Conduction of liquids

Class practical

If students have already passed an electric current through solid materials, they can now test the conductivity of liquids and solutions.

Apparatus and Materials

For each student group

• Crocodile clips, 2
• Cells, 1.5 V, with holders, 3
• Lamps with holders, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 7
• Beaker, 250 ml
• Electrodes, either 15 cm lengths of bare copper wire (20 SWG) or carbon rods (pencil leads), 2
• Hardboard disc (to cover beaker) with holes (to take electrodes)
• Other items to be made readily available for testing e.g. distilled water (or de-ionized water), tap water, salt (NaCl), copper sulfate crystals, sugar, dilute sulfuric acid 0.1 M, and oil (any type, e.g. cooking oil)

Health & Safety and Technical Notes

Modern dry cell construction uses a steel can connected to the positive (raised) contact. The negative connection is the centre of the base with an annular ring of insulator between it and the can. Some cell holders have clips which can bridge the insulator causing a short circuit. This discharges the cell rapidly and can make it explode. The risk is reduced by using low power, zinc chloride cells not high power, alkaline manganese ones.

Children have been known to taste copper sulfate crystals. A supply of drinking water should be to hand to wash a mouth if this is deemed a possibility.

Read our standard health & safety guidance

The carbon rods can be pencil leads.

Procedure

1. Connect up the circuit shown, with three cells, one or two lamps and an ammeter, all in series.
2. Half-fill the beaker with distilled water. Pass the two bare copper wires through the holes in the lid so that they dip into the liquid. Use crocodile clips to connect up to the copper wires (‘electrodes’).
3. Note whether any current flows in the circuit. Is distilled water an electrical conductor?
4. Now add a few crystals of common salt to the water and stir carefully. (Salt will dissolve more readily than copper sulfate crystals, though these can be tried in place of salt.) Is salty water an electrical conductor?
5. Replace the liquid with fresh distilled water, having washed the beaker carefully. Add several drops of dilute sulfuric acid. Observe what happens to the current.
6. Repeat with fresh distilled water and add sugar. Then try with oil. After washing the beaker thoroughly, try again with tap water. A more sensitive meter could be offered, under supervision.

Teaching Notes

• Students should take care not to let the electrodes touch, especially if the meter is sensitive to high currents.
• Students should see that bubbles of gas appear at the electrodes when using sulfuric acid. The bubbles only appear at the electrodes and are not seen in the body of the liquid.
• Copper is deposited on the electrodes when using copper sulfate solution; brown copper is only seen at the electrodes and not in the blue liquid.
• Adding a salt or acid to pure water allows the solution to conduct electricity. Tap water also conducts a low current, indicating that it contains soluble impurities.
• Students should be able to compare the sizes of the currents through the different liquids.
• Oil will not register a current as far as the students can tell with the equipment being used.
• As a teacher demonstration, you could use larger potential differences and more sensitive meters. This approach avoids saying that the liquid will never conduct, because some materials may conduct at high enough potential differences (up to 24 V).
• If students know that resistance is equal to V/I then the resistance could be calculated.

This experiment was safety-tested in January 2007