Experiment: Comparing properties of ionic and covalent compounds

Ionic and covalent compounds behave very differently, and this experiment gathers the main differences into one comparison so the pattern is easy to remember and explain. By testing the same properties, melting point, solubility and conductivity, on an ionic and a covalent compound side by side, you build the table that the Chemical Bond chapter asks you to know and justify. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To compare the physical properties of an ionic compound, for example sodium chloride, and a covalent compound, for example naphthalene, in terms of melting point, solubility in water and in an organic solvent, and electrical conductivity, and to relate the differences to the type of bonding.

Apparatus and materials

• Sodium chloride (ionic compound)
• Naphthalene (covalent compound)
• Distilled water and an organic solvent, for example tetrachloromethane
• Test tubes and a test-tube rack
• Bunsen burner, tripod and heating apparatus
• Carbon electrodes, cells, bulb and connecting wires
• Spatula and stirring rod

Procedure

1. Melting point. Place a small, equal amount of sodium chloride and of naphthalene in separate ignition tubes and heat each gently with the same flame; note which melts easily and which does not melt.
2. Solubility in water. Add a spatula of sodium chloride to water in a test tube, shake, and note whether it dissolves; repeat with naphthalene.
3. Solubility in organic solvent. Add a spatula of sodium chloride to the organic solvent, shake, and note whether it dissolves; repeat with naphthalene.
4. Conductivity of the solutions. Where a compound dissolved, test the solution with the carbon electrodes, cells and bulb, and note whether the bulb lights.
5. Record every result in a comparison table for the two compounds.

Expected observations

Sodium chloride does not melt with gentle heating, showing it has a high melting point, while naphthalene melts easily, showing it has a low melting point. Sodium chloride dissolves in water but not in the organic solvent; naphthalene does not dissolve in water but dissolves in the organic solvent. The sodium chloride solution conducts electricity and the bulb lights, while naphthalene (which does not dissolve in water) does not give a conducting aqueous solution.

Inference and conclusion

The ionic compound has a high melting point because strong electrostatic forces act between the oppositely charged ions throughout the giant lattice, and a large amount of heat energy is needed to overcome them. The covalent compound has a low melting point because it is made of molecules held together only by weak forces between molecules, which need little energy to overcome. Ionic compounds dissolve in water, a polar solvent, but not in organic solvents; covalent compounds dissolve in organic solvents but not in water. Ionic compounds conduct when dissolved because their ions are free to move, while covalent compounds do not conduct because they have no free-moving ions. All these differences follow from the type of bonding.

Science process skills (Paper 3 style)

Making a hypothesis. An ionic compound has a higher melting point, dissolves in water and conducts electricity in solution, whereas a covalent compound has a lower melting point, dissolves in an organic solvent and does not conduct.

Identifying variables. The manipulated variable is the type of compound (ionic or covalent); the responding variables are the melting behaviour, the solubility and the conductivity; the controlled variables include the amount of substance, the volume of solvent and the heating conditions.

Tabulating data. Draw a comparison table with rows for melting point, solubility in water, solubility in organic solvent and conductivity, and a column for each compound, so the two can be compared at a glance.

Making inferences. From the high melting point and water solubility of sodium chloride, infer strong ionic bonding; from the low melting point and organic solubility of naphthalene, infer weak forces between covalent molecules.

Operational definition. A high melting point is operationally defined here as one at which the compound does not melt under the gentle heating that easily melts the covalent compound.

Safety precautions

• Handle the organic solvent in a well-ventilated area away from flames, because its vapour can be harmful and some organic solvents are flammable.
• Wear safety goggles, because heated substances and solvents can spit or splash.
• Heat the tubes gently and point them away from people, so hot material is not ejected towards anyone.
• Switch off the electricity supply before adjusting the conductivity apparatus, to avoid a shock.

Common errors

• Using unequal amounts. Different amounts of the two compounds make the melting comparison unfair; use small, equal amounts.
• Confusing the solvents. Testing solubility in the wrong solvent gives a misleading result; label the water and organic solvent clearly.
• Testing conductivity of a solid. Conductivity here is tested on the solution, not the dry solid; make sure the compound has dissolved first.
• Heating the organic solvent over a flame. This is a fire risk; keep the flammable solvent away from the Bunsen burner.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this comparison to build the ionic-versus-covalent table that students must reproduce and justify. A student who can link each property to the bonding, strong lattice forces or weak forces between molecules, can answer the structure-and-properties questions that recur across SPM Chemistry. Because comparing these properties is a standard Paper 2 and Paper 3 task (Paper 3 is a practical test assessing science process skills), understanding the reasons here, not just the results, protects marks throughout the Chemical Bond topic.