Why are decomposition reactions called the opposite of combination reactions? Write an equation for this reaction.

What is Decomposition Reaction and Combination Reaction?

In a decomposition reaction, a single reactant breaks down to form two or more products. On the other hand in combination reactions two or more reactants combine to form a single product.

That is the reason why decomposition reaction is called the opposite of combination reaction.

Examples of Decomposition Reaction

Example 1
Decomposition reaction of Calcium Carbonate

Calcium carbonate on heating decomposes to give calcium oxide and carbon dioxide.


CaCo3 (s)————- CaO (s) + CO2

In this reaction calcium carbonate is decomposed into calcium oxide and carbon dioxide.

Example 2
Decomposition Reaction of water molecule

When electric current is passed through water, it decomposes to give Oxygen and hydrogen.

             Electric current

2 H2O (l)————————— 2H2 (l) + O2 (g)

In this reaction water is decomposed into hydrogen and oxygen.

Examples of Combination Reaction

Example 1
Combination Reaction of Calcium oxide and water to give Slaked lime

CaO (s) + H2O (l)————— Ca(OH)2 (aq) + heat

In this reaction calcium oxide combines or reacts vigorously with water to form calcium hydroxide( slaked lime). Lot of heat is produced during this reaction.

Example 2
Combination Reaction of hydrogen and oxygen molecule

Reaction between hydrogen gas and Oxygen gas

2H2 (g) + O2 (g) —————- 2 H2O (l)

In this reaction hydrogen and oxygen combine to form water.

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