What actually happened to the atoms of Cu and Zn when the penny was put into the zinc and sodium hydroxide solution?

All you need are a couple of common chemicals to turn your normal copper-colored pennies (or another mainly-copper object) from copper to silver and then to gold. No, the coins won't really be silver or gold. The actual metal involved is zinc. This project is easy to do. While I don't recommend it for very young kids, I'd consider it appropriate for kids ages third grade and older, with adult supervision.

• Clean pennies
• Zinc metal (preferably powder)
• Sodium hydroxide or sodium hydroxide solution
• Tweezers or tongs
• Container of water
• Source of heat/flame

Note: Supposedly you can substitute galvanized nails for the zinc and Drano™ for the sodium hydroxide, but I was unable to get this project to work using nails and drain cleaner.

1. Pour a spoonful of zinc (1 to 2 grams) into a small beaker or evaporating dish containing water.
2. Add a small quantity of sodium hydroxide.
3. Alternatively, you could add zinc to a 3M NaOH solution.
4. Heat the mixture to near-boiling, then remove it from heat.
5. Add clean pennies to the solution, spacing them so that they are not touching each other.
6. Wait 5 to 10 minutes for them to turn silver, then use tongs to remove the pennies from the solution.
7. Rinse the pennies in water, then set them on a towel to dry.
8. You can examine the pennies once you have rinsed them.

This chemical reaction plates the copper in the penny with zinc. This is called galvanization. The zinc reacts with the hot sodium hydroxide solution to form soluble sodium zincate, Na2ZnO2, which is converted to metallic zinc when it touches the surface of the penny.

1. Grasp a silver penny with tongs.
2. Gently heat the penny in the outer (cool) part of a burner flame or with a lighter or candle (or even set it on a hotplate).
3. Remove the penny from heat as soon as it changes color.
4. Rinse the gold penny under water to cool it.

Heating the penny fuses the zinc and copper to form an alloy called brass. Brass is a homogeneous metal that varies from 60% to 82% Cu and from 18% to 40% Zn. Brass has a relatively low melting point, so the coating can be destroyed by heating the penny for too long.

Please use proper safety precautions. Sodium hydroxide is caustic. I recommend conducting this project under a fume hood or outdoors. Wear gloves and protective eyewear to prevent getting splashed by the sodium hydroxide solution.

In this teacher demonstration students observe a copper coin as it is dipped into a solution of sodium zincate in contact with zinc metal. The coin becomes plated with zinc, appearing silver in colour. Students then witness the coin being heated in a Bunsen flame, forming an alloy of brass which makes the coin appear gold

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

The demonstration takes about 10–15 minutes.

Equipment

Apparatus

• Eye protection (goggles)
• Beaker, 250 cm3
• Electric heating plate
• Pair of tongs or forceps
• Glass stirring rod
• Bunsen burner
• Access to a top-pan balance

Chemicals

• Sodium hydroxide solution, 0.4 M, 100 ml (IRRITANT), 24 g
• Zinc powder (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT), 5 g
• Steel wool (note 7)
• Deionised or distilled water, 100 cm3
• Copper coins (note 8)

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear goggles and take care to avoid skin contact.
3. Sodium hydroxide, NaOH(s), (WARNING: IRRITANT) – Refer to CLEAPSS Hazcard HC091a.
4. Zinc powder, Zn(s), (HIGHLY FLAMMABLE, DANGEROUS FOR THE ENVIRONMENT) – Refer to CLEAPSS Hazcard HC107. Any solid zinc remaining in the solution (as fine powder or any clumps that have formed) 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.
5. Hydrogen gas, H2 (g), (EXTREMELY FLAMMABLE) – Refer to CLEAPSS Hazcard HC048.
6. Since hydrogen is evolved from a hot solution of zinc in sodium hydroxide an electric heating plate should be used to heat the solution and turned off before the zinc is added.
7. Hot coins could cause burns. Allow to cool for at least five minutes before handling.
8. If steel wool isn’t available a proprietary mild abrasive material (for example, ‘Brillo’ soap pads) can be used instead.
9. 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.

Procedure

Before the demonstration 

1. Measure 100 ml of 0.4 M sodium hydroxide solution in a 250 cm3 beaker.
2. Heat the solution to boiling point on an electric heating plate.
3. Turn the electric heating plate off.
4. Add 5 g of zinc powder carefully. The solution will fizz as some of the zinc dissolves forming sodium zincate and giving off hydrogen.
5. Clean a ‘copper’ coin with steel wool until it is shiny.

The demonstration

1. Drop the cleaned coin into the hot solution containing sodium zincate and the remaining zinc powder.
2. 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.
3. 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.
4. Remove the plated coin with tongs or forceps and rinse it under running tap water to remove traces of sodium hydroxide and sodium zincate.
5. Show the ‘silver’ coin to the audience.
6. 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.
7. 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.

You can weigh the coins before and after coating to find the mass of zinc added.

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.

Introduction

A copper penny appears to turn to silver after heating in an evaporating dish with a mixture.  The penny then appears to turn to gold when heated in a flame.

In evaporating dish:

Zn + NaOH → Na2ZnO2

                                                                                                            sodium zincate

Cu in penny reduces Na2ZnO2 to metallic zinc:

Cu + Zn2+ → Cu2+ + Zn

Heat causes a fusion of zinc and copper to form brass (gold-colored).  Brass is 60-82% Cu and 18-40% Zn.

To Conduct Demonstration

1.  Heat NaOH/Zn mixture to near boiling in a crucible (3.5 in in diameter with handle)  on a hot plate.
2. Place one or more cleaned pennies into the mixture and continue to warm for 3 or 4 minutes.
3. Remove the pennies when they are completely and evenly coated with zinc.  Wash them and blot them dry.
4. Using tongs, hold each penny in a Bunsen burner flame (3-4 sec.). Students in first few rows report being able to see the zinc turn grey while in the flame.
5. Wash and dry the "gold" penny.

 Demo Time: ~ 5 minutes

Notes:

1. Clean the pennies in concentrated HCl.  Watch zinc filled pennies (after 1983) to make sure the HCl doesn’t get to the zinc and dissolve it.
2. Stir NaOH/Zn mixture with a glass rod.
3. Zn will clump up on the bottom of the crucible in the NaOH.  The Zn compound can stick to the pennies, so experiment with the placement of the pennies to minimize this occurrence.
4. This demonstration is not recommended for large audience viewing unless a video camera or document camera can be used to project the demonstration on the screen.

 Safety 

Safety:  Weigh the zinc dust in a hood.  Heat sodium hydroxide and zinc dust using the ventilation grating in Chem 140 or in a chemical fume hood. Use tongs to handle the pennies.  Use caution with hot sodium hydroxide and hot pennies.  Brass pennies are safe to handle (when cool) and may be given away to students.

Reference:

L.R. Summerlin and J.L. Ealy, Chemical Demonstrations: A Sourcebook for Teachers, 1985.