Give the characterization of phosphoric acid.

Pure phosphoric acid is a non-flammable, colorless, odorless and hygroscopic crystal. Commercial phosphoric acid comes as a viscous solution in water that contains 75-85% phosphoric acid. The liquid can solidify at lower temperatures. Phosphoric acid can be prepared using two different methods. One method is the direct reaction of the phosphate rock with sulfuric acid, which also produces large amounts of gypsum (calcium sulfate) as a waste by-product. The second method is to burn elemental phosphorus and then hydrate the phosphorus oxide. Phosphoric acid is a corrosive acid that can form three different classes of salts, namely primary phosphates, dibasic phosphates and tribasic phosphates.