Amphoteric - a substance that acts as either acid or base, depending on what it reacts with. Water is the most common amphoteric substance. In the presence of a proton donor, it acts like a base. In the presence of a proton acceptor, it acts like an acid. Anhydrous - without water.
Anhydrides are substances that have had water removed.
2 Acid anhydride - an oxide that produces an acid when dissolved in water. Oxides of nonmetals are acid anhydrides. P 2 O 3 NO 2 Basic anhydride - an oxide that produces a base in when dissolved in water. Oxides of metals are basic anhydrides. Al 2 O 3 Strong acids and bases ionize completely in water solution.