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

Class practical

An opportunity to investigate the deposition of solids and gases at the electrodes immersed in a solution.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Crocodile clips, 2
• Lamps with holders, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 7
• Beaker, 250 ml (or 150 ml, tall form)
• Electrodes - pairs of carbon rods and strips of copper foil, width 1 cm
• Hardboard disc (to cover beaker) with holes (to take electrodes)
• Copper sulfate solution, 0.5 M

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.

Read our standard health & safety guidance

The carbon rods can be pencil ‘leads’.

The strips of copper foil should be 2 cm longer than the depth of the beaker.

A cheaper alternative to the copper sulfate solution is a 50:50 mixture of 0.1 M copper sulfate and 0.2 M sodium chloride.

Procedure

1. Fit the two strips of copper foil inside the beaker as shown, with the top 2 cm bent back over the edge of the beaker.
2. Use two crocodile clips to keep the foils in place. Connect leads to the clips.
3. Half-fill the beaker with copper sulfate solution.
4. Complete the circuit as shown. Let the current run for some minutes and then look at the strips to see if there is any difference.
5. Repeat using the carbon electrodes. These pencil leads will slip through the holes drilled in the beaker lids, and can be kept in place with the crocodile clips.

Teaching Notes

• Bare copper wire electrodes are not satisfactory for copper plating. The initial current density would likely be so high that the oxygen produced would then reduce the current.
• Copper sulfate is a chemical compound containing copper, sulfur and oxygen. By using first copper electrodes and then carbon electrodes, the students can be encouraged to conclude that copper is being deposited at one side. This may prompt them to ask what is happening at the other side.
• If the terminals of the copper electrodes are reversed, copper will go into solution at the positive electrode and be deposited at the negative electrode. Some form of copper is travelling from one electrode to the other, through the solution. Electric current is also passing between the electrodes. Say, there may be some things made of copper making up the current in the solution.
• Scientists believe that there are charged carriers in a conducting solution. They give these the old Greek name for travellers: ions. If the charged particle is travelling from the red terminal (also called the positive terminal) of the cell through the liquid to the black terminal (also called the negative terminal) of the cell, then it is said to be a positive particle. This is the origin of the convention that electric current flows from positive to negative; we call this conventional current as a short-hand.

This experiment was safety-tested in December 2004

Class practical

A useful application of electrolysis.

Apparatus and Materials

For each student group

• Cells, 1.5 V, with holders, 3
• Lamps with holders, 2
• Crocodile clips, 2
• Ammeter (0 - 1 amp), DC
• Leads, 4 mm, 7
• Beaker, 250 ml
• Strip of copper foil, 1 cm wide
• Copper sulfate solution, 0.5 M
• Coin or other object to be plated
• Silver nitrate plating solution (see technical notes)

Health & Safety and Technical Notes

Read our standard health & safety guidance

To make the silver nitrate plating solution dissolve 1.6 g of silver nitrate and 32 g of potassium iodide in 100 ml of distilled water. Add 3 drops of concentrated sulphuric acid.

The strip of copper foil should be 2 cm longer than the depth of the beaker.

Procedure

1. Fit the strip of copper foil inside the beaker as shown, with the top 2 cm bent back over the edge of the beaker.
2. Use one crocodile clip to keep the foil in place.
3. Attach a second crocodile clip to a coin, and dangle the coin in the beaker at the opposite side to the copper foil. Ensure that the coin is attached to the negative terminal.
4. Connect leads to the clips.
5. Half-fill the beaker with copper sulfate solution.
6. Complete the circuit as shown. Let the current run for some minutes and then look at the coin and the copper strip to see if there are any differences.
7. Repeat with coins made of different metals.

Teaching Notes

• Avoid objects made of zinc or iron - these metals displace copper of their own accord from the solution and so can confuse the story badly. Try the materials yourself beforehand.
• Copper-plating coins is a useful way to use up small change from foreign travel. Some students will want to see what happens to ‘silver coins’, and after a ‘disaster’ in copper sulfate solution, a little silver nitrate solution (expensive!) can be tried.
• An old iron bedstead thrown into a river with copper salts in it proves to be an easy way of getting at the copper. Nickel-plated iron will also show this substitution.

This experiment was safety-tested in June 2007

Demonstration

A delightful demonstration of electrolysis.

Apparatus and Materials

• Variable voltage supply, 0–12 V
• Leads, 4 mm, 2
• Ammeter, 0-1 A
• Cell to contain liquid - sized to suit projector
• Electrodes, one pair
• Lead acetate (ethanoate) solution
• Projection microscope or slide projector

Health & Safety and Technical Notes

Lead salts are toxic and must be handled with care. Hands must be washed thoroughly after handling them and their solutions.

Read our standard health & safety guidance

The cell should have flat sides. It should be of glass or Perspex.

If you use a projection system, it should be an old optical projector or flexcam and display screen.

The electrodes can be of carbon (for example, two pencil leads) but platinum wire is to be preferred. Lead electrodes can also be used.

A suitable strength is 30 g of lead acetate to 100 g of water. Should difficulty be experienced in dissolving the lead acetate, add a few drops of glacial acetic acid.

Procedure

1. Put the solution of lead acetate into the cell and arrange the two electrodes suitably. In the case of wire electrodes, one wire should run down one side and round the bottom as shown. This is the anode. The central wire is the cathode.
2. Place the projection microscope next to the cell, and ensure that a clear image of the electrodes is projected. If using a slide projector, remove the slide holder and support the cell in its place.
3. Connect the power supply to the electrodes. (Positive terminal to anode, negative terminal to cathode.)
4. Pass a small current, preferably less than 50 mA.. About 10 volts DC may be necessary. A beautiful ‘tree’ of crystalline lead will be grown.
5. The tree can be made to dwindle away by reversing the current. A new tree will grow at the other electrode.

Teaching Notes

• A forest of lead crystals grows on the negative electrode. On reversing the potential difference, that forest shrinks and a new one grows on the other electrode.
• Make sure that students can correctly identify the electrodes by attaching a ‘+’ label to the anode, and ensure that this is visible in the projected image. Check carefully for optical reversion of the image.

This experiment was safety-tested in January 2007

Demonstration

The production of hydrogen and oxygen from water.

Apparatus and Materials

• Retort stand, boss, and clamp
• Power supply, variable, 0–12 V
• Voltameter
• Demonstration ammeter (0-1 A)
• Leads, 4 mm, 4
• Rheostat, 10 - 15 ohms rated at about 5 A
• Dilute sulfuric acid (0.1 M)
• Syringe, plastic, 20 - 25 ml, 2
• Rubber tubing to connect syringes to burette outlets
• Test tubes, 2
• Glass trough, small
• Electrode assembly (platinum or carbon electrodes, two, with suitable holder)

Health & Safety and Technical Notes

Keep flames at least 1 m from the voltameter. Keep students 4 m from exploding bubbles, which are ignited with a splint taped to a meter rule.

Read our standard health & safety guidance

A voltameter improvised from two burettes (as illustrated) is perfectly adequate for this demonstration.

Alternatively, a Hoffman voltameter could be used but the gas pressure may be too low to blow bubbles.

Procedure

1. Fill the large gas jar with 0.1 M sulfuric acid almost to the top.
2. Hang the electrode assembly over the rim.
3. Lower the burettes into the acid with the taps open until the open ends rest on the bottom of the jar. If the acid level does not reach the taps, connect a syringe to the top of the outlet, and transfer the remaining air to the syringe.
4. Close the top and lift the burette to surround one electrode.
5. Repeat with the other burette and support them in clips as shown.
6. Connect the voltameter into a series circuit of rheostat, ammeter and DC supply.
7. Switch on. A current will now flow. The rheostat can be adjusted to give a suitable current of about 0.5 A. Bubbles will be seen at both electrodes and gas can be collected in the inverted burettes.

Teaching Notes

• The relatively high solubility of oxygen in water makes it preferable to run the equipment for some time before the demonstration if acceptable volumes of oxygen are to be obtained. If the current is too low, the inverted burettes should be raised a little.
• With a Y-piece and thin rubber tubing, the two gases can be mixed in a soap bubble. (To help form the bubble, the burette taps are opened and the burettes lowered in the voltameter so that the gases are forced out.) If the bubble is exploded, the energy transfer will be apparent.
• To test the gases, they can be extracted quite easily in separate syringes as follows. Connect the syringe to the top of the burette, open the tap, and withdraw the plunger of the syringe until the syringe is full of gas. Then fill test tubes with the gas. Do this by putting a test tube full of water upside down in a trough of water and connecting a rubber tube to the syringe. This can be put under the mouth of the test tube. Gas can be propelled into the test tube by gently pushing the plunger inwards. Test the gases in the usual way: for hydrogen using a lighted splint, and for oxygen using a glowing splint.
• Miniature versions of this demonstration using test tubes instead of burettes will turn it into a class experiment.
• The appearance of bubbles at both electrodes suggests that there may be ions travelling both ways; the hydrogen carrying positive charge and the oxygen negative charge.

This experiment was safety-tested in January 2007

Demonstration

A magic stylus for colourful writing.

Apparatus and Materials

• Copper plate, about 20 cm x 20 cm (or similarly sized sheet of aluminium foil)
• Stylus (see technical note)
• Power supply, variable, 0-12 V, with smoothing capacitor of high ripple rating
• Rheostat, (10 - 15 ohms), rated at 4 or 5 A
• Filter papers, large
• Potassium iodide and starch solution (see technical note)
• Potassium sulfate and phenolphthalein solution (see technical notes)

Health & Safety and Technical Notes

Read our standard health & safety guidance

The stylus is made from a 15 cm length of 1 cm dowel with two 0.5 cm screw-eyes screwed into one end. A 75 cm length of 26 SWG insulated copper wire is secured to each screw-eye and taped to the dowel with Sellotape or rubber bands. (The copper wire must make good electrical contact with the screw-eye.)

0.1 M potassium iodide solution should be adequate. Weigh out 1.66 g of KI in a 250 ml beaker; add 70 ml of distilled water and stir. When dissolved, add 70 ml of distilled water to make up to 100 ml.

Starch indicator is best prepared on the day of use. Mix 1 g of soluble starch with distilled water to form a thin paste in a 250 ml beaker. Bring 80 ml of distilled water to the boil and add the paste, stirring as it goes in. Allow to cool and make up to about 100 ml. Add about 1 ml of starch indicator to 100 ml of KI solution.

0.1 M potassium sulfate solution: Weigh out 1.74 g of K2SO4 in a 250 ml beaker. Add 70 ml of distilled water and stir until dissolved. Pour into measuring cylinder and make up to 100 ml.

Phenolphthalein indicator: Weigh out 0.1 g of solid and dissolve 60 ml industrial methylated spirit; pour into a measuring cylinder and make up to 100 ml with water. Add 0.5 ml to 100 ml of potassium sulfate solution.

Procedure

1. Soak a filter paper in the solution of starch and potassium iodide.
2. Lay it flat on the metal sheet.
3. Connect the leads from the screw-eyes in the stylus into a simple series circuit with the smoothed DC supply and a rheostat, set initially at its maximum value.
4. Draw the stylus across the dampened paper. One of the screw eyes will leave a brown trace (caused by the arrival of iodine ions), or blue if starch is added.
5. Try reversing the DC voltage.
6. Try using an AC supply. Remember to remove the smoothing unit before switching to AC.
7. Repeat using paper soaked with the potassium sulfate and phenolphthalein solution.

Teaching Notes

• This is a pretty experiment. Electrolysis on wet filter paper containing an indicator shows the product of electrolysis. A full explanation requires a lot of chemical knowledge which might not be appropriate here. Treat the colours as indicators of an electric current, whether AC or DC.
• Iodine will make a brown stain where its ions arrive. Phenolphthalein (dissolved in alcohol) is an indicator which, when added to the solution, will create a crimson stain where the potassium ions arrive.
• The stylus can be used to write and draw patterns on the filter paper. The cell terminals can be reversed and the colours will swap over.

This experiment was safety-tested in January 2007