Chemistry
Physical and chemical changes
Class 6
Intermediate
About 20 min
Premium
Physical or chemical change?
Heat and cool ice, wax, sugar and egg white. Find out which changes can be reversed and which make a new substance.
Core concept
In short: Sort changes into ones you can undo and ones that make something new.
Physical changes can be reversed; chemical changes make new substances
Topic: Physical and chemical changes
When ice melts it is still water: only its state has changed, and cooling turns it back into ice. That is a physical change. In a chemical change, a new substance forms, and simply cooling it down does not bring the old substance back. Heating, then cooling, is a good way to tell the two apart.
What you will learn
- Define physical and chemical changes.
- Use reversibility as evidence for the type of change.
- Classify melting, freezing, cooking and caramelising.
Before you start
- Know the three states of matter: solid, liquid and gas.
Materials
- Ice
- Candle wax
- Sugar
- Raw egg white
Equipment
- 4 heat-proof dishes
- Hot plate
- Freezer
- Thermometer
Procedure
- Dish A holds ice from the freezer (-10 °C). Dishes B, C and D hold candle wax, sugar and raw egg white at room temperature (20 °C).
- Heat each dish. Try different temperatures. The sugar needs a much higher temperature.
- Watch what each substance turns into.
- Cool every dish back down to room temperature, or into the freezer, and check whether each change reverses.
How this simulation is simplified
- Each dish reaches the chosen temperature straight away.
- Caramelising is shown starting at 160 °C and egg white setting at 70 °C. In reality these changes happen gradually over a range of temperatures.
- 1 g of a melted solid is treated as 1 mL of liquid.
- Dishes A, B and D are heated to at most 90 °C, so the water never boils.