Borax is a white crystalline solid. It is ?

Borax, also known as sodium tetraborate decahydrate (Na₂B₄O₇·10H₂O), is a white crystalline solid that occurs naturally as a mineral and is also prepared industrially. The key physical property of borax is its solubility behavior — it is more soluble in hot water than in cold water. This means its solubility increases with temperature.

When borax dissolves in hot water, it forms a clear alkaline solution. This happens because borax undergoes hydrolysis, producing boric acid (H₃BO₃) and sodium hydroxide (NaOH). This alkaline nature makes borax useful in many chemical processes and laboratory experiments.

Na2B4O7⋅10H2O  +  H2O→4H3BO3  +  2NaOHNa_2B_4O_7·10H_2O \; + \; H_2O \rightarrow 4H_3BO_3 \; + \; 2NaOHNa2B4O7⋅10H2O+H2O→4H3BO3+2NaOH

The solubility property of borax is important in its purification. Crystallization from hot solutions is a common method used to obtain pure borax crystals. As the hot saturated solution cools, borax crystallizes out due to reduced solubility at lower temperatures.

Borax has many applications. It is used in the manufacture of glass and ceramics, as a water softener, in detergents, in fluxes for metallurgy, and as a buffer in chemical analysis. In the borax bead test, borax helps identify metal ions based on the color of the bead formed upon heating with metal salts.

Being ionic in nature, borax is not soluble in organic solvents but dissolves well in polar solvents like water, especially at higher temperatures. It is chemically stable, non-volatile, and has mild antiseptic properties, which is why it is also used in some household and cosmetic products.

Thus, borax is a white crystalline solid that is more soluble in hot water than in cold water, a characteristic that defines its practical and laboratory uses.