If the oxoacid generated is weakly acidic, the oxide is usually but not always insoluble in water. Some acidic oxides such as those of sulfur and nitrogen are air pollutants because they react with moisture in the air to produce acid rain. Water insoluble oxide are classed as acidic if they react with bases to form salts. There are a few oxides, such as NO 2 and ClO 2 in which the central atom's oxidation state does not correspond to the oxidation state of that element in a stable or known oxo acid. Such oxides generate a mixture of oxo acids or anions by disproportionation.
Some oxides exhibit both acidic and basic properties.
In general, the electropositive character of the oxide's central atom will determne whether the oxide will be acidic or basic. The more electropositive the central atom the more basic the oxide. The more electronegative the central atom, the more acidic the oxide. Electropositive character increases from right to left across the periodic table and increases down the column. A resultant borderline between basic and acidic oxides occurs along a diagonal.