Question

When orthoboric acid ${ H }_{ 3 }{ BO }_{ 3 }$ is heated, the residue is?
a.) Boron
b.) Metaboric acid
c.) Boric anhydride
d.) Borax

Answer 1

You should know that it appears as colorless, semi-transparent glassy lumps or hard white colorless crystals and you should know that it is an excellent solvent for metallic oxides at high temperatures.

Complete step by step answer:
In brief, a heating borax undergoes several transition forms. In the beginning it will lose all of the possessed water molecules and then it starts to swell.
After which it will get turned into a transparent liquid and solidifies to form a glass like material known as borax bead.
Here we can see that borax leads to the formation of sodium metaborate after which it leads to the formation of boric anhydride.

Let’s understand the process in detail.
First let’s place a few grams of pure boric acid crystals in about 100-cc in a platinum dish and heated over a meker burner to a temperature of about ${{600}^{{}^\circ }}C$.
Now, the dish containing the clear viscous liquid is then electrically heated for about an hour.
At the end of this, the dish is removed and the contents are poured on a marble slab, where it immediately hardens to a clear glass.
Generally the boric acid melts at ${{577}^{{}^\circ }}C$, but a temperature of ${ 800 }^{ ° }C$ is necessary for sufficient fluidity to permit easy pouring.
Samples of the anhydride were dehydrated at ${{800}^{{}^\circ }}C,{{900}^{{}^\circ }}C\text{ }and\text{ }{{1000}^{{}^\circ }}C$

We can write the reaction as-
$\begin{aligned} & {{H}_{3}}B{{O}_{3}}\to HB{{O}_{2}}+{{H}_{2}}O \\\ & 2HB{{O}_{2}}\to {{B}_{2}}{{O}_{3}}+{{H}_{2}}O \\\ \end{aligned}$
Therefore, when orthoboric acid ${ H }_{ 3 }{ BO }_{ 3 }$ is heated, the residue is boric anhydride.
So, the correct answer is “Option C”.

Note: You should know that it has been stated that boric acid anhydride vaporizes at a red heat, due to this property for the preparation of this substance it is obtained in a finely divided form and thus avoiding the use of ball mill. Boric anhydride is used as an insecticide. It is also the starting material for synthesis of many boron compounds.