Combination reactions are of three types. These are discussed as follows:
1. Combination between two elements : In these reactions, two elements combine under suitable conditions to form a compound. A few examples of this type of combination reactions are:
(a) Carbon element burns in oxygen to form carbon dioxide
$$\text { C }+\mathrm{O}_2 \stackrel{\text { combustion }}{\longrightarrow} \mathrm{CO}_2$$
Carbon($C$)
Oxygen($O_2$)
Cabon dioxide($CO_2$)
(b) Iron and sulphur elements when heated form iron sulphide
$$\mathrm{Fe}+\mathrm{S} \stackrel{\text { heat }}{\longrightarrow} \mathrm{FeS}$$
Iron($Fe$)
Sulphur($S$)
Iron Sulphide($FeS$)
2. Combination between an element and a compound : In these reactions, one of the combining substances is an element whereas the other is a compound. A few examples of this type of combination reactions are:
(a) Carbon monoxide combines with oxygen to form carbon dioxide.
$$2 \mathrm{CO} \quad+\mathrm{O}_2 \longrightarrow 2 \mathrm{CO}_2$$
-Carbon monoxide($CO$)
Oxygen($O_2$)
Carbon dioxide($CO_2$)
(b) Sulphur dioxide combines with oxygen upon heating to form sulphur trioxide.
$$2 \mathrm{SO}_2+\mathrm{O}_2 \stackrel{\text { heat }}{\longrightarrow} 2 \mathrm{SO}_3$$
Sulphur dioxide, Oxygen, Sulphur trioxide
3. Combination between two compounds : In these combination reactions, two compounds take part to form a new compound. For example,
(a) Ammonia reacts with hydrogen chloride to form ammonium chloride.
$$\mathrm{NH}_3+\mathrm{HC} 1 \longrightarrow \mathrm{NH}_4 \mathrm{Cl}$$
Ammonia, Hydrogen, chloride Ammonium chloride
(b) Carbon dioxide combines with calcium oxide (quick lime) to form calcium carbonate.
$$\mathrm{CaO}+\mathrm{CO}_2 \longrightarrow \mathrm{CaCO}_3$$
Calcium oxide, Calcium carbonate, (Quick lime)
