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Grades 6–9 Intermediate About 20 min Version 1 Awaiting review Premium

Physical or chemical change?

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

  1. 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).
  2. Heat each dish. Try different temperatures. The sugar needs a much higher temperature.
  3. Watch what each substance turns into.
  4. 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.