Chemical & Physical Change

D. Crowley, 2008

Chemical & Physical Change

To know the difference between a chemical and physical change

Wednesday, April 19, 2023

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

Brainstorm all the chemical reactions you can think of…

Cooking LivingMetal makingBurningStickingRusting

Useful

Many chemical reactions are useful

Not So Useful

However, some chemical reactions are not so useful

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

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

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

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

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)

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…

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

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)