Conditions for Rusting of Iron

• The presence of water and oxygen is essential for the rusting of iron.

• Impurities in the iron, the presence of water vapor, acids, salts and carbon dioxide hastens rusting.

• Pure iron does not rust in dry and carbon dioxide free air. It also does not rust in pure water, free from dissolved salts.

• Metals like chromium, zinc and magnesium prevent rusting to a great extent and alkalis also help to prevent rusting.

Take some clean iron nails and place them in the three test tubes prepared as under:

Test tube A: Contains boiled water and thin layer of oil.

Test tube B: Contains a small cloth bag containing soda lime.

Tent tune C: Contains ordinary water. Some nails are kept under water and some more are suspended above the water surface.

All the test tubes are tightly closed after placing the nails. Wait for a few days and then observe what has happened (Fig.13.18).

Test tube A

The nails do not rust. They have water, but no oxygen. Boiled water does not contain any dissolved oxygen. The thin layer of oil prevents oxygen in air from reaching the water in the test tube.

Test tube B

The nails do not rust. They have air, but no water and no carbon dioxide, for soda lime absorbs all the carbon dioxide and water vapor.

Test tube C

The nails placed in the water, as well as the ones suspended in the air rust. The water contains dissolved air, and the air contains water vapor.