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Decomposition reactions are very common, and
this word is used so much that many chemists just say "decomp". When
people use this word in the lab, they might just mean that something didn't
work, or that a chemical reacted in an unwanted way, especially while
it was sitting in a bottle for a long time. The official meaning of
decomposition is a little bit more specific, and means a reaction in
which one chemical splits into two or more chemicals, like this:
A → B + C
Decomposition reactions are often undesirable, but not always. For
instance, many explosions are decompositions, and explosives are very
important for many purposes other than weapons. Decomposition
reactions might be hard to predict at first.
The decomposition reactions in intro chemistry classes often result
from heating a substance. For instance, when heated or struck, a salt of a
complex anion (chlorate, carbonate, azide) may lose a gas (oxygen,
carbon dioxide or nitrogen) leaving behind a simpler salt or
metal. This could happen explosively, depending on the compound. Or, when heated, a metal hydroxide loses water to form the
metal oxide (the reverse of the basic anhydride combination
reaction). Here are some examples:
CaCO3(s) → CaO(s) + CO2(g)
2NaN3(s) → 2Na(s) + 3N2(g)
2KClO3(s) → 2KCl(s) + 3O2(g)
Cu(OH)2(s) → CuO(s) + H2O(g or l)
Last year some of my students got confused and thought that combination reactions, the opposite of decomposition reactions, were called composition reactions. Actually, composition is not a type of reaction, but has a different meaning. Composition means the ratio of elements in a compound, such as 75% C, 25% H.