c WHAT COMBINES WITH IRON DURING RUSTING Moisten the inside of a testtube with water sprinkle into it a spatula measure of iron filings and rotate it horizontally so that the filings spread and adhere to the walls Alternatively push a small plug of moistened iron wool to the bottom of the tube Invert the testtube in a beaker about one third full of water Use the beaker lip to support the tube The water levels inside and outside the tube should be the same and the level should be marked on the tube Leave the tube in this position for a few days The iron will rust and the water level will rise up inside the tube finally becoming steady Again add water to the beaker until the levels inside and outside the tube are the same and mark the new level It will be seen that one fifth of the air volume has been used up suggesting that oxygen has been used up in the rusting of iron The residual gas does not support combustion of a lighted splint