What it shows
Black tarnish on silver is silver sulphide, and it can be turned back to shiny silver without scrubbing, using aluminium foil, baking soda and hot water. It is a small electrochemical cell in which aluminium is more reactive than silver.
The science
Tarnish is silver sulphide, Ag2S, formed when silver reacts with sulphur compounds in air. Aluminium is higher than silver in the reactivity series, so it gives up electrons more easily. In hot baking soda solution, which conducts electricity, electrons pass from the aluminium to the silver sulphide, turning silver ions back into silver metal while the sulphur goes into solution. Aluminium is used up slowly.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A tarnished silver coin, ring or spoon (or a clean cheap silver-plated item)
- Aluminium foil
- Baking soda, 2 spoons
- A steel or glass bowl
- Hot water, about 300 ml
- Tongs
- A soft cloth
- Gloves
Safety
For this project: An adult must heat and pour the hot water. Use tongs to remove the item. Work in a ventilated room because a mild egg smell of hydrogen sulphide can form. Do not use valuable family jewellery or jewellery with stones.
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.
Line a bowl with aluminium foil, shiny side up.
Place the tarnished silver on the foil so that it touches the foil firmly in as many places as possible.
Sprinkle two spoons of baking soda over it.
An adult pours hot, not boiling, water over it until the item is covered, and the water fizzes slightly.
Wait 5 to 10 minutes while the black coating fades, and keep the bowl in an open place.
Lift the item with tongs, rinse it in cool water and dry it with a soft cloth.
Repeat for a second item without foil contact as a control, and see that it stays dark.
Draw the cell: aluminium foil, the solution and the silver, and write the electron flow with an arrow.
What you should see
The black coating disappears from the touching silver and the metal looks bright, while the control item without foil contact remains tarnished.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is tarnish on silver?
Silver sulphide formed by reaction with sulphur compounds in air.
2Why is aluminium used?
It is more reactive than silver and supplies electrons.
3What is the role of baking soda?
It makes the water a conducting electrolyte.
4Why is there an egg-like smell?
A little hydrogen sulphide gas is formed.
Take it further
Time how long a standard tarnished piece needs to turn clean at 50 °C, 60 °C and 70 °C.
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.
Free ebooks for this project
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