Change

Evidence of Chemical Change

Chemists are interested in two types of change - physical and chemical.

Physical Changes such as melting, freezing and dissolving do not involve any change in the chemical composition of a substance so you can get back what you started with by reversing the process, e.g. freezing and melting or dissolving and evaporating.

Chemical Changes involve a change in chemical composition in the substances which are being investigated so you cannot easily get back your original substances as something new has been formed. (Ever try to unfry an egg?)

Some signs (or evidence) of chemical change are:

a gas is produced,
the temperature changes,
a substance disappears,
a solid is formed
a colour change occurs,
a new odour is produced.

Which of the activities you have done involve physical changes? Which ones involve chemical changes? Think carefully about your reasons in each case.

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