Do liquids conduct electricity?
Materials that let current pass easily are good conductors; those that don't are poor conductors (insulators). Metals are good conductors. What about liquids?
Making a tester
Connect a cell, a bulb and two wires, leaving the two free ends apart. Dip the free ends into a liquid, keeping them apart. If the bulb glows, the liquid conducts.
If the current is weak, a torch bulb may not glow even though some current flows. A light-emitting diode (LED) glows with much smaller currents, so it makes a more sensitive tester. An LED has two leads: the longer one goes to the positive terminal. A compass placed near the wire also works: its needle deflects whenever current flows.
Results
Good conductors | Poor conductors |
|---|---|
lemon juice, vinegar | distilled water |
salt water | vegetable oil |
tap water (weakly) | honey, sugar solution (very weakly) |
solutions of acids and bases | kerosene |
Most liquids that conduct electricity are solutions of acids, bases or salts.
Distilled water is pure and contains no dissolved salts, so it is a poor conductor. Tap, pond and well water contain small amounts of dissolved salts, which is why they conduct. That is why touching electrical appliances with wet hands, or standing in water near a fallen wire, is dangerous.
Chemical effects of current
When current passes through a conducting solution, it causes chemical reactions. You may see:
bubbles of gas forming on the electrodes,
deposits of metal on an electrode,
a change in colour of the solution.
These are the chemical effects of electric current.
Example. Pass current through water, with carbon rods (taken from used cells) as electrodes. Bubbles form on both rods. In 1800, the British chemist William Nicholson showed that when electrodes are placed in water and current is passed, oxygen forms at the electrode connected to the positive terminal and hydrogen at the one connected to the negative terminal.
Example. A potato with two wires from a battery pushed into it develops a greenish-blue spot around the wire connected to the positive terminal. Such effects can be used to identify a battery's positive terminal.
Electroplating
Electroplating is depositing a thin layer of a desired metal on another material by electricity.
Copper plating example:
Dissolve copper sulphate in water, with a little dilute sulphuric acid.
Connect a clean copper plate to the positive terminal and the object to be plated, say a carbon rod or iron key, to the negative terminal.
Pass current for about 15 minutes.
A layer of copper forms on the object at the negative terminal. Copper from the solution is deposited there, and an equal amount of copper dissolves from the plate at the positive terminal, so the solution's strength stays the same.
Uses of electroplating
Chromium plating on car parts, bath taps, bicycle handlebars and wheel rims gives a shiny finish that does not corrode or scratch easily. Chromium is expensive, so objects are made of a cheaper metal and only coated.
Gold and silver plating on jewellery makes cheaper metals look like precious ones.
Tin plating on iron makes tin cans for food. Tin is less reactive than iron, so food does not touch the iron and spoil.
Zinc coating (galvanising) protects iron in bridges and cars from rusting.
The used solutions from electroplating factories contain metals that pollute water, so they must be disposed of safely.
Summary
Many liquids, mainly solutions of acids, bases and salts, conduct electricity; distilled water does not.
An LED or compass detects weak currents.
Current through a solution can produce gas bubbles, metal deposits or colour changes.
In electroplating, the object to be plated is connected to the negative terminal.