Demonstrations designed to capture the student's imagination

Demonstrations to capture the student's imagination by Adrian Guy of Blundell's School. In this issue: Turning copper into gold and silver

In this 'alchemical' practical copper coins are first coated with zinc, giving a silver appearance, and then heated to diffuse the zinc into the copper, forming a brass coin with a golden hue. 

EXHIBITION-CHEMISTRY-400

EXHIBITION-CHEMISTRY-400

Kit

  • copper coins
  • household malt vinegar
  • table salt
  • 250 ml beaker
  • zinc powder
  • 4 M aqueous sodium hydroxide
  • evaporating basin
  • hot plate or Bunsen burner, tripod and pipe-clay triangle
  • tongs
  • 50 ml measuring cylinder
  • distilled water
  • heat-resistant mat

Procedure

Dissolve 5 g of salt in 20 ml of vinegar, add three coins and swirl for ca 30 s or until clean. Remove the coins and rinse with distilled water before placing on a paper towel to dry. To avoid grease contamination do not touch the coins with your fingers. Keep one clean copper coin for comparison. 

Zinc plating. Add 3 g of powdered zinc to 30 ml of 4 M NaOH in an evaporating dish. Place the two remaining coins in the dish so that they are in contact with the zinc powder and fully submerged by the NaOH. Heat the dish on a hot plate set at 200oC for ca 10 min or until the coins turn a silver colour. Alternatively, place the dish on a pipe-clay triangle on a tripod and heat using a Bunsen burner. This latter method is quicker but requires more care, owing to the possibility of spitting. Using tongs, turn the coins over periodically to ensure an even coating of zinc. Note - bubbles of hydrogen gas should be seen forming on the coins' surface. Once shiny silver in appearance remove the coins with the tongs and rinse with distilled water. Keep one clean 'silver' coin. 

Brass Plating. Heat the remaining zinc-plated coin by holding in a blue Bunsen flame, using tongs (photo (a)). Heat until the brass colour is just seen to appear and then remove the coin from the flame to allow the diffusion of the zinc to continue. Do not overheat the coin because the brass coating will discolour and darken. Allow to cool on a heat-resistant mat. 

You can now compare the colour of the three coins (photo (b)). 

Special tips

Strictly speaking it is illegal to 'deface coins of the realm' so use foreign coins, tokens or pieces of copper instead.

Repeat the experiment using tin powder, or tin granules instead of zinc. The coating process is identical but slower (> 1 h) because tin has a much less negative electrode potential than zinc. The resulting coin has a dull bronze appearance. 

Weigh the coins before and after coating to find the mass of zinc added.

Teaching goals

This demonstration is suitable for any age and can be used to illustrate the formation and properties of some alloys. The change in appearance when the heated zinc and copper form brass is helpful to reinforce the different properties alloys have compared to their constituent metals. This is also an useful experiment to reinforce the constitution of brass and bronze. With older students the complexing of zinc ions to form zincate, and a discussion of electrode potentials can be introduced.

The exact electrochemical process is debateable but some proposed steps are :

  • hydrated zinc ions form at the zinc electrode and react with aqueous sodium hydroxide to give aqueous zincate ions (Zn(OH)42-) with the evolution of hydrogen gas:

    Zn(s) + 2OH-(aq)  + 2H2O(l) → Zn(OH)42-(aq)  + H2(g) 

  • at the copper electrode the zincate ions are reduced to zinc:

    Zn(OH)42-(aq) + 2e- → Zn(s) + 4OH-(aq) 

Quite where the electrons come from to reduce the zincate ions is not clear. One possibility is that they come from hydrogen formed by the reduction of hydroxide ions by zinc.

Safety

Hot 4 M sodium hydroxide is corrosive and can cause severe burns. Eye protection (goggles, not safety spectacles) and gloves are essential. Use a safety screen to protect pupils when plating the coins with zinc.

Hot coins must be handled with care - leave for at least five minutes to cool before touching to avoid burns.