Chemical reactions usually involve a change in appearance (e.g. colour) or a detectable energy change (e.g. involving heat, light or sound).
All chemical reactions involve the formation of one or more new substances.
Rusting involves shiny, grey coloured iron becoming red and crumbly iron oxide
Another example of change in appearance is seen when adding iodine solution to starch. This will cause a reaction which changes the iodine solution from orange-brown to blue-black.
Car engines release the chemical energy trapped in petrol to allow cars to move:
hydrocarbons + oxygen → carbon dioxide + water
However, even though energy is released, it is omitted from the equation as only the names of chemicals are included in equations.
carbon + oxygen → carbon dioxide
In living organisms, respiration is the reaction where sugar reacts with oxygen to release energy. Carbon dioxide and water are products of the reaction:
sugar + oxygen → carbon dioxide + water
Another common example that illustrates both change in appearance and formation of a new substance is cooking, as demonstrated in this video clip.
Video: Cooking eggs
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