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Dissociation of Sodium Chloride in Water

Dissociation of Sodium Chloride in Water

It is the polar nature of water that allows ionic compounds to dissolve in it. In the case of sodium chloride (\(\text{NaCl}\)) for example, the positive sodium ions (\(\text{Na}^{+}\)) are attracted to the negative pole of the water molecule, while the negative chloride ions (\(\text{Cl}^{-}\)) are attracted to the positive pole of the water molecule. When sodium chloride is dissolved in water, the polar water molecules are able to work their way in between the individual ions in the lattice. The water molecules surround the negative chloride ions and positive sodium ions and pull them away into the solution. This process is called dissociation.

Note that the positive side of the water molecule will be attracted to the negative chlorine ion and the negative side of the water molecule to the positive sodium ions. A simplified representation of this is shown in the figure below. We say that dissolution of a substance has occurred when a substance dissociates or dissolves. Dissolving is a physical change that takes place. It can be reversed by removing (evaporating) the water.

Definition: Dissociation

Dissociation is a general process in which ionic compounds separate into smaller ions, usually in a reversible manner.

Definition: Dissolution

Dissolution or dissolving is the process where ionic crystals break up into ions in water.

Definition: Hydration

Hydration is the process where ions become surrounded with water molecules.

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Sodium chloride dissolves in water

The dissolution of sodium chloride can be represented by the following equation:

\[\text{NaCl (s)} \rightarrow \text{Na}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)}\]

The dissolution of potassium sulfate into potassium and sulfate ions is shown below as another example:

\[\text{K}_{2}\text{SO}_{4}\text{(s)} \rightarrow 2\text{K}^{+}\text{(aq)} + \text{SO}_{4}^{2-}\text{(aq)}\]

Remember that molecular substances (e.g. covalent compounds) may also dissolve, but most will not form ions. One example is glucose.

\[\text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(s)} \rightarrow \text{C}_{6}\text{H}_{12}\text{O}_{6}\text{(aq)}\]

There are exceptions to this and some molecular substances will form ions when they dissolve. Hydrogen chloride for example can ionise to form hydrogen and chloride ions.

\[\text{HCl (g)} + \text{H}_{2}\text{O (l)} \rightarrow \text{H}_{3}\text{O}^{+}\text{(aq)} + \text{Cl}^{-}\text{(aq)}\]

You can try dissolving ionic compounds such as potassium permanganate, sodium hydroxide and potassium nitrate in water and observing what happens.

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