Question

When borax is heated strongly it gives:
A.${B_2}{O_3}$
B.$N{a_2}{B_4}{O_7}$
C.$NaB{O_2} + {B_2}{O_3}$
D.None of the above

Answer 1

We know that oxygen is essential for burning. For example: If you want to burn magnesium oxide then if it does not provide oxygen (air) to the system then it will not burn. Hence magnesium oxide will only burn when you provide oxygen for the reaction.

Complete step by step answer:
First of all let us talk about combustion.
Combustion is defined as a process in which a substance reacts with oxygen and gives heat and light.
Exothermic reactions: They are defined as the reaction in which energy is evolved after completion of reaction or during reaction in the form of heat and energy. They also require energy to initiate a reaction but the amount of energy released is greater than the amount of energy used to start a reaction.
Endothermic reactions: They are defined as the reaction in which energy is used to start a reaction and on completion on reaction they do not evolve energy. If they evolve then the amount of energy evolved is very less than that the amount of energy consumed to start a reaction.
Combustion is an example of exothermic reaction.
Now borax is a compound of boron (which is an element of the group $13$ i.e. p-block element). Formula of borax is as: $N{a_2}{B_4}{O_7}.10{H_2}O$. So when borax is heated then first the water molecule of borax is eliminated from the compound and then a mixture of $NaB{O_2} + {B_2}{O_3}$ is formed. The complete reaction is as follows:
${\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}{\text{.10}}{{\text{H}}_{\text{2}}}{\text{O}}\xrightarrow{{{\Delta }}}{\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\xrightarrow{{{\Delta }}}{\text{NaB}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{B}}_{\text{2}}}{{\text{O}}_{\text{3}}}$
When borax is heated then ${\text{N}}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}$ is formed and when it is strongly heated then $NaB{O_2} + {B_2}{O_3}$ is formed.

Hence option C is correct.

Note:
There are two types of combustion: Complete combustion and incomplete combustion.
Complete combustion: It is defined as the process in which supply of oxygen is sufficient for the substance to react completely.
Incomplete combustion: It is defined as the process in which supply of oxygen is insufficient for the substance to react completely.