insoluble and is just floating or sitting in the water.
- Separate the sand first. Fold a piece of filter paper into a funnel and pour the mixture through it. The sand is too big to pass through the tiny holes in the paper, so it stays behind. The liquid that comes through (the filtrate) is clear salty water.
- Rinse the sand in the filter paper with a little clean water and leave it to dry. Now you have the sand back, and it hasn’t changed.
- Recover the salt. Pour the salty water into a shallow dish and leave it somewhere warm, such as a sunny windowsill, for a few days. In class the teacher may heat it gently, which children never do alone. The water evaporates into the air but the salt does not, so crystals are left in the dish.
- Check the reasoning. Every step used a property: filtering used the fact that sand is insoluble, and evaporating used the fact that salt is soluble but does not turn into a gas.
Filter the mixture to remove the sand, then evaporate the water from the clear salty water to leave the salt. This shows that mixing and dissolving are reversible changes, because both materials are recovered unchanged.
Common mistakes and how to help
Thinking dissolving means the solid has melted or disappeared
The sugar vanishes from sight, and children often use “melt” in everyday talk for anything that goes soft or goes away.
What helps: Let a little salty water evaporate on a saucer so the salt reappears. Explain that melting is caused by heat, while dissolving needs a liquid.
Believing a clear liquid has to be colourless
Children link “see-through” with “no colour”, so they say squash or weak tea is not transparent.
What helps: Hold a glass of coloured squash up to the light. You can see through it, so it is transparent. Compare it with milk, which is not.
Thinking all metals are magnetic, or that metal feels cold because it is cold
Children often meet magnets with iron and steel, so they group all shiny things together. Metal feels cold because it conducts heat away from your hand quickly.
What helps: Test a steel paper clip, a piece of kitchen foil and a clean, empty drinks can with a fridge magnet. Only the steel clip is attracted, because foil and most drinks cans are aluminium. Then feel a metal spoon and a wooden spoon from the same drawer and ask why they feel different.
Believing that all changes can be reversed if you try hard enough
Melting and freezing are so familiar that children expect to be able to undo any change, including toast or a baked cake.
What helps: Ask “can we get the bread back?” before and after a change. If it made a new material (heat, a fizz, a burn or rust), it is not usually reversible.
How to help at home
- Dissolve or not? Stir a spoonful each of salt, sugar, sand, flour and coffee granules into separate glasses of cool or lukewarm water. Predict first, watch what happens and sort them into soluble and insoluble. Don’t drink or taste any of the mixtures.
- Get the salt back. Dissolve salt in warm tap water, pour a little onto a saucer and leave it on a windowsill. After a few days the water has gone and salt crystals remain, which shows dissolving can be reversed.
- Filter and sieve. Pour muddy or sandy water through a paper coffee filter held in a funnel or sieve, and use a kitchen sieve to separate rice from flour or dried peas. Ask your child which method suits which mixture and why.
- Material detectives. Choose five objects and test each with a fridge magnet, then ask which are transparent and which are hard to scratch. Ask why a kettle handle, a window and a raincoat are made from different materials.
- Fizzy change in the sink. With an adult present, add a spoonful of bicarbonate of soda to a little vinegar in a tray or the sink and watch it fizz. Talk about the gas being a new material that you cannot easily turn back into the powder and vinegar.
Related topics
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