What it shows
Dull old copper or brass coins shine again in a dip of vinegar and salt, because the acid dissolves the dark copper oxide layer. If the coins are then left wet with vinegar in the air, a blue green coating forms, the same patina seen on old temple bells and statues.
The science
Copper slowly reacts with oxygen to form a dark layer of copper oxide, which makes coins dull. Acetic acid in vinegar reacts with that oxide and dissolves it as copper acetate, and the salt adds chloride ions that speed the attack, so clean metal is exposed. When a coin coated with the acid solution is left in air, the dissolved copper reacts with oxygen and the air’s carbon dioxide to form green copper salts, the patina. Rinsing stops this, which is why one coin stays shiny.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Five dull copper or brass coins or copper pieces (old one-paisa coins, copper wire scraps)
- Quarter cup of vinegar
- One teaspoon of salt
- A small bowl and a saucer
- Paper towels
- Lemon juice and plain water for comparison
Safety
For this project: Vinegar can sting cuts and eyes; wash hands after handling and keep the coins away from the mouth. Do this on a tray; the blue green crust is not to be touched by small children.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Mix the vinegar and salt in the bowl and stir until the salt dissolves.
Drop in three dull coins and watch them; within a minute the colour brightens.
Take one coin out after two minutes, rinse it in water and dry it; it stays shiny.
Take a second coin out and place it wet, without rinsing, on the saucer.
Leave the third coin in the vinegar for ten minutes and compare with the first.
Dip the fourth coin in lemon juice and the fifth in plain water as comparisons, and note which cleaned.
Check the unrinsed coin after one day and after three days for blue green spots.
Record all five coins in a table: liquid used, rinsed or not, final look.
What you should see
The vinegar and lemon coins turn bright copper within minutes while the water coin stays dull; the unrinsed coin develops a blue green patina in one to three days and the rinsed one stays shiny.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why are old coins dull?
A layer of dark copper oxide forms slowly on the surface in air.
2What does the vinegar do?
Its acid dissolves the oxide layer and exposes clean metal.
3Why does the unrinsed coin turn green?
Dissolved copper reacts with air to form green copper salts, called patina.
4Where have you seen green copper outside?
On old temple bells, statues and copper roofs.
Take it further
Put a clean iron nail into the used blue vinegar for an hour and look for a copper coating forming on it.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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