Boric oxide (B2O3). Boron forms one well-known oxide, B2O3, called boric anhydride. It is formed as a glassy mass by heating boric acid to a high temperature. It absorbs water very readily, uniting with it to form boric acid again:
B2O3 + 3H2O = 2H3BO3.
In this respect it differs from silicon dioxide, which will not combine directly with water.
Boric acid (H3BO3). This is found in nature in considerable quantities and forms one of the chief sources of boron compounds. It is found dissolved in the water of hot springs in some localities, particularly in Italy. Being volatile with steam, the vapor which escapes from these springs has some boric acid in it. It is easily obtained from these sources by condensation and evaporation, the necessary heat being supplied by other hot springs.
Boric acid crystallizes in pearly flakes, which are greasy to the touch. In the laboratory it is easily prepared by treating a strong, hot solution of borax with sulphuric acid. Boric acid being sparingly soluble in water crystallizes out on cooling:
Na2B4O7 + 5H2O + H2SO4 = Na2SO4 + 4H3BO3.
The substance is a mild antiseptic, and on this account is often used in medicine and as a preservative for canned foods and milk.
Metaboric and polyboric acids. When boric acid is gently heated it is converted into metaboric acid (HBO2):
H3BO3 = HBO2 + H2O.
On heating metaboric acid to a somewhat higher temperature tetraboric acid (H2B4O7) is formed: