What would happen with non-electrolytes?
3 marks · NormalA student wants to find out whether substance X is an ionic compound, a covalent compound (non-electrolyte), or an acid using an experiment. She sets up the following experiment in which two carbon electrodes are placed in a solution of substance X, connected to a power supply and a bulb, as shown in the diagram:
In three separate experiments she uses:
| Test | Substance in solution | Observation |
|---|---|---|
| A | X dissolved in water | Bulb does not light up |
| B | Molten X | Bulb lights up |
| C | Aqueous acid | Bulb lights up |
Using the data above, calculate whether substance X is ionic, covalent (non-electrolyte), or an acid. Explain your reasoning.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
Ionic. Substance X is ionic because it only conducts electricity when molten, not when dissolved in water.
Also accept: X is ionic because it does not conduct in solution but does when molten.; It is an ionic compound, as only molten ionic substances conduct electricity, not covalent or non-electrolyte.
Common mistakes: Suggesting X is covalent because it does not conduct in solution.; Suggesting X is an acid because an aqueous acid conducts electricity.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Identifies from the data that molten X conducts electricity but solution of X does not.
M2 · 1 mark · States that this is behaviour expected of ionic compounds: ionic solids conduct when molten, not when in solution if insoluble.
A1 · 1 mark · Conclusion: substance X is ionic.
Worked solution
In test A, when X is dissolved in water, the bulb does not light up, so X in solution does not conduct electricity.
In test B, molten X conducts electricity (bulb lights up). In test C, an aqueous acid conducts electricity.
Since X conducts only when molten and not in solution, it must be an ionic compound that is insoluble or only molten ionic compounds conduct. Covalent (non-electrolytes) do not conduct in any state, and acids conduct when in solution.
Therefore, X is ionic.