The Arrhenius Concept of Bases

The Arrhenius theory of ionization attributed the characteristic properties of basic solutions to the hydroxide ion, OH^-. Thus, a base was defined as a compound containing OH groups which can become hydroxide ions when the base is dissolved in water.

The Arrhenius definitions do not include such compounds as ammonia (NH₃) and calcium oxide (CaO). It was assumed for example, that when ammonia dissolves in water, it reacts with the solvent to form ammonium hydroxide (NH₄OH), which can be regarded as a base because it contains an OH group that become a hydroxide ion in the solution; in terms of the Arrhenius concept it is the NH₄OH, not the NH₃, that is the base.

Similarly, CaO reacts with H₂O to give the base calcium hydroxide (Ca(OH)₂), which furnishes hydroxide ions to the solution; the CaO is termed a basic oxide but not a base. Further examples of bases according to the Arrhenius concept are sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH)₂) and potassium hydroxide (KOH).

The Hydroxides

As we shall see later, the hydroxides are strong bases - they combine very firmly with H⁺ ions to form almost non-ionizable water molecule - water serves here as a very weak acid. The hydroxides ions, OH^- are produced by metallic oxides and hydroxides - these are known as basic oxides and hydroxides.

Definition: A basic oxide or hydroxide is a metallic oxide or hydroxide, which contains ions O^2- or OH^-, which will react with an acid to form a salt and water only.

Note: a basic oxide or hydroxide will form only salt and water with an acid - nothing more. For this reason lead(IV) oxide, PbO₂ is not a basic oxide. Due to the closeness of lead to hydrogen on the electrochemical series, it oxidizes an acid very strongly, producing other products apart from salt and water.

Example, 4HCl(aq) + PbO₂(s) → PbCl₂(aq) + 2H₂O(l) + Cl₂(g)

Therefore, PbO₂ is not a basic oxide. The nature of metallic hydroxides is summarized below:

Hydroxides of potassium, sodium and calcium are soluble in water and are alkalis. The hydroxides of potassium and sodium are not decomposed to their oxides by heat but that of calcium is.

The hydroxides of magnesium, aluminium, zinc, iron, lead, and copper are insoluble in water. The hydroxides decompose to their oxides when heated. The hydroxides of aluminium, zinc, and lead are amphoteric. That is, they can react with acids and bases.

Mercury, Hg, silver, Ag, and gold, Au do not form hydroxides.