Cleaning Coppers

Public summary: 

Those grubby coppers in your pocket or purse can be shined up in minutes, give it a try and find out how it works.

Removing copper oxides from coins.
Useful information
Kit List: 

Plastic containers (e.g. disposable cups) - broad shallow containers are generally better.
Acids e.g. vinegar, cola, lemon juice, ketchup - at least a couple of different ones
Copper coated coins
Waste bin
Paper towels

Packing Away: 

NB: There is no box for this experiment.
Weak copper solutions can be poured down a drain, with excess running water.
Clean and dry non-disposable items (if there are any).
Wipe down surfaces.
Return unused solvents (vinegar etc.) to stores (box/pile).
Return money.

Frequency of use: 

In a nutshell
Copper coins are cleaned by putting them in vinegar/other acids to dissolve the copper oxide/copper chloride corrosion products, which are a lot more soluble in acid than the plain copper.

How to run the experiment
1) Make up cups of vinegar, cola, ketchup etc. before the event. Dissolving some salt in the acids makes them clean the coins faster, vinegar is a good one to add salt to as it is clear so you can watch the coin get cleaner.
2) Ask children/parents for copper coins - the dirtier the better (which will be returned, clean!)
3) Get kids to drop a coin into each liquid, for ketchup you can try to only cover half of a coin to see a before/after comparison
4) Wait a couple of minutes for coins to get clean. It may be better start trying to explain some science at this point rather than before the coins have been put in to avoid running out of things to say.
Some potential discussion points:
- Would the coins come clean in water? Could have a cup of water as a 'control'. Why are these liquids better at cleaning the coins?
- What do all of the liquids have in common? All acidic
- Which of the liquids would they expect to clean the coins fastest? We might expect this to be the strongest acid. Acids can make things taste sour, which tastes the most sour?
- What metal are the coins made of?
- Why does the surface copper oxide get removed, but not the copper making up the coin? Copper oxide more soluble in acids than copper
5) Take coins out and see if their predictions on what liquids clean best were correct

What you need to know during the experiment
- Corrosion is a result of the reaction of the metal with oxygen in air, forming the black solid copper oxide. You can compare it with rusting of iron and, for older kids, you can talk about the reaction being faster in the presence of moisture.
- The black copper oxide reacts with the acetic acid in vinegar to form copper acetate, which is soluble in water and forms a pale green solution.

Want to know more?
- You can see the green copper acetate by putting lots of really dirty copper coins in vinegar. This should be done in a container with a lid.
- The green colour of many copper salts can be seen on the surface of copper roofs and constructions like the Statue of Liberty. Although it is soluble in acid, under normal conditions it is very stable, preventing corrosion of the rest of the copper underneath the surface.
- Reaction with cola is remarkable from the point of view of making the copper clean - it's very acidic - but you can't see it happening very well. What would happen if you put a tooth in it? Can relate this to drinking lots of fizzy drinks being bad for your teeth. Do they know what metal is found in teeth? Lots of kids have heard of needing to eat calcium for strong teeth/bones but don't necessarily know it's a metal.

Risk Assessment
Date risk assesment last checked: 
Tue, 07/01/2020
Risk assesment checked by: 
Holly Smith
Date risk assesment double checked: 
Fri, 10/01/2020
Risk assesment double-checked by: 
Risk Assessment: 

Clean coppers using vinegar, orange juice, coke, water etc.
Possibly dissolving enough copper to make the vinegar go green.

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Copper dissolved in liquid Ingestion of copper salts can lead to the creation of free radicals in the body (which can damage DNA). Public 3 4 12 Concentrations are very low, the tolerable ingestion limit of copper is about 10mg, much less than would be present in a container used to clean a couple of coppers.
Change acid every 10 coins and dispose of used solution down a drain immediately. Empty and rinse container.
If cleaning large numbers of coppers to observe green colour of solution use a closed (plastic) jar and tape down lid. Keep control of jar (at busy events may be advisable to tape jar to a length of string to prevent it "walking off").
Call a first aider in the event of ingestion.
2 2 4
Acids Use of lemon juice, coke, other culinary acids - will sting if they get into people's eyes or cuts. All 4 2 8 Ensure children lower coins gently into solution. Encourage them to check for scratches before putting hands near solutions and to wipe fingers afterwards.
Rinse out cuts, call first aider if required. If trained and confident to do so use sterile eye wash to clean out splash to eye.
2 1 2
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