Decomposition reactions are reactions in which one reactant breaks down at least two products. 

AB -> A + B

This may be because the reactant is energetically unstable under particular conditions. A decomposition reaction is spontaneous if it does not require an energy input for it to occur. 

Combustion reactions are exothermic reactions between reactants, usually a fuel and an oxidant. Common combustion reactions involve hydrocarbons. 

The general way of balancing hydrocarbon combustion reactions is: 

C(x)H(y) + (x + 0.25y)O2 -> xCO2 + (y/2) H2O

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In decomposition reactions a single reactant is broken down into two or more products, there are all sorts of decomposition reactions such as thermal decomposition where heat causes a reactant to break down or photo decomposition where light causes the break down.

Combustion is when oxygen reacts with another reactant causing it to combust or burn. Combustion of hydrocarbons (compounds made up of only carbon and hydrogen) produce water and carbon dioxide when they undergo combustion.

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Decomposition reactions see the reactant(s) break down into the product(s)
Combustion reactions involve the reactant(s) reacting with oxygen

A composition reaction produces a single substance from multiple reactants. A decomposition reaction produces multiple products from a single reactant.Combustion reactions are the combination of some compound with oxygen to make oxides of the other elements as products (although nitrogen atoms react to make N2).



Chemistry

Compare decomposition reactions with combustion reactions?

Aside from gas composition, exhaled air is at a higher temperature than inhaled air as a result of being warmed by the body. 

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