Orthoboric acid is 2.6% soluble in water at 40°C and __________ at 107°C.

Orthoboric acid (H₃BO₃), commonly known as boric acid, is a weak, crystalline compound of boron that shows strong temperature-dependent solubility in water. At 40°C, only 2.6% of boric acid dissolves in water, indicating low solubility at moderate temperatures. However, as the temperature rises, more molecules gain the energy required to overcome intermolecular forces and interact with water molecules, leading to higher solubility.

At 107°C, the solubility of orthoboric acid increases dramatically to 37%, which is more than a tenfold increase compared to its solubility at 40°C. This behavior results from the breaking of hydrogen bonds in the boric acid crystal lattice and formation of strong interactions between hydroxyl groups of H₃BO₃ and water molecules. The increased dissolution is further aided by the slight formation of borate ions, which occurs at elevated temperatures and contributes to the solute’s greater solubility.

This property of boric acid is essential in industrial and laboratory processes. For instance, in crystallization and purification, knowledge of its solubility at different temperatures allows controlled formation of boric acid crystals. Additionally, high-temperature solubility is exploited in glass production, antiseptics, flame retardants, and ceramics, where boric acid acts as a flux or additive.

Understanding the solubility behavior of orthoboric acid also illustrates important inorganic chemistry concepts, such as temperature dependence of solubility, lattice energy, and hydrogen bonding, providing students with practical insight into solute-solvent interactions.

Hence, the correct solubility of orthoboric acid at 107°C is 37% (Option C