it to air, which improves its flavor and removesunpleasantodors. Thismethod
ofpurifying water is used when largequantitiesofwater are involvedandin
places where thenaturaltaste andodorofthe water areunpleasant.
Distillation.Distilled water is the only kind ofwater thatis 100 per cent
chemically pure H20.Distillation,a process you will learnaboutin the experi-
ment on page 39, occurs in two stages:evaporationandcondensation.After
distillation no contaminantsofany kind remain in the water. Water goes
througha naturaldistillationprocess every time itevaporatesfrom the sea and
then condenses into clouds. Rain water,therefore,is always pure if itdoesn't
fallthroughdirty air.
HOW YOU CAN SHOW THAT SAND AND GRAVEL ARE USEFUL IN
FILTERING
Gather thesematerials:Three funnels; anuprightstandand3 ringsupports;
piecesofgauze; 3pintjars;sand from a nearby beach or sandpit;soil from the
backyardorgarden;gravel from the drivewayofyour home, from somenearby
area,or from a pet shop.
Follow this procedure:Arrangetheuprightstandand ringsupportsas shown
in thediagram. Place a funnel containinga pieceofgauze in each one. In
the first funnel put 1 inch ofordinarysoil; in the second put 1 inchofsand;
and in thethirdput 1 inchofgravel.Pour1 cupfulofvery muddy water slowly
througheach funnel, and catch the waterthatgoesthrougheach inseparate
pint jars.Comparethe results.
~.
~
, I
~"':-c-a-,Results: The amountofmud that camethrougheach filter differed. The
gravel, whichpermittedthe most mud to getthrough,was the least effective
filter. If the soil you used as a filtercontainedclay, then the mud not only
passedthroughit but also dragged some of the clay with it. The sand did the
bestjobof filtering out the solid particles. Large cities filter waterthroughsand
in filter beds or settling tanks. The water still needsfurthertreatment,though,
because eventhoughit is clear, it is not necessarily free from germs orpleasant
to the taste.