100ml of air was passed repeatedly over heated ____ turnings. The ____ of air kept reducing until all of the ____ had ____ it to form black copper ____. Then the remaining ____ was allowed to cool back down to the original ____, because gas ____ when it is heated. Then its volume was measured and was found to have reduced by from 100 ml to ____ml). This is how we know that air contains ____% oxygen.

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