Aqueous solutions

Many ionic or covalent compounds are soluble in water; others are insoluble. When ionic substances dissolve in water, the ions break away from the crystal and become surrounded by water molecules. We indicate this by writing (aq) beside each ion: NaCl(s) → Na + (aq) + Cl - (aq) Solutions containing dissolved ions will conduct electricity — the greater the concentration of ions, the greater the conductivity.

When a molecular substance dissolves in water, the molecules move apart and become surrounded by water molecules. C 6 H 12 O 6 (s) → C 6 H 12 O 6 (aq) Dissolving a molecular substance in water will not increase the number of ions in solution. Some molecular compounds react with water, producing ions. In this case the solution formed will conduct electricity to some extent.

Measuring conductivity We can tell how many ions are in solution by how well it conducts electricity. This conductivity tester uses mains electricity and different sizes of light bulbs to estimate conductivity. The brighter the bulb the better the conductor. If a 150 W bulb glows brightly it is an excellent conductor. This 60 W bulb is only half lit. This solution is only partially ionised.

Distilled water 150 W bulb 60 W bulb 25 W bulb 5 W bulb Water is a molecular substance, but the fact that the 5 W bulb does glow tells us that it contains a few ions. 2H 2 O H 3 O + (aq) + OH – (aq)

Since water has a pH of 7 we know that it contains 10 –7 mol L –1 H 3 O + and 10 –7 mol L –1 OH –.

Ethanol Ethanol is a molecular substance which is liquid at room temperature. It dissolves readily in water.

This sample of ‘pure’ ethanol is conducting electricity very slightly. That’s probably because it contains a trace of water. If it was completely pure it would not conduct electricity at all.

A mixture of ethanol in distilled water conducts electricity in the same way that distilled water itself does – because of the already present in the distilled water. Distilled water

Sodium chloride is an ionic compound which dissolves easily in water. Sodium chloride

The solution formed conducts electricity well. NaCl(s) → Na + (aq) + Cl – (aq)

Calcium sulfate Calcium sulfate is an ionic compound which is not very soluble in water. However, the solution formed is a relatively good conductor of electricity, so there are quite a few ions in solution: CaSO 4 (s) Ca 2+ (aq) + SO 4 2– (aq)

Lead carbonate Lead carbonate appears to be insoluble, but the 25 W bulb glows, indicating that even this ‘insoluble’ substance dissolves a little: PbCO 3 (s) Pb 2+ (aq) + CO 3 2– (aq)

Hydrogen chloride/hydrochloric acid Pure hydrogen chloride, HCl, is a gas at room temperature. It is a molecular substance which dissolves rapidly in water. The solution formed is known as hydrochloric acid. It is an excellent conductor of electricity, which indicates that although HCl(g) is molecular, HCl(aq) is ionic.

Hydrogen chloride reacts with water, producing H 3 O + : HCl(g) + H 2 O(l) → H 3 O + (aq) + Cl – (aq) The pH of a 0.1 mol L –1 solution of HCl is 1, which tells us that [H 3 O + ] is 0.1 mol L –1 and that therefore HCl is fully ionised in water. We say HCl is a strong acid because it is fully ionised.

Ethanoic acid Ethanoic acid (acetic acid) is a colourless liquid with a strong smell. It is a molecular compound and does not conduct electricity at all. Even the 5 W bulb does not glow.

Ethanoic acid dissolves readily in water. The solution is conducts electricity to a limited extent. The 60 W bulb glows.

For ions to be present, the ethanoic acid must react with water: CH 3 COOH(aq) + H 2 O(l) CH 3 COO – (aq) + H 3 O + (aq) We know this is an equilibrium reaction because the poor electrical conductivity tells us that there are not many ions in solution. Ethanoic acid is a weak acid because it is only partially ionised in water. Strong acid: fully ionised Weak acid: partially ionised

Sodium hydroxide Sodium hydroxide is an ionic solid which dissolves rapidly in water, forming a solution which conducts well. NaOH(s) → Na + (aq) + OH – (aq)

We say that sodium hydroxide is a strong base because it is fully ionised in water.

Ammonia Ammonia, NH 3, is a gas with a characteristic smell. It is a molecular compound and is very soluble in water. Ammonia solution conducts electricity to a limited extent.

Molecular NH 3 reacts with water to form ions: NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH – (aq) We know that this is an equilibrium because: there are few ions in solution the pH of a 0.1 mol L –1 solution is about 11, indicating that [H 3 O + ] = 10 –11 and [OH – ] = 10 –3. we can smell NH 3 in an open bottle of ammonia solution, which means there must be many molecules of NH 3 present. We say ammonia is a weak base because it is only partially ionised in water.

Sodium ethanoate Sodium ethanoate (sodium acetate) is the salt formed when ethanoic acid reacts with sodium hydroxide. It is a white crystalline substance which dissolves readily in water.

Ethanoic acid is molecular, and sodium hydroxide is ionic, but what kind of substance is sodium ethanoate? The solution is a good conductor of electricity: sodium ethanoate is an ionic substance: CH 3 COONa(s) → CH 3 COO – (aq) + Na + (aq)