100ml of air was passed repeatedly over heated [copper] turnings. The [volume] of air kept reducing until all of the [oxygen] had [oxidised] it to form black copper [oxide]. Then the remaining [air] was allowed to cool back down to the original [temperature], because gas [expands] when it is heated. Then its volume was measured and was found to have reduced by from 100 ml to [79]ml). This is how we know that air contains [21]% oxygen..

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