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Chemical reactions happen every day:
from the burning of petrol in car engine...
...to the rusting of iron.
There are millions of reactions taking place every second of every day.
Chemical reactions involve the production of one or more new substances called products. Word equations are used in chemistry to show the things that react, called reactants, and the new substances produced, which are called products.
Reactants 
Products
For example, if magnesium metal is burned in oxygen, magnesium oxide is produced. This can be represented as a word equation:
Magnesium + Oxygen 
Magnesium oxide
Word equations like these are used a lot in science as a quick way of representing a chemical reaction.
Most chemical reactions are irreversible. This means that once the reaction has taken place it is very difficult to get the reactants back.
Think how difficult it would be to turn the fumes from the exhaust pipe of a car back into petrol!
