Experiment: Electrical conductivity of ionic and covalent compounds

Electrical conductivity is the clearest single test that separates ionic bonding from covalent bonding, which is why it sits at the heart of the Chemical Bond chapter. By testing the same substances as solids and again when molten or dissolved, you can show that conduction depends on free-moving ions, and connect a physical observation directly to the model of bonding. This guide sets out the method, the observations, and the Paper 3 skills the practical assessment rewards.

Aim

To investigate the electrical conductivity of an ionic compound and a covalent compound in the solid state and in the molten or aqueous state, and to relate the results to the presence of free-moving ions.

Apparatus and materials

• Carbon (graphite) electrodes
• Dry cells or a low-voltage power supply
• A bulb or an ammeter, and connecting wires with crocodile clips
• Crucible and Bunsen burner with a tripod and pipe-clay triangle
• Lead(II) bromide (ionic solid)
• Naphthalene (covalent solid)
• Copper(II) sulfate crystals and distilled water
• Beakers

Procedure

1. Set up a simple circuit with the two carbon electrodes, the cells, the bulb or ammeter and the connecting wires, leaving a gap for the sample to be tested.
2. Place solid lead(II) bromide so the electrodes dip into it, complete the circuit, and note whether the bulb lights.
3. Transfer the lead(II) bromide to a crucible and heat it until it melts, then dip the electrodes into the molten compound and note whether the bulb lights.
4. Repeat the solid test with solid naphthalene, and again after melting it gently, noting the bulb in each case.
5. Test dry copper(II) sulfate crystals with the electrodes and note the result.
6. Dissolve copper(II) sulfate in distilled water and test the solution with the electrodes, noting the result.
7. As a control, test distilled water alone.
8. Record for every sample whether it conducts (bulb lights) or does not conduct (bulb stays off).

Expected observations

Solid lead(II) bromide does not conduct: the bulb does not light. When the lead(II) bromide is molten, the bulb lights, and bubbles or a deposit may be seen at the electrodes. Solid naphthalene does not conduct, and molten naphthalene also does not conduct: the bulb stays off in both states. Dry copper(II) sulfate crystals do not conduct, but copper(II) sulfate solution conducts and the bulb lights. Distilled water alone does not light the bulb noticeably.

Inference and conclusion

The ionic compounds, lead(II) bromide and copper(II) sulfate, conduct electricity only when molten or dissolved, not as dry solids. This is because in the solid the ions are held in fixed positions in the lattice and cannot move, but when the compound is melted or dissolved the ions become free to move and can carry charge. The covalent compound, naphthalene, does not conduct in any state, because it is made of neutral molecules with no free-moving ions or electrons. Conductivity therefore depends on the presence of mobile ions, which is a feature of ionic, not covalent, bonding.

Science process skills (Paper 3 style)

Making a hypothesis. An ionic compound conducts electricity when molten or in solution but not as a solid, whereas a covalent compound does not conduct in any state.

Identifying variables. The manipulated variable is the compound and its state (solid, molten or aqueous); the responding variable is whether the compound conducts electricity; the controlled variables include the voltage, the electrodes and the distance between them.

Tabulating data. Draw a table with a row for each compound and columns for the solid state and the molten or aqueous state, entering “conducts” or “does not conduct” in each cell.

Making inferences. From the fact that lead(II) bromide conducts only when molten, infer that free-moving ions are needed; from naphthalene never conducting, infer that it has no free-moving ions.

Operational definition. A substance is operationally defined as conducting electricity when it completes the circuit and lights the bulb (or gives a reading on the ammeter).

Safety precautions

• Heat lead(II) bromide in a fume cupboard, because molten lead(II) bromide gives off toxic bromine vapour during electrolysis.
• Wear safety goggles and handle the hot crucible with tongs, to avoid burns.
• Switch off the supply before changing samples, to avoid a short circuit or shock.
• Melt naphthalene gently over a water bath where possible, because its vapour is flammable.

Common errors

• Testing only the solid. Concluding that an ionic compound never conducts because the solid does not; it must also be tested molten or in solution.
• Damp “dry” samples. Moisture on crystals lets a little current flow and confuses the solid result; use genuinely dry samples.
• Poor electrode contact. Loose clips or electrodes not touching the sample give a false “does not conduct” result.
• Dissolving the covalent compound. Naphthalene does not dissolve in water, so it should be tested molten, not in a water solution.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to tie a simple observation to the bonding model: a student who can explain why lead(II) bromide conducts only when molten can answer the structure-and-bonding questions that recur across SPM Chemistry. Because conductivity is a standard way to distinguish ionic from covalent compounds and a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills), understanding both the results and the reason here supports the whole Chemical Bond topic.