To find out if rusting can take place in the absence of moisture.
Put a few dry, clean nails on the wire gauze of a desiccator. Dip a few more in water and hang them on the outside of the desiccator, to act as a control. Keep the apparatus in a convenient place for a few days and watch what happens. Also place a few bright nails in a dry test tube, insert a loose plug of cotton wool, place a few pieces of calcium chloride on top and a further plug of cotton wool as shown in the diagram. Set up a control experiment consisting of a few wet bright nails in a test tube exposed to both air and moisture.
The nails hung outside the desiccator and those in tube which were exposed to both air and moisture rust within a short time, those in the desiccator and in the other tube remain bright and will remain so for months.
The calcium chloride absorbed all the moisture in the desiccator and in the tube. Under this condition the iron nails failed to rust. We may therefore conclude that rusting does not occur in the absence of oisture. On the other hand, the control experiments, in which the nails are exposed outside the desiccator and the other tube to both air and moisture, show again that both air and water are necessary for rusting.