1. Chemical & Physical Change
D. Crowley, 2008

2. Chemical & Physical Change
Wednesday, April 19, 2017
Chemical & Physical Change
To know the difference between a chemical and physical change

3. Chemical Changes
Neutralisation was an example of a chemical reaction, but there are many different types
Chemical reactions happen anywhere that new substances are made
Cooking
Rusting
Sticking
Burning
Metal making
Living
Brainstorm all the chemical reactions you can think of…

4. Useful
Many chemical reactions are useful

5. Not So Useful
However, some chemical reactions are not so useful

6. Observations
There are usually some obvious changes during a chemical reaction, including: -
A change in colour
A gas coming off (you may see fizzing or bubbling)
A change in temperature (the reaction mixture may get hotter)
A solid may be formed when two solutions are mixed together

7. Physical Changes
Ice melting into water is an example of a physical change
No new substances are formed during physical changes
What other physical changes are there?
Freezing, boiling and melting

8. Chemical vs Physical
Chemical reactions make new substances, physical reactions do not
For example: -
Burn magnesium in oxygen and we produce magnesium oxide, a new substance, so this is a chemical reaction
Melt ice and we produce water (but ice is just frozen water), so melting is a physical reaction

9. Chemical vs Physical
Chemical reactions are usually irreversible (once you make a new product, it is difficult to get the original reactants back)
Physical reactions are usually reversible (the changes are not permanent)
For example: -
Once magnesium oxide has been formed in the reaction between magnesium and oxygen, it is very difficult to split it back into magnesium and oxygen again
However water can easily be turned back to ice again by freezing it

10. Summary Chemical Changes Physical Changes New substances are formed
No new substances are formed
Changes are usually permanent (irreversible)
Changes are usually not permanent (reversible)

11. Experiment
Your task is to look at the five different reactions, and decide if they are chemical change or physical changes
You need to record:
If any gas has been produced;
If there was any colour change;
If there was any temperature change;
If it was easy to reverse
You can then say if you think the reaction is a chemical change or physical change
There are five different reactions, and each group must look at every one…

12. Experiment Reaction Gas produced? Colour change? Temperature change?
Easy to reverse?
Chemical or physical?
Heating copper in oxygen
Dissolving salt in water
Magnesium + hydrochloric acid
Melting ice
Sodium thiosulphate + hydrochloric acid

13. Results
Heating copper in oxygen – chemical change (copper oxide produced)
Dissolving salt in water – physical change (easily separated by evaporation)
Magnesium + hydrochloric acid – chemical change (magnesium chloride + hydrogen produced)
Melting ice – physical change (easily frozen back to ice)
Sodium thiosulphate + hydrochloric acid – chemical change (sodium chloride + sulphur + sulphur dioxide + water produced)