Question

Choose the example of chemical combination reaction from the following:
A. Coal burns in air
B. Water freezes on cooling
C. Oxygen and hydrogen react to give water
D. Ice melts on heating

Answer 1

Hint: A combination reaction is a reaction in which one product is formed when two reactants combine together. For example, oxygen and halogens being highly reactive elements can undergo combination reactions with other elements.

Complete step-by-step answer:
A combination reaction is a reaction in which two or more substances combine to form a single new substance and thus they are also called synthesis reactions. There general form can be written as
 \[A + B \to AB\]
These reactions involve the formation of new bonds and release of a large amount of energy in the form of heat. Therefore, most of the combination reactions are exothermic in nature. These are of three types:
Non-metals when react with highly active metals they form a covalent compound. Such as, when sulphur reacts with oxygen sulphur dioxide is produced.
The reaction between a metal and non-metal to form ionic compounds. Such as, when oxygen combines with carbon monoxide, carbon dioxide is released.
Reaction between elements and compounds or between two compounds, forming a new compound. Such as, calcium oxide reacts with carbon dioxide to give calcium carbonate.
Now, let us look at the options provided. When coal burns in air, it produces carbon dioxide gas and the reaction is called combustion reaction. Water when freezes on cooling becomes ice and this is not a chemical reaction, it is a mere physical change. Ice melts on heating is also the same i.e. phase change from solid to liquid. No chemical change takes place in both.
 \[C(s) + {O_2}(g) \to C{O_2}(g)\]
 \[{H_2}O(s) \rightleftharpoons {H_2}O(l)\]
When oxygen and hydrogen react to form water, they basically combine with each other and produce a new compound. So, it is a combination reaction.
 \[{H_2} + {O_2} \to 2{H_2}O\]

Hence, the correct option is (C).

Note: It is not necessary that all combination reactions will be exothermic in nature. They can be endothermic also, for example, when nitrogen and oxygen combine to form nitric oxide at high temperature it is an endothermic reaction.