When hydrogen burns in oxygen, water is formed and when water is electrolysed then hydrogen and oxygen are produced. What type of a reaction takes place?
(i) In the first case,
(ii) In the second case?
A. In both the cases, i.e. (i) and (ii), a combination reaction takes place.
B. In the first case, a combination reaction takes place and in the second case, a decomposition reaction takes place.
C. In the first case, a combination reaction takes place and in the second case, a displacement reaction takes place.
D. In the first case, a displacement reaction takes place and in the second case, a decomposition reaction takes place.

Hint: When hydrogen burns in air, ${H_2}$ reacts with ${O_2}$ to form water molecules. Here two reactants combine to form a single product. And, when water is electrolysed into hydrogen and oxygen, here a single compound decomposes into more than one product.

Complete step by step answer:
In the First case:
When hydrogen burns in oxygen, water is formed.
Hydrogen + Oxygen = Water
$2{H_2} + {O_2} \to 2{H_2}O$
When two reactants elements combine to form a single component, then this reaction is called the combination reaction. So, in this case, two reactants i.e. Hydrogen and oxygen combine to form a single product i.e. water molecules and heat is also released.
In the Second case:
When water is electrolysed then hydrogen and oxygen are produced.
$2{H_2}O \to 2{H_2} + {O_2}$
When a single reactant is decomposed or broken down into two or more products, it is known as decomposition reaction. Here, a single reactant i.e. water, breaks or decomposes up to form two or more simple products i.e. hydrogen and oxygen. This decomposition reaction takes place by the action of electricity. So, we can see that in the first case, a combination reaction takes place and in the second case, a decomposition reaction takes place.

Therefore, the correct answer is option (B).

Note: Hydrogen is combustible and it burns with a popping sound in oxygen with an almost colourless flame to form water molecules and release heat. This reaction is highly exothermic. Hydrogen must be handled very carefully as the mixture of hydrogen and oxygen can be explosive when two are present in a particular ratio.