Phosphoric acid, << fos FAWR ihk, >> is the most common acid of phosphorus. Manufacturers use it to make inorganic phosphate compounds, fertilizers, soft drinks, and flavoring syrups. Its normal sodium salt is an excellent water softener. Metal surfaces treated with phosphoric acid are highly resistant to corrosion.
Most phosphoric acid is made by one of two processes. In the wet process, naturally occurring phosphate minerals are treated with sulfuric acid. In the thermal process, pure phosphorus is burned to make phosphorus pentoxide, which produces the acid when dissolved in water. Pure phosphoric acid forms colorless crystals that melt at about 42.4 °C. It is very soluble in water. Phosphoric acid is called orthophosphoric acid.
