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Ions in aqueous solution

Water is seldom pure. Because of the structure of the water molecule, substances can dissolve easily in it. This is very important because if water wasn't able to do this, life would not be possible on Earth. In rivers and the oceans for example, dissolved oxygen means that organisms (such as fish) are able to respire (breathe). For plants, dissolved nutrients are available in a form which they can absorb. In the human body, water is able to carry dissolved substances from one part of the body to another.

Dissociation in water

Water is a polar molecule. If we represent water using Lewis structures we will get the following:

Figure 1
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You will notice that there are two electron pairs that do not take part in bonding. This side of the water molecule has a higher electron density than the other side where the hydrogen atoms are bonded. This side of the water molecule is more negative than the side where the hydrogen atoms are bonded. We say this side is the delta negative (δ-) side and the hydrogen side is the delta positive (δ+) side. This means that one part of the molecule has a slightly positive charge (positive pole) and the other part has a slightly negative charge (negative pole). We say such a molecule is a dipole. It has two poles. Figure 2 shows this.

Figure 2: Water is a polar molecule
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Special properties of 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 (NaCl) for example, the positive sodium ions (Na+) are attracted to the negative pole of the water molecule, while the negative chloride ions (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 Figure 3. 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 1: Dissociation

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

Definition 2: Dissolution

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

Definition 3: Hydration

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

Figure 3: Sodium chloride dissolves in water
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The dissolution of sodium chloride can be represented by the following equation:

NaCl(s)Na+(aq)+Cl-(aq)

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

K2SO4(s)2K+(aq)+SO42-(aq)

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

C6H12O6(s)C6H12O6(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.

HCl(g)+H2O()H3O+(aq)+Cl-(aq)

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

Exercise 1: Ions in solution

For each of the following, say whether the substance is ionic or molecular.

  1. potassium nitrate (KNO3)

  2. ethanol (C2H5OH)

  3. sucrose (a type of sugar) (C12H22O11)

  4. sodium bromide (NaBr)

a) Ionic

b) Molecular

c) Molecular

d) Ionic

Write a balanced equation to show how each of the following ionic compounds dissociate in water.

  1. sodium sulphate (Na2SO4)

  2. potassium bromide (KBr)

  3. potassium permanganate (KMnO4)

  4. sodium phosphate (Na3PO4)

Draw a diagram to show how KCl dissolves in water.