An aqueous solution of borax above 62°C gives crystals of which compound?

Borax, chemically known as sodium tetraborate (Na₂B₄O₇·10H₂O), is a well-known inorganic compound that forms colorless crystalline solids. It is a hydrated salt containing sodium, boron, and oxygen. The degree of hydration in borax crystals depends on the temperature at which the crystals are formed from aqueous solution.

At normal room temperature, borax crystallizes as Na₂B₄O₇·10H₂O (decahydrate). However, when an aqueous solution of borax is heated above 62°C, it loses some of its water of crystallization and forms a new crystalline compound — Na₂B₄O₇·5H₂O (pentahydrate). This change occurs because higher temperatures reduce the stability of the decahydrate form, causing partial dehydration.

The decahydrate form is stable only below 62°C. When the solution is cooled again, the pentahydrate crystals may absorb water and revert to the decahydrate form. This reversible transformation between decahydrate and pentahydrate forms demonstrates the relationship between temperature and hydration state in inorganic salts.

Borax pentahydrate appears as white crystalline granules and is less soluble in water compared to the decahydrate form. In industry, borax is used in the manufacture of glass, ceramics, detergents, and fluxes. It also serves as a source of boron in laboratory and industrial chemical processes.

The study of borax crystallization provides insight into hydration equilibria, crystal structure, and thermal stability — essential topics in inorganic chemistry. Therefore, when an aqueous solution of borax is maintained above 62°C, it crystallizes as sodium tetraborate pentahydrate (Na₂B₄O₇·5H₂O), while below this temperature, it forms the decahydrate crystals.