Most elements of the periodic table form at least one oxide. The oxides of the representative metals are ionic in character. They dissolve in water to form basic solutions. The oxides of the nonmetals are generally covalent in character. Many of them react with water to yield acidic solutions. Many nonmetals form more than one oxide. In general, the acidity of the acid formed from the oxide increases with increasing oxidation state of the nonmetal. For example, sulfur forms two common oxides: sulfur dioxide, in which sulfur is in the +4 oxidation state and sulfur trioxide in which it is in the +6 oxidation state. Sulfur dioxide reacts with water to form sulfurous acid, H2SO3, a weak acid. Similarly, SO3 reacts with water under appropriate conditions to yield sulfuric acid, a strong acid. In summary, the oxides of the strongly metallic elements form basic solutions, while the oxides of the nonmetals generally for acidic solutions. The oxides of many other elements may exhibit either acidic or basic character and are said to be amphoteric. The acid-base properties of transition element oxides must be discussed separately.
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