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How to Distinguish Electrolytes from Nonelectrolytes

Electrolytes are substances that conduct electricity in the molten state or when dissolved in water. Nonelectrolytes are substances that don’t conduct electricity when in these states.

When an ionic compound such as sodium chloride is put into water, the water molecules attract both the cations and anions in the crystal and pull them into the solution (see the crystal shown in the figure). The cations and anions get distributed throughout the solution.

The crystal structure of sodium chloride.
The crystal structure of sodium chloride.

You can detect the presence of these ions by using an instrument called a conductivity tester. A conductivity tester tests whether water solutions of various substances conduct electricity. It’s composed of a light bulb with two electrodes attached.

The light bulb is plugged into a wall outlet, but it doesn’t light until some type of conductor (substance capable of transmitting electricity) between the electrodes completes the circuit. (A finger will complete the circuit, so this experiment should be done carefully.)

When you place the electrodes in pure water, nothing happens, because there’s no conductor between the electrodes. Pure water is a nonconductor. But if you put the electrodes in the NaCl solution, the light bulb lights, because the ions conduct the electricity (carry the electrons) from one electrode to the other.

You don’t even need the water. If you were to melt pure NaCl (it requires a lot of heat!) and then place the electrodes in it, you’d find that the molten table salt also conducts electricity. In the molten state, the NaCl ions are free to move and carry electrons, just as they are in the saltwater solution.

Scientists can get some good clues as to the type of bonding in a compound by discovering whether a substance is an electrolyte or a nonelectrolyte. Ionically bonded substances act as electrolytes. But covalently bonded compounds, in which no ions are present, are commonly nonelectrolytes.

Table sugar, or sucrose, is a good example of a nonelectrolyte. You can dissolve sugar in water or melt it, but it won’t have conductivity. No ions are present to transfer the electrons.

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