Turning copper coins into 'silver' and 'gold'

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Lesson organisation

A simple demonstration involving electroplating and the chemistry of alloys, this is suitable for any age group depending on the sophistication of the theoretical treatment used - if any.

The demonstration takes about 10 -15 minutes.

Apparatus and chemicals

For each demonstration:

Eye protection: goggles
Disposable gloves (preferably nitrile)
Beaker (250 cm³)
Bunsen burner (see note 1)
Tripod and gauze
Pair of tongs or forceps
Glass stirring rod
Access to a top-pan balance

Zinc powder (Highly flammable), 5 g
Sodium hydroxide pellets (Corrosive), 24 g
Steel wool (see note 2)
Deionised or distilled water, 100 cm³
Copper coins (see note 3)

Technical notes

Zinc powder (Highly flammable) Refer to CLEAPSS Hazcard 107
Sodium hydroxide (Corrosive) Refer to CLEAPSS Hazcard 91
Hydrogen (Extremely flammable) Refer to CLEAPSS Hazcard 48

1 Since hydrogen is evolved from a hot solution of zinc in sodium hydroxide, an alternative source of heating is to be preferred, e.g. an electric heating plate. If a Bunsen burner is to be used then it should be turned off before the zinc is added.

2 If steel wool isn't available a proprietary mild abrasive material (for example, ‘Brillo’ soap pads) can be used instead.

3 Copper foil could be used instead, but coins are better since they are everyday articles, and there are bound to be requests from the audience to turn copper into 'gold'. Strictly speaking it is illegal to "deface coins of the realm", so the law-abiding teacher might prefer to use foreign coins instead. It would be wise under these circumstances to ensure that the plating works, since many other alloys are used in foreign coinage.

4 Any remaining finely powdered zinc should not be left to dry because it can ignite spontaneously. Dispose of it by rinsing with water, dissolving in excess dilute sulfuric acid and washing the resulting zinc sulfate solution down the sink.

Procedure

HEALTH & SAFETY: Wear goggles and disposable gloves.

Before the demonstration

a Dissolve 24 g of sodium hydroxide in 100 cm³ of deionised/distilled water in a 250 cm³ beaker, stirring continuously. The solution will get warm and is corrosive.

b Heat the solution to boiling point on a Bunsen burner (caution: the hot solution is Highly Corrosive).

c Turn the Bunsen off.

d Add 5 g of zinc powder carefully. The solution will fizz as some of the zinc dissolves forming sodium zincate and giving off hydrogen.

e Clean a ‘copper’ coin with steel wool until it is shiny.

The Demonstration

a Drop the cleaned coin into the hot solution containing sodium zincate and the remaining zinc powder.

b The coin must make contact with the powdered zinc at the bottom of the solution. If necessary use a glass rod to move the coin until this is so.

c Leave the coin until it is plated with a shiny coat of zinc. This will take about 2-3 minutes. Leaving the coin too long may cause lumps of zinc to stick to it.

d Remove the plated coin with tongs or forceps and rinse it under running tap water to remove traces of sodium hydroxide and sodium zincate.

e Show the 'silver' coin to the audience.

f Using tongs or forceps, hold the plated coin in the upper part of a roaring Bunsen flame for a few seconds until the surface turns gold. Turn the coin so that both sides are heated equally. Overheating will cause the coin to tarnish.

g Allow the coin to cool and show it to the audience.

Teaching notes

It may be sensible to carry out a trial experiment before performing the demonstration in front of an audience.

If the mixture of sodium zincate solution and zinc is cloudy, allow to cool, and then filter off the zinc to leave a clear filtrate. Place a small piece of zinc foil in the liquid as a substitute for the powder.

Younger students might want to have their own coins plated.

The theory is as follows:

The reaction between zinc and sodium hydroxide to form sodium zincate is as follows:

Zn(s) + 2NaOH(aq) + 2H₂O(l) → Na₂[Zn(OH)₄](aq) + H₂(g)

The plating reaction involves an electrochemical cell; it will not take place unless the copper and the zinc are in contact, either directly (as here) or by means of a wire.

The electrode reactions are:

At the zinc electrode: Zn(s) → Zn²⁺(aq) + 2e^–
followed by complexing of the zinc ions as [Zn(OH)₄]^2–(aq)

At the copper electrode: [Zn(OH)₄]^2–(aq) + 2e^– → Zn(s) + 4OH^–(aq)

The coating of zinc gives the impression that the coin is now coated with silver.

On heating the coin in the Bunsen flame, brass is formed by the zinc migrating into the surface layer of the copper. This gives a gold appearance to the coin.

Brass is an alloy of copper containing between 18% and 40% of zinc.

A similar zinc plating process is used industrially, but with cyanide ions rather than hydroxide ions as the complexing agent.

Health & Safety checked, August 2008