CHEMICALS IN THE AIR
Perhapsyou knowthatair is amixtureofmany gases andthatwe live at the
bottomofa great ocean of it.You'veprobablyheardthis or similarstatements
before. Butwhatdo you knowaboutthe gases the air is made of? First, the
combinationof gases in the air forms amixture.Like acompound,a mixture
containstwo or more differentsubstances.But in amixturethesubstancesare
notcombinedchemically. Theproportionsof differentsubstancesin amixture
may vary from place to place and from time to time; they can also beseparated
from eachothermore easilythanthe differentsubstancesin acompound.
The mixturecalled air containsmolecules ofnitrogen (N2),oxygen(0 2 ),
carbondioxide (C0 2 ) ,watervapor(H20),and theinert gases:helium (He),
radon(Rn), argon(A), neon (Ne),krypton(Kr) and xenon (Xe). The air con-
tainsmorenitrogenthananything;about78 per centofthe air isnitrogen.Itis
a very inactive (chemists sayinert)gas, andsothequantityof it in itspureform
hardlyever varies. Pure oxygen, which is the most importantto us because we
use it forbreathing,makes up onlyabout 20 per cent of the air. Oxygen is an
active element and eventhoughthere are moreatomsof oxygen onearththan
ofanyotherkind, they are usually found incombinationwithotherelements
ratherthanas oxygen alone. Oxygencombineseasily with manysubstancesand
isconstantlydoing so, sothatthequantityofit in the air does vary slightly.
Carbondioxide in the air is only a smallpercentageof themixturecompared
with theothergases in air,buttheamountofit varies to a muchgreaterdegree.
Thepercentageofwatervaporalso varies greatly.
Whataccountsfor the changes in thepercentageofwatervaporandcarbon
dioxide inair?A few examples will make this clear. Where would you expect to
find moremoisture-ina forest, or adesert?In a forestofcourse. Butwhy?
Onereasonis thatthere are so many plants and trees in a forest. Whenplants
grow, theyabsorbwater from theground.Some of it becomespartoftheplant,
buta great deal of water is given offintotheatmospherethrough"pores"in
the leaves. This yielding ofmoistureto the air does notoccurin the desert,
because there are so fewplantsthere. The fewplantsthatdo thrive in the
desertdon'tyield muchmoistureeither. Inorderto survive they have evolved
in such a way as topreventthe escape ofwhateverpreciousmoisturethey con-
tain. In places where theatmosphereis moist, it usually rains. Where theatmos-
phereis dry, it doesnotrain.