Question

When orthoboric acid $\left( {{H_3}B{O_3}} \right)$ is heated, the residue left is:
A.Boron
B.Metaboric acid
C.Boric anhydride
D.Borax

Answer 1

The compound obtained on heating orthoboric acid is a colorless, semi- transparent glassy solid. It also appears as colorless hard crystals. This residue is also a good solvent for metallic oxides at high temperatures.

Complete answer:
The heating of boric acid proceeds through three major steps. When heated mildly, at a temperature of around 370 Kelvin, orthoboric acid loses a molecule of water to form metaboric acid.
${H_3}B{O_3}\xrightarrow{\Delta }HB{O_2} + {H_2}O$
Considering that we are required to tell the first residue formed, metaboric acid should be our answer.

Hence, the correct answer is B.

Additional information:
When further heated on a comparatively higher temperature, the metaboric acid forms loses water further and results into the formation of tetra boric acid. When tetra boric acid is subjected to red heat, it further undergoes loss of water to result in the formation of boron trioxide or borax.
When heated, borax undergoes various transition forms. To start with, it begins to lose the water molecules that it possesses and then it begins to swell. After this, it turns into a transparent liquid which further solidifies forming a glass like looking material called a borax bead. Here it can be seen that borax on heating results in the formation of metaborate which later forms boric anhydride.

Note:
For the formation of borax, it is necessary to heat tetra boric acid at red heat. Borax will not be obtained as the first product when ortho boric acid is heated. Borax is used as a buffer in industries and also for the regulation of viscosity.