The air outside,therefore,pressed down on the water in the basin more strongly
thandid the air inside the glass. As a result, itpushedthe water up into the glass
inproportionto the difference in pressure. This gives you amathematicalwayof
measuringtheamountofoxygen used up. Since all the oxygen originally con-
tainedin the air inside the glass was used up, what you arecalculatingis the
proportionofoxygenoriginallycontainedinthatair.
Supposeyourglass was 10 inches tall.Measurethedistancebetween the rim
and thepositionoftherubberband. Itshould beabout2 inches.Thatmeans
thatoxygenaccountedfor2/10 or 1/5ofthe original air in the glass. Since
1/5 is 20 per cent, you can seethat 20 per centofthe air was oxygen.HOWYOU CAN SHOWTHATTHEGAS YOU EXHALECONTAINS
CARBON DIOXIDE
Gather these materials: Powderedcalciumhydroxide(Ca(OH)2); 1 testtube
i-filled with waterand 1emptytesttube;analcoholburner;and adrinking
straw.
Follow this procedure:Place!teaspoonfulofcalcium hydroxidein the test
tubecontainingthe water. Light the alcoholburnerandheatthe test tube gently
until all thepowderdissolves, or until no more will dissolve.Ifitdoes dissolve
completely,adda little more. Set the tube aside tocool;as it cools, the excess
calciumhydroxidewill settle to thebottom.When the calciumhydroxidehas
cooledandsettled,pour^2 inchesofthe clear liquid above it into theothertest
tube. Now blow into the clear liquidthroughthe straw.Continueto do this for
3 minutes. Now light amatchandblow itoutwithyourbreath.
Results:As soon asyourbreathtouches thesolutionofcalcium hydroxide,
the solutionbegins toturna milky white. As youcontinue,it becomes even
cloudier.If you now allow thesolutiontostand,a whiteprecipitatewill settle
down. Aprecipitateis aninsolublesubstancethatseparatesoutfrom asolution
as the resultofcertaintypesofchemical reactions.