Question

Hydrogen acts an oxidizing agent in the reaction with:
A.$B{r_2}$
B.$Ca$
C.${N_2}$
D.$S$

Answer 1

Oxidised: The substance which takes oxygen or loses hydrogen during the reaction, is known as oxidised. Or we can say that the molecule or atom which loses one or more electrons, is known as an oxidised molecule.
Reduced: The substance which loses oxygen or takes hydrogen during the reaction, is known as reduced molecule. Or we can say that the molecule or atom which gains one or more electrons, is known as reduced molecule.

Complete step by step solution:
First of all let us talk about oxidised and reduced compounds.
Oxidised: The substance which takes oxygen or loses hydrogen during the reaction, is known as oxidised. Or we can say that the molecule or atom which loses one or more electrons, is known as an oxidised molecule.
Reduced: The substance which loses oxygen or takes hydrogen during the reaction, is known as reduced molecule. Or we can say that the molecule or atom which gains one or more electrons, are known as a reduced molecule.
Oxidising agent: Those compounds which oxidize others and reduce themselves are known as oxidising agents. Generally the compound which is reduced in the reaction will act as an oxidising agent.
Reducing agent: Those compounds which reduce others and oxidise themselves are known as reducing agents. Generally the compound which is oxidised in the reaction will act as a reducing agent.
Oxidation number of an element: It is defined as the total number of electrons that an atom accepts or loses in order to make a chemical bond which results in the formation of a chemical compound.
We know that when metal reacts with hydrogen then hydrogen acts as an oxidizing agent and when hydrogen reacts with non-metals then hydrogen acts as a reducing agent.
Here in the question we are given with the elements as bromine, calcium, nitrogen and sulphur.
Among all these elements only calcium is a metal. Hence in the reaction of hydrogen with calcium hydrogen will act as an oxidizing agent.
The reaction is as: $Ca + {H_2} \to Ca{H_2}$.

Hence option B is correct.

Note:
The compounds in which the oxidation number of metal increases after the reaction are known as oxidized compounds or reducing agents. And the compounds in which the oxidation number of metal decreases after the reaction are known as reduced compounds or oxidizing agents. Here in the reaction the oxidation number of hydrogen decreases from $0$ to $- 1$ hence it acts as an oxidizing agent.