If a spoonful of salt is put into a tumbler of cold water and the water
is stirred, the salt swiftly vanishes from view; and, after a time, so
far as our sense of vision goes, the water appears to be just what it
was before. But if the water in the tumbler at first weighed five ounces
and the salt weighed two ounces, the water in the tumbler will now weigh
seven ounces; the water will now taste salt, the salt is said to be
=dissolved=, and the =solution= is called =brine=. Moreover, the
solution is said to be =saturated=, for if you put more salt in it will
remain unchanged. Water, in fact, will dissolve two-fifths of its weight
of salt and no more. If the brine thus formed is put into a wide dish,
so that the water may evaporate; or if it is heated and the water boiled
away; as fast as the water diminishes, a quantity of salt, equal to
two-fifths of the water which is converted into steam, returns to the
solid state and falls to the bottom of the vessel. And when all the
water is driven off, the salt which remains will have exactly the
weight, and all the other properties which it had before it was
dissolved by the water.
Thus, contact with water has had a very singular effect upon the salt.
It appears to have changed one of the properties of the salt, namely,
its =solidity=, but to have left all the rest unaltered. We saw just now
that powdered ice does not mix with ice-cold water, but that the
fragments of ice remain solid. The moment, however, that the temperature
rises, the =cohesion=, or sticking together of the molecules, which is
the characteristic of the solid state, comes to an end; they become
loose and free to move, and they mingle with the surrounding water. Or
we may say that the ties which held the molecules of the solid together
are dissolved, so that the solid water becomes fluid.
The resemblance of this process to the dissolving of salt in water is so
obvious that, in common language, it is often said that a lump of salt
or of sugar =melts= away in water; but if you try to make salt fluid by
heat, you will have to expose it to a very high temperature, so that the
conversion of salt from the solid state into the liquid state by
solution in cold water is obviously a very different process from
liquefaction by heat. Nevertheless the result is the same so far as the
condition of the salt is concerned. The cohesion between its molecules
is destroyed, and they distribute themselves evenly among the molecules
of the water, just as the molecules of steam distribute themselves among
the molecules of air. And, when you study chemistry, you will learn how
it may be proved that the smallest drop of the solution of salt contains
exactly the same proportion of salt as the whole does.
If brine is allowed to evaporate slowly, the molecules of the salt
arrange themselves, as the water leaves them, in beautifully regular
cubical crystals. You may see them form easily enough if you watch a
drop of brine gradually dry up under a microscope. The salt crystals
contain nothing but salt. If they are heated till they become red-hot
they pass into the fluid state; and when still further heated, the fluid
salt becomes a vapour or gas and, as such, flies off into the air, or
=volatilizes=.
Thus we see that when salt and water are brought into contact, the salt
undergoes a certain amount of change, while the water does not remain
wholly unchanged. For brine no longer boils at 212°, but requires a
considerably higher temperature. The salt, as it were, holds the water
back, and prevents it from assuming the gaseous state under the same
conditions as if it were pure, just as, in the previous case, the water
held the alcohol back; or we may say that the force of heat which drives
the molecules of liquid water apart, when steam is formed, has a greater
resistance to overcome when salt is dissolved in the water. And just as
the presence of alcohol lowers the freezing point of the water with
which it is mixed, so does the presence of salt lower the freezing point
of water. Sea water, which is a weak brine, begins to freeze at about
27°; and the ice which is formed is quite pure, while the remainder of
the sea water becomes richer in salt.
If we mean by attraction that which opposes any force which tends to
separate bodies, then we may say that the molecules of salt and those of
water attract one another. And such attraction between molecules of
matter of different kinds is called =chemical attraction=.