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Home > Chemistry > Elements and reactions > Chemical reactions


Chemical reactions

Chemical changes

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.

Change in appearance

rusty truck

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.

iodine added to a peeled potato causes the potato to change in colour from white to black

Detectable energy change

  • 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.

Formation of a new substance

  • chops on top of charcoal barbeque
    Burning charcoal has a main reaction of:


    carbon + oxygen → carbon dioxide

Reactions in our life

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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