Borax (from Arabic beetroot - saltpeter), sodium tetraborate, Na2B4O7 - 10H2O, crystalline hydrate of the sodium salt of tetraboric acid. In nature, it occurs in the form of a mineral - borax (or tinkal), white, less often gray and yellow; density 1690-1720 kg / m3 (see. Natural borates). B. is soluble in water (1.6 g of anhydrous salt in 100 g of H2O at 10 ° C).
Aqueous solutions of B. are alkaline and absorb CO2 from the air. B. decomposes with strong acids. It is used in soldering, since molten metal dissolves metal oxides and thereby cleans the metal surfaces to be soldered.