Introduction and concepts

Electrolytes, ionisation and conductivity

Conductivity in aqueous solutions, is a measure of the ability of water to conduct an electric current. The more ions there are in the solution, the higher its conductivity.

Conductivity

Conductivity is a measure of a solution's ability to conduct an electric current.

Electrolytes

An electrolyte is a material that increases the conductivity of water when dissolved in it. Electrolytes can be further divided into strong electrolytes and weak electrolytes .

Electrolyte

An electrolyte is a substance that contains free ions and behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions.

1. Strong electrolytes A strong electrolyte is a material that ionises completely when it is dissolved in water:
   $$\mathrm{AB\; (s,\; l,\; g)}\to {\mathrm{A}}^{+}\mathrm{(aq)}+{\mathrm{B}}^{-}\mathrm{(aq)}$$
   This is a chemical change because the original compound has been split into its component ions and bonds have been broken. In a strong electrolyte, we say that the extent of ionisation is high. In other words, the original material dissociates completely so that there is a high concentration of ions in the solution. An example is a solution of potassium nitrate:
   $${\mathrm{KNO}}_3\mathrm{(s)}\to {\mathrm{K}}^{+}\mathrm{(aq)}+{\mathrm{NO}}_3^{-}\mathrm{(aq)}$$
2. Weak electrolytes A weak electrolyte is a material that goes into solution and will be surrounded by water molecules when it is added to water. However, not all of the molecules will dissociate into ions. The extent of ionisation of a weak electrolyte is low and therefore the concentration of ions in the solution is also low.
   $$AB(s,l,g)\to AB\left(aq\right)\rightleftharpoons A^{+}\left(\mathrm{aq}\right)+B^{-}\left(\mathrm{aq}\right)$$
   The following example shows that in the final solution of a weak electrolyte, some of the original compound plus some dissolved ions are present.
   $$C_2{H}_3{O}_{2}H\left(l\right)\to C_2{H}_3{O}_{2}H\rightleftharpoons C_2{H}_3{O}_2^{-}\left(\mathrm{aq}\right)+H^{+}\left(\mathrm{aq}\right)$$

Non-electrolytes

A non-electrolyte is a material that does not increase the conductivity of water when dissolved in it. The substance goes into solution and becomes surrounded by water molecules, so that the molecules of the chemical become separated from each other. However, although the substance does dissolve, it is not changed in any way and no chemical bonds are broken. The change is a physical change . In the oxygen example below, the reaction is shown to be reversible because oxygen is only partially soluble in water and comes out of solution very easily.

Factors that affect the conductivity of water

The conductivity of water is therefore affected by the following factors:

• The type of substance that dissolves in water. Whether a material is a strong electrolyte (e.g. potassium nitrate, ${\mathrm{KNO}}_3$ ), a weak electrolyte (e.g. acetate, ${\mathrm{CH}}_3\mathrm{COOH}$ ) or a non-electrolyte (e.g. sugar, alcohol, oil) will affect the conductivity of water because the concentration of ions in solution will be different in each case.
• The concentration of ions in solution. The higher the concentration of ions in solution, the higher its conductivity will be.
• Temperature. The warmer the solution, the higher the solubility of the material being dissolved and therefore the higher the conductivity as well.