Acidic and basic oxides

An oxide is a compound of oxygen with another element. Oxides can be classified by how they react with acids and bases: an acidic oxide reacts with a base, while a basic oxide reacts with an acid. These reactions can produce a salt and water.

Acidic oxides

Non-metal oxides are generally acidic oxides. Sulfur dioxide, $$\ce{SO2}$$, and carbon dioxide, $$\ce{CO2}$$, are examples.

For example, sulfur dioxide reacts with the base sodium hydroxide to produce sodium sulfite, a salt, and water:

Basic oxides

Metal oxides are generally basic oxides. Copper(II) oxide, $$\ce{CuO}$$, and calcium oxide, $$\ce{CaO}$$, are examples.

For example, copper(II) oxide reacts with hydrochloric acid to produce copper(II) chloride, a salt, and water:

The metallic or non-metallic character of the element combined with oxygen therefore gives a useful guide to an oxide’s behaviour. These are general patterns, however, rather than rules that apply to every oxide.