What it shows
Passing a small current through copper sulphate solution deposits a layer of copper on the object connected to the negative terminal, while the copper anode slowly dissolves to replace the copper ions. A clean steel key or coin comes out with a bright copper coating.
The science
Copper sulphate solution contains copper ions carrying a positive charge. The key is connected to the negative terminal, making it the cathode; copper ions are attracted to it, gain two electrons each and become copper metal on its surface. The copper anode, connected to the positive terminal, loses electrons and releases fresh copper ions into the solution, so the concentration stays steady. The thickness of the coating depends on the current and the time, and a clean, grease-free surface is essential for the copper to stick.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Copper sulphate from the school lab, about 25 g dissolved in 250 ml water (teacher prepares)
- A 250 ml beaker or glass jar
- A piece of thick bare copper wire or copper strip as the anode
- A clean steel key or old coin as the object to plate
- A 1.5 V to 4.5 V battery pack with two leads and crocodile clips
- Fine sandpaper, a little detergent and a cloth
- Gloves and safety goggles
Safety
For this project: A teacher or adult must prepare the copper sulphate solution and supervise throughout; copper sulphate is harmful if swallowed and irritates skin and eyes, so wear gloves and goggles and wash hands. Use only batteries, never mains, and give the used solution back to the teacher for disposal rather than pouring it down the drain.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Rub the key with sandpaper until it is bright all over, wash it with detergent, rinse and dry, and from now on hold it only by a wire hook.
The teacher pours the copper sulphate solution into the beaker.
Bend the copper wire into a loop and hang it on one side of the beaker, connected with a clip to the positive terminal.
Hang the key on the other side from a wire hook connected to the negative terminal, making sure the key and anode do not touch.
Connect the battery and watch the key darken as copper starts to deposit within a minute.
Leave it for 10 to 15 minutes, keeping the current low; a high current gives a dull, powdery coating.
Disconnect, lift out the key, rinse it with water and dry it gently with the cloth.
Compare the plated key with an unplated one, and try a second key with the terminals swapped to see that nothing deposits on the anode side.
What you should see
The key is coated in a smooth pink copper layer that stays on when rubbed, the copper anode looks slightly eaten away, and the key connected as the anode receives no copper at all.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which electrode is the key connected to?
The negative terminal, making it the cathode where copper ions gain electrons and deposit.
2What happens at the copper anode?
Copper atoms lose electrons and go into solution as copper ions, replacing those used up.
3Why must the key be cleaned first?
Grease or rust stops the copper from bonding, so the coating flakes off.
4Name a use of electroplating.
Chrome plating on taps, gold plating on jewellery, and tin plating inside food cans.
Take it further
Plate two keys for 5 and 15 minutes and compare the colour and thickness of the coating.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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