1. Physical and Chemical Changes
20/09/2018
Physical and Chemical Changes
Objective
To describe the energy changes when a substance changes state and the change in energy in chemical reactions.
Get pupils to discuss which they think is a chemical reaction and which one is physical. How can they tell?

2. Chemical Reactions
Citric acid solution and powdered sodium hydrogen carbonate. Add a spatula of the carbonate at a time to 25 cm3 of acid solution and use a thermometer to note any change in temperature.
Use magnesium ribbon and 20 cm3 1.0 M hydrochloric acid. Add a 2 cm strip of the ribbon to the acid. Again make a note of any change in temperature.
For each experiment write 2 sentences describing what you saw and what happened to the temperature.
During the experiment ask students to describe the reactions noting the transfer of chemical energy to cause heating (exothermic) and the rarer thermal energy into chemical energy (endothermic).

3. Physical Changes
Heat a beaker of water with crushed ice in it until it boils while recording the temperature every 30 seconds. Use a table to record your results.
Write two sentences describing what happens in the beaker and what happens to the temperature as the ice is melting and as the water boils.
What are the main differences you observed between the chemical reactions and the physical changes? Are they reversible?
This will show that the temperature does not change during the changes of state (melting and boiling).
Remind students that the material is still gaining energy during these changes of state and ask them to suggest why the temperature does not change. You may have to lead them to the idea that the energy is being used to ‘break bonds’ between particles.

4. Plenary – True/False F Ice melting is a chemical change.
The temperature stays the same as ice is melting.
An exothermic reaction is when the temperature increases.
Chemical changes are reversible.
Dissolving sugar in a cup of hot water is a physical change. T T F T
Get students to explain their answers.