What it shows
An iron nail wrapped with zinc stays bright in salt water because the more reactive zinc corrodes instead, while a nail wrapped with copper rusts faster than a bare nail because copper makes the iron the more reactive partner. This is the chemistry of galvanised sheets and the zinc blocks bolted to ship hulls.
The science
When two different metals touch in a conducting liquid, they form a cell in which the more reactive metal loses electrons and corrodes while the less reactive one is protected. Zinc is more reactive than iron, so a zinc wrap becomes the anode and dissolves slowly, feeding electrons to the iron and keeping it from forming iron ions. Copper is less reactive than iron, so with a copper wrap the iron becomes the anode and corrodes faster than it would alone. Zinc protection of this kind is called sacrificial protection and is the reason galvanised iron stays rust-free even when scratched.
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.
- Six plain iron nails, sanded bright
- Galvanised (GI) wire or a strip of zinc from the school lab
- Bare copper wire
- Three jars with 3 percent salt water (30 g salt per litre)
- Sandpaper, a balance reading 0.01 g
- A magnifier
- Labels and a notebook
Safety
For this project: Nails and wire ends are sharp; an adult should cut the wire. Only salt water is used. Wash hands after handling.
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.
Weigh each of the six nails and record the masses with numbers scratched on their heads.
Wrap two nails tightly with several turns of galvanised wire so the metals touch firmly.
Wrap two nails the same way with copper wire, and leave two bare as controls.
Place each pair in its own jar of salt water, half submerged, and label the jars zinc, copper and bare.
Observe every two days with the magnifier, noting where rust or white zinc corrosion appears, and sketch what you see.
After 14 days unwrap the nails, brush off loose rust, dry and weigh them again.
Compare the average mass loss of the three groups in a bar chart.
Examine the zinc wire for white powdery deposits and the copper wire for any change, and explain both in the report.
What you should see
The zinc-wrapped nails stay almost bright with white zinc corrosion on the wire, the bare nails rust moderately, and the copper-wrapped nails rust the most, giving mass losses in the order copper-wrapped greater than bare greater than zinc-wrapped.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does zinc protect the iron?
Zinc is more reactive, so it corrodes first and supplies electrons that stop the iron forming ions.
2Why does copper make rusting worse?
Iron is more reactive than copper, so the iron becomes the anode of the cell and corrodes faster.
3What is galvanising?
Coating iron with a layer of zinc that protects it even where the coating is scratched.
4Where are sacrificial anodes used?
On ship hulls, underground pipelines and water heater tanks.
Take it further
Connect a nail to a zinc strip with a wire instead of a wrap, both in the same jar, to show that protection works even when the metals are not touching directly.
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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