3. Investigating irreversible changes
Here, we build on the first two activities by now considering ways in which matter can be changed. In science there are two types of changes pupils should consider:
- reversible changes – called physical changes;
- irreversible changes (changes that cannot easily be reversed) – called chemical changes.
Wax melting with heat and then solidifying again is a physical change. Dissolved sugar can be regained if the liquid is evaporated, so this is also a physical change. (Resource 4: Disappearing ice cube race gives one idea for a classroom activity around a physical change.) But glass cannot be easily turned back into sand – so this is a chemical change.
Case Study 3 shows one way that pupils can be challenged to think about chemical changes through a series of guided demonstrations. (SeeKey Resource: Using explaining and demonstrating to assist learning.)
When iron and steel rust (a chemical change) the metal loses its shape and strength. In the Key Activity you use a competition to get your pupils thinking about how they could slow down this chemical change.
Case Study 3: Showing irreversible changes
Sam is an unqualified teacher volunteering in his rural village school. He believes that learning should be a combination of fun and seriousness. When the time comes to look at irreversible changes he sets up a series of activities.
First, he demonstrates what happens when a small cube of bread on the end of a wire pushed into a cork handle is toasted to a cinder over a flame. He asks his pupils to observe carefully. Eventually the bread gets dry enough to catch light and burn with a flame. He then asks: ‘When did it stop being bread?’ ‘What do you think has happened?’ ‘What solid substance are we left with if we grind the remains to a powder?’ Some of the pupils have heard of carbon and Sam explains this is what is left. He burns a small piece of wood and shows this, too, leaves carbon. He listens carefully to their answers and encourages questions based on their observations. In this way, he is able to assess their learning and thinking.
Next, he shows them a more obviously chemical change. He mixes tartaric acid powder and baking powder (sodium bicarbonate) and shows that no change happens. But then he adds water and asks them to observe. This gave rise to lots of questions. Why all the fizzing and bubbling? What is in the bubbles? What gas is given off? Have the substances changed? If we evaporated the water what would we get? He talks about how new substances have been made. For more details see Resource 5: Molecules and atoms.
Sam finishes the lesson by asking his pupils to each find three examples of chemical changes for tomorrow’s lesson. He is very pleased with the examples they give – some of the pupils even bring in materials to show how they have changed.
Key Activity: Slowing down an irreversible change
Show your class some metal articles that have rusted. Ask them: Where have they seen rust? What makes the iron or steel rust? What sort of change is this? Then tell them their challenge is to find out how to stop iron rusting.
Divide your class into groups. Give each group two iron nails (or other small pieces of iron) and tell them to clean the nails with sandpaper.
Then ask them to think how they will protect the iron from rusting. They should plan their investigation; what they will do, the equipment they will need and make predictions. Why do they think they have been given two nails?
The next day, ask the groups to set up their experiments. You will need to plan time for them to make observations over the next few weeks. After a few weeks, ask each group to report back on their investigation. Was their method successful? Where might it be used?