True solutions: Matter which, when mixed with water or other liquids,
forms true solutions, is divided into:
• Non-ionic solutions. When lactose is dissolved in water,
no important changes occur in the molecular structure of
the lactose.
• Ionic solutions. When common salt is dissolved in water,
cations ( Na+) and anions (Cl–) are dispersed in the water,
forming an electrolyte.
Acidity of solutions
When an acid (e.g. hydrochloric acid, HCl) is mixed with water it releases
hydrogen ions (protons) with a positive charge (H+). These quickly attach
themselves to water molecules, forming hydrogen (H30+) ions.
When a base (a metal oxide or hydroxide) is added to water, it forms a
basic or alkaline solution. When the base dissolves it releases hydroxide
(OH–) ions.
• A solution that contains equal numbers of hydroxide and
hydrogen ions is neutral. Figure 2.8.
• A solution that contains more hydroxide ions than hydrogen
ions is alkaline. Figure 2.9.
• A solution that contains more hydrogen ions than hydroxide
ions is acid. Figure 2.10.
pH
The acidity of a solution is determined as the concentration of hydrogen
ions. However, this varies a great deal from one solution to another. The
symbol pH is used to denote the hydrogen ion concentration.
Mathematically, pH is defined as the negative logarithm to the base 10 of
the hydrogen ion concentration expressed in molarity, i.e. pH = – log [H+].
This results in the following scale at 25 °C: