Boric acid heated to red hot gives

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$H_{2} {BO}_{2}$

$B_{2} O_{3}$

Borax

${HBO}_{2}$

answer is C.

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Detailed Solution

Heating of Boric Acid: When boric acid (orthoboric acid, H₃BO₃) is heated to red hot, it undergoes thermal decomposition through several stages, ultimately forming boron trioxide (B₂O₃), also known as boric anhydride.

Thermal Decomposition Stages

Upon initial heating around 100°C, boric acid loses water and converts to metaboric acid (HBO₂).
On further heating (around 160°C), it forms tetraboric acid (H₂B₄O₇).
Finally, when heated to red hot (approximately 600°C or more), tetraboric acid decomposes to boron trioxide (B₂O₃), releasing more water.

Overall Reaction

2H₃BO₃ → B₂O₃ + 3H₂O

So, the residue left after heating boric acid to red hot is boron trioxide (B₂O₃), and water is released as a byproduct.

This boron trioxide is a white solid known as boric anhydride.

Hence, heating boric acid to red hot gives boron trioxide (B₂O₃) as the final residue.

This process is well established in inorganic chemistry and important in understanding the thermal behavior of boric acid.