What it shows
Rusting is an electrochemical process that speeds up when the water conducts better. Nails kept in water with 0, 1, 3.5 and 10 percent salt for two weeks, then cleaned and weighed, show that salt accelerates corrosion, which is why vehicles and ships near the sea rust faster, and also that very strong brine slows it again.
The science
When iron rusts, atoms at one spot lose electrons and become iron ions, while at another spot oxygen dissolved in the water gains those electrons. The two spots are connected through the water, so the rate depends on how well the water conducts. Dissolved salt supplies ions that carry current, so corrosion speeds up as salt is added. At very high salt content, however, less oxygen can dissolve in the water, so the rate falls again; sea water at about 3.5 percent salt is close to the fastest. Mass lost from the cleaned nail measures how much iron was converted to rust.
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.
- Eight identical plain iron nails, not galvanised
- Table salt and a balance reading 0.01 g
- Four jars with lids and distilled or boiled cooled water
- A measuring cylinder
- A plastic brush or coarse cloth for cleaning rust
- Sandpaper
- Labels and a notebook
Safety
For this project: Nails are sharp; carry them point down. Rust-cleaning should be done over newspaper and hands washed afterwards. No chemicals other than salt and water are used.
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.
Sand all eight nails bright, wash, dry and weigh each to 0.01 g, recording the mass with the nail number.
Prepare 200 ml each of 0, 1, 3.5 and 10 percent salt solutions by mass and label the jars.
Place two nails in each jar, half submerged if possible by leaning them against the side, and loosely lid the jars.
Keep the jars together at the same temperature and photograph or sketch them every two days.
After 14 days remove the nails, brush off all loose rust under running water, dry fully and weigh again.
Calculate the mass loss for each nail and average the two nails from each jar.
Plot average mass loss against salt percentage.
Discuss the shape of the graph, especially if the 10 percent jar shows less loss than the 3.5 percent jar.
What you should see
Mass loss and visible rust increase from plain water to 1 and 3.5 percent salt, with the 3.5 percent nails usually the worst affected, while the 10 percent brine nails often rust a little less than those at 3.5 percent.
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 salt speed up rusting?
It makes the water conduct better, so the electron flow between rusting spots is easier.
2Why might very salty water rust nails less?
Less oxygen dissolves in strong brine, and oxygen is needed for rusting.
3Why weigh the nails after brushing off the rust?
Rust is iron that has left the metal; removing it shows how much iron was lost.
4Where is this important in daily life?
Cars, bridges and ships near the coast rust faster because of salty air and water.
Take it further
Repeat with one jar of boiled, cooled water sealed with a layer of oil on top to show that removing oxygen almost stops rusting.
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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