(a) Most of the metals acquire a dull surface when exposed to air . Name the chemical phenomenon responsible for this process.
(b) State the conditions under which the iron articles get rusted . Design an activity to investigate the conditions necessary for rusting .

Explanation

Rusting happens in the presence of air and moisture.
An investigation can be carried out to study the conditions required for rusting, where three test tubes are taken with one iron nail in each.
In the first tube anhydrous calcium chloride is added which acts as a dehydrating agent and removes moisture from the tube, in the second tube boild and cooled water (boiling removes the dissolved gases from water) is added with an oil seal on the top, and in the third tube tap water (normal) is added till two-third of the nail gets immersed and one third is exposed to moist air above.
The tubes are lecft undisturbed for one week. Observations

n the first and second tube no rusting takes place, indicating that only air and onloy water do not support rusting. Only in the third tube the iron nail rusts, indicating that for rusting both air and water are required.
Rusting results in weakening of the iron structures in buildings and brides, which is dangerous. Hence preventing rusting is vital.