Pennies
are made from zinc and coated with a layer of copper. Copper can
react with the air to form copper oxide and this is the dark tarnish
you see on old pennies. Acidic liquids like coke and lemon juice
can remove the copper oxide to give bright shiny colored pennies
- the acid acts to strip off the top layer of the pennies and in
time it could strip off the entire layer of copper to show the silver
zinc underneath.
Coke contains phosphoric and carbonic acid, fruit juices contain
citric acid. When pennies are soaked in vinegar they go green
because they become coated with a layer of copper acetate, produced
by the reaction between the copper and the acetic acid in vinegar.
This is a similar effect to that seen on the Statue of Liberty,
which, when brought over from France, was a dark copper color
but has turned green over the years. This green layer is known
as Patina, or copper carbonate. This layer protects the copper
from further corrosion.
Adding salt to the vinegar increases the ionic strength of the
weak acetic acid solution and speeds up the removal of the copper
oxide tarnish on the pennies. This is why ketchup is so good at
removing this darkened layer- it contains a huge amount of salt.
If you measure the pH of ketchup
you will see that its not very acidic compared with some of the
other liquids you looked at. In the vinegar and salt solution
the pennies should turn a blue-green color because of the deposition
of copper acetate (from the acetic acid) and copper chloride (from
the salt).
Note - This coating on pennies is not dirt - the coins probably
have a lot of horrible stuff lurking on them that could be classed
as dirt, but copper oxide is not! The reason that people do not
use these substances to clean their dishes is that they are no
good at cleaning stuff. If you look at the experiments on pH
you will see that most cleaning agents are in fact alkali, not
acidic, and this is because alkalis are much better at cleaning.
When you add an iron nail to the solution containing vinegar
(acetic acid) and salt along with the penny, the nail will be
coated in a thin layer of copper. This experiment would work much
better if you used copper wire instead of the penny as there isn't
much copper on a penny and the zinc can get in the way too.
