Physical and chemical reactions

Compare physical & chemical changes and recognise signs of a chemical reaction

How do you know if a reaction is a physical one?Is it reversible? Yes: then it is a physical

reactionDoes a new substance appear? If no, then it is

a physical reaction Does a substance disappear? If it has just

melted then it is a physical reaction Has there been an energy change? Has it got

hotter or colder? If no then it is a physical reaction

Simple examples of physical changes:Ice melting: this is a change in state and is a

common physical change.All changes in state are physical reactions.

Sugar dissolving: this is a physical reaction as it can be reversed. The water just needs to be evaporated off.When a substance is dissolved, it can be

undissolved by removing the water through evaporation

MovieBending a nail: this can be easily reversed by

unbending itA change in shape is just a physical reaction.

How do you know a reaction is a chemical reaction?Is the reaction reversible? If no then it is a

chemical reactionDoes a new substance appear? If yes, then it

is a chemical reaction Does a substance disappear? Has it

dissolved? Then it is a chemical reaction, if it has just melted then it is a physical reaction

Has there been an energy change? Has it got hotter or colder? If yes then it is a chemical reaction

Examples of chemical reactionsBaking a cake: this can’t be reversed,

you can’t unbake a cake.Baking soda reacting with vinegar: a gas

is given off, this is a new substancemovie

When the baking soda reacts with the vinegar it disappears: it is reacting.

When some chemicals are mixed there is a change in temperature: think about an instant ice pack: the chemicals are mixed and it gets very cold.

Basic experiments:Aim: to observe a candle burning and decide

where chemical and physical reactions are occuring

Method: 1. Light a candle2. Write down your observations3. Label a candle diagram showing what

chemical and physical reactions are occuring

Results:Observations: …

Conclusion: both chemical and physical reactions occur when a candle burns

physical

physical

chemical

chemical