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.