Salt
Have you heard that iron articles used in homes near the sea rust faster than those used elsewhere? Activity 12.3.5 looks into this.
Activity 12.3.5
You will need: cleaned iron nails, boiling tubes, solid sodium chloride
- Take two new iron nails and clean them.
- Put the nails in two boiling tubes. Add water with sodium chloride dissolved in it to one tube, and ordinary cold water to the other.
- Leave for about a day, observe and record.
Figure to be added
The observation
The nail in tube (B) has rusted more than the nail in tube (A). So sodium chloride has made rusting faster. Sodium chloride is a salt, and most salts increase the rate of rusting. In coastal areas the salt concentration is high, so iron articles used there rust quickly.
Why salt speeds it up
Salt does not take part in the rusting equations — you will not find Na+ or Cl- in them. What it does is make the water an electrolyte. Rusting is an electrochemical cell: iron is oxidised at one spot, oxygen is reduced at another, and ions have to move through the water between them to complete the circuit. Pure water has very few ions, so this is slow; salt water is full of ions, so the cell runs quickly. This is why sea spray, and the salt spread on roads in cold countries, are so hard on cars and bridges.
Salt speeds up rusting
Salt makes water a better electrolyte, so iron rusts faster in salt water and near the sea.