We experience examples of materials changing when we stir sugar into a cup of tea, make toast, bake a cake or drop ice cubes into a drink. Children will be aware of the consequences of changes in material when they eat ice cream on a warm day or hold a chocolate biscuit in their hand for too long before eating it. Knowing a little of the science behind materials changing will help you to make the most of interesting experiences of this sort whenever they occur.
What do I need to know?
Use the information below to help you support young children's scientific learning and to plan experiences that children can draw on to build their own scientific understanding. (See also 'That's the stuff', Nursery World, 16 November 2006, which looked at the properties of materials.)
Physical and chemical change
The wide range of changes that different materials can undergo is divided into two broad categories: physical change and chemical change.
Physical change happens when a material is converted from one form to another, for example, from a solid to a liquid. Physical changes are usually reversible - the material can be changed back again to a solid just by changing the conditions.
Chemical change of a material alters the structure of the material in some way, resulting in the creation of something new. For this reason, chemical change is permanent and not reversible.
Physical change
When materials are changed physically, the individual particles that they are made up of remain the same; they are just arranged in a different way.
Ice is an excellent example of this. In 'That's the stuff' we looked at the three states of matter - solid, liquid and gas - and discussed how the particles in all three are arranged. Ice melting and turning from a solid to a liquid is an example of physical change. If we collect the liquid water and put it back into the freezer, we can convert it back into solid ice again.
Dissolving
Adding sugar to water is an interesting example of physical change. When the solid sugar is stirred into the water, it seems to disappear. We can establish that it is still there because if we taste the water, it will be sweet. What has happened is that the crystals of sugar have broken down in the water and the tiny individual sugar molecules have dispersed throughout the water, creating a sugar solution. The sugar is still there and can be recovered by leaving the solution exposed to the air, in a warm place, for a few days. Over time the water will evaporate from the surface of the solution and the sugar crystals will begin to crystallise into a solid again.
Evaporation and condensation
Evaporation and condensation of water are also examples of physical change.
When water evaporates - for example, from the surface of a puddle on a sunny day - it changes from a liquid to a gas called water vapour. When the sun shines on the puddle it warms up the water, giving the water molecules more energy, so they move around faster and more escape from the surface as vapour. The hotter the day, the faster the puddle will dry up. Condensation is the reverse of this process. On a cold day, water vapour will change back from a gas to a liquid. We notice this on the inside of windows and on the car windscreen after a cold night.
Chemical change
Chemical changes are not reversible. They result in the creation of something new which is different from the original material.
Burning
When materials burn they undergo a chemical change. A simple example of this is making bread from toast. When the bread is heated, water is lost and the carbohydrate on the surface of the bread changes colour and is converted to carbon.
When a candle burns, the wax melts to form a liquid and then converts to a gas as it burns in the candle flame. As it burns, the gas changes to water and carbon dioxide, while at the same time releasing energy as heat and light.
Cooking
Most of the changes that take place during cooking are examples of irreversible chemical change. For example, when eggs, sugar, flour and butter are combined to make a cake and then put in the oven, the sloppy mixture is converted into a solid, which is firm and retains its shape. All of the ingredients of the cake have been chemically changed by combining with one another and then by being heated to a high temperature.
Investigating
Ice stories
What you need
A large plastic tray and supply of ice cubes; selection of small-world resources (people, boats, animals) for children to set up a small-world scenario.
What to do
* Many children will know a lot about ice already, as they will have experienced it at home. Use this activity as an opportunity to find out what children know about ice. What does it feels like? What is it made of? How is it formed?
* Set up a large tray, either indoors or outdoors, adding ice cubes and some water. If you are making your own ice cubes, try adding blue food colouring to the water before you freeze it.
* Encourage the children to create an imaginary world with the ice, water and small-world play resources.
* As they are playing, talk to the children about what the ice feels like, what it sounds like and what is happening to it over time. Blue ice cubes will obviously be changing; the coloured ice will have a very visual effect as it melts and changes the colour of the water already in the tray. What shape/size are the ice cubes? What colour is the water? Are the ice cubes changing shape/size? Is the water changing colour? Why do you think this is happening? What happens when you hold an ice cube in your hand? Why do you think this happens?
- Talk to the children about what they think would happen if they were playing with the ice cubes in bright sunshine.
- When the children have finished playing with the ice cubes, look at how much water there now is in the tray. Talk to them about what they think would happen if they put the water back into a freezer. Re-freeze the water using balloons or rubber gloves as moulds and have more fun.
Celebration cakes
What you need
The ingredients for making some small cakes - flour, sugar, butter, eggs, milk; mixing bowl and spoons; measuring jug; cake cases and bun tins; recipe book What to do
- This is a good opportunity to talk to the children about the importance of washing their hands before starting to cook. Talk with the children about the different ingredients. What do they look like? What do they think they will taste like? How will we know how much of each ingredient to add?
- Help children to measure out the ingredients and mix them together. Draw their attention to the changes happening - solids turning to liquids, solids and liquids combining together.
- Ask what they think will happen when the mixture goes into the oven.
- Bake the cakes and when they are cool, look at them and then break one open and look inside. What shape are the cakes now? Are they the same as they were before they went into the oven? Can you see the air holes in the cake? Can you see the eggs or the sugar or the butter inside the cake?
- Take photographs throughout the process so children can reflect on the different stages of the process. NW
Vocabulary to introduce
hot water measure size
cold ice mix shape
melt liquid pour cube
freeze solid stir change
Further information
- The Really Useful Science Book by Steve Farrow (Routledge Falmer)
- Primary Science: Knowledge and Understanding - Achieving QTS by Robert Jones et al (Learning Matters Ltd, http://www.learningmatters.co.uk)
Linda Thornton and Pat Brunton are education consultants with a special interest in science and technology for young children. They can be contacted at www.alcassociates.co.uk.