CHAPTER XXVIII THE CHEMISTRY OF WATER.

135. Pure Water.--Review the experiments for electrolysis, and
for burning H. Pure water is obtained by distillation.

Experiment 80.--Provide a glass tube 40 or 50 cm long and 3 or 4
cm in diameter. Fit to each end a cork with two perforations,
through one of which a long tube passes the entire length of the
larger tube (Fig. 32a). Connect one end of this with a flask of
water arranged for heating; pass the other end into an open
receptacle for collecting the distilled water. Into the other
perforations lead short tubes,-- the one for water to flow into
the large tube from a jet; the other, for the same to flow out.
This condenses the steam by circulating cold water around it. The
apparatus is called a Liebig's condenser. Put water into the
flask, boil it, and notice the condensed liquid. It is
comparatively pure water; for most of the substances in solution
have a higher boiling-point than water, and are left behind when
it is vaporized.

(Fig. 32a.)

136. Test.

Experiment 81.--Test the purity of distilled water by slowly
evaporating a few drops on Pt foil in a room free from dust.
There should be no spot or residue left on the foil. Test in the
same way undistilled water. 137. Water exists in Three States,--
solid, liquid, and vaporous. It freezes at 0 degrees, suddenly
expanding considerably as it passes into the solid state. It
boils, i.e. overcomes atmospheric pressure and is vaporized, at
100 degrees (760 mm pressure). If the pressure is greater, the
boiling-point is raised, i.e. it takes a higher temperature to
overcome a greater pressure. If there be less pressure, as on a
mountain, the boiling-point is lowered below 100 degrees. Salts
dissolved in water raise its boiling-point, and lower its
freezing-point to an extent depending on the kind and quantity of
the salt. Water, however, evaporates at all temperatures, even
from ice.

Pure water has no taste or smell, and, in small quantities, no
color. It is rarely if ever found on the earth. What is taken up
by the air in evaporation is nearly pure; but when it falls as
rain or snow, impurities are absorbed from the atmosphere. Water
falling after a long rain, especially in the country, is
tolerably free from impurities. Some springs have also nearly
pure water; but to separate all foreign matter from it, water
must be distilled. Even then it is liable to contain traces of
ammonia, or some other substance which vaporizes at a lower
temperature than water.

138. Sea-Water.--The ocean is the ultimate source of all water.
From it and from lakes, rivers, and soils, water is taken into
the atmosphere, falls as rain or snow, and sinks into the ground,
reappearing in springs, or flowing off in brooks and rivers to
the ocean or inland seas. Ocean water must naturally contain
soluble salts; and many salts which are not soluble in pure water
are dissolved in sea-water. In fact, there is a probability that
all elements exist to some extent in sea-water, but many of them
in extremely minute quantities. Sodium and magnesium salts are
the two most abundant, and the bitter taste is due to MgSO4 and
MgCl2. A liter of sea- water, nearly 1000 g., holds over 37 g. of
various salts, 29 of which are NaCl. See Hard Water.

139. River Water.--River water holds fewer salts, but has a great
deal of organic matter, living and dead, derived from the regions
through which it flows. To render this harmless for drinking,
such water should be boiled, or filtered through unglazed
porcelain. Carbon filters are now thought to possess but little
virtue for separating harmful germs.

140. Spring Water.--The water of springs varies as widely in
composition as do the rocks whence it bubbles forth. Sulphur
springs contain much H2S; many geysers hold SiO2 in solution;
chalybeate waters have compounds of Fe; others have Na2SO4, MgSO4
NaCl, etc.