Question

When nitrogen reacts with hydrogen under high temperature and pressure, ammonia which is also a gas is formed. This is an example of:
A.Chemical combination
B.Chemical decomposition
C.Physical change
D.None of these

Answer 1

We know that when two elements combine with each other, a new product (compound) would be formed. We can call such chemical reactions as synthesis reactions.

Complete step by step answer:

At higher temperature and pressure, the reaction of nitrogen with hydrogen to form ammonia is Haber’s process.
The chemical reaction of ammonia formation is written as,
${{\text{N}}_{\text{2}}}\left( {\text{g}} \right){\text{ + 3}}{{\text{H}}_{\text{2}}}\left( {\text{g}} \right) \rightleftharpoons {\text{2N}}{{\text{H}}_{\text{3}}}\left( {\text{g}} \right)$
In combination reactions, the reactant combines to form a single product. This type of reaction occurs between two elements, an element and a compound or two compounds. It is recognized easily because it involves two reactants and only a single product.
Decomposition reactions are the opposite of a combination reaction because decomposition reaction involves the breaking apart of a substance into simpler substances. We can easily recognize decomposition reactions because there is one reactant and more than one product.
An alteration of substance without changing its composition is called physical change. The change in state of a substance is the most common physical change.
We can see that nitrogen combines with hydrogen to form ammonia, so the reaction would be a synthesis reaction. We can call synthesis reactions as combination reactions.
Therefore, Option (A) is correct.
Option (B) is incorrect because the product ammonia formed is not produced as a result of decomposition of nitrogen and hydrogen.
Option (C) is incorrect because there would change in the composition of nitrogen and hydrogen to form ammonia, though the physical state of the elements would be the same.
Therefore, the option A is correct.

Note:
The reaction of sodium chlorate to sodium chloride and oxygen is an example of decomposition reaction. The reaction of sodium chlorate to sodium chloride and oxygen is,
${\text{NaCl}}{{\text{O}}_{{\text{3(s)}}}}\xrightarrow{{}}{\text{NaC}}{{\text{l}}_{\left( {\text{s}} \right)}}{\text{ + }}{{\text{O}}_{{\text{2(g)}}}}$
A single reactant sodium chlorate decomposes to form two products namely sodium chloride and oxygen.
The reaction of formation of calcium sulphate from calcium oxide and sulphite is a combination reaction. The reaction of formation of calcium sulphate from calcium oxide and sulphide is,
${\text{Ca}}{{\text{O}}_{\left( {\text{s}} \right)}}{\text{ + S}}{{\text{O}}_{{\text{3(g)}}}}\xrightarrow{{}}{\text{CaS}}{{\text{O}}_{{\text{4(s)}}}}$
The process of making ice cubes is identified as a physical change because there is a change in the physical state of the ice with no change in its composition.