In this demonstration we appear to
change a base metal, copper, firstly into silver, and then into
gold. The silver is actually just a layer of zinc
plating and the gold is brass, an alloy of the
copper and the zinc. The first thing we need to do is clean
off a copper coin. I’m going to use some wire wool to clean the copper coin off. Be careful when you use wire wool because the small pieces can break off and get into the skin. You might prefer
to wear a pair of gloves or to use a different mild
abrasive. So we need to get the copper coin nice and clean. The next step is to make a
solution of sodium zincate. I’m going to do that by
adding five grams of zinc powder into 100 cm
cubed of 6 mol per dm cubed sodium
hydroxide solution. I then need to heat the solution to boiling. The bubbling you can see is partly due
to the hydrogen being given off from the reaction between the zinc and the sodium hydroxide which is quite unusual and shows the amphoteric nature of zinc, the ability to react with both acids and alkalis. Take the Bunsen out while
there’s still some zinc left in the bottom of the beaker. That’s important because the plating stage of the process is actually an electrochemical reaction and the copper needs to be in direct
contact with the zinc in the bottom of the beaker. Pop the coin into the solution so it’s in contact with the zinc at the bottom. After a few minutes the copper coin will be coated in
a silvery grey layer which is the zinc. If I just lift it out. The next step is to rinse the coin off in some cold water. This cools the coin down and gets rid of the sodium zincate
which is important for the next stage. If possible try and do this stage under a running tap and then just dab the coin dry, and we make sure that we take off any lumps of zinc that have stuck to it. You can see that we’ve changed our copper coin into silver The silver is actually a plating of zinc which if
you think about it is quite unusual because the electrode potentials, the order of the reactivity series of the
metals, would lead us to predict that zinc should displace copper from its compounds not the other way around, but by complexing the zinc we’ve actually changed the order of the
electrode potentials. The next stage is to change the silver into gold. Put the coin in the top part of the Bunsen flame and very gently heat it. Don’t overheat it because the
metal will tarnish and you can see before your eyes the silver changing into gold. The gold is actually a layer of brass which is formed by the zinc
diffusing or migrating to the surface layer of the copper.