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The name borax (disodium tetraborate) generally describes a number of closely related compounds with different amounts of crystal water:

  • borax decahydrate, Na2B4O710H2O
  • borax pentahydrate, Na2B4O75H2O
  • anhydrous borax, Na2B4O7

Borax forms soft colorless/white crystals which dissolve easily in water and which effloresce in dry air.

Wikimedia Commons, user Aramgutang

When heated to temperatures beyond 350°C borax decahydrate loses its crystal water and forms anhydrous borax. Molten borax (m.p. 743°C) forms a glass-like bead which can readily dissolve metal oxides, developing a characteristic color which can be used in analytical chemistry for the detection of certain metals ("borax bead").

Borax is easily converted to boric acid by reaction with hydrochloric acid:

Na2B4O710H2O + 2 HCl 4 H3BO3 + 2 NaCl + 5 H2O

The "decahydrate" is sufficiently stable to find use as a primary standard for acid base titrimetry. Borax occurs naturally in evaporite deposits of seasonal lakes (California, Turkey, Chile, Tibet, Romania). The biggest borax producer is California. Most of the borax world production is used in the glass and ceramics industry (for ceramic glazes, optical glasses, and laboratory glassware).