How does soap make your hands clean?
Why will gasoline take a grease spot out of your clothes?
If we were to go back to our convenient imaginary switchboard to turn
off another law, we should find near the heat switches, and not far
from the chemistry ones, a switch labeled SOLUTION. Suppose we turned
it off:
The fishes in the sea are among the first creatures to be surprised
by our action. For instantly all the salt in the ocean drops to the
bottom like so much sand, and most salt-water fishes soon perish in
the fresh water.
If some one is about to drink a cup of tea and has sweetened it just
to his taste, you can imagine his amazement when, bringing it to his
lips, he finds himself drinking tasteless, white, milky water. Down
in the bottom of the cup is a sediment of sugar, like so much fine
gravel, with a brownish dust of tea covering it.
To see whether or not the trouble is with the sugar itself, he may
take some sugar out of the bowl and taste it,--it is just like white
sand. Wondering what has happened, and whether he or the sugar is
at fault, he reaches for the vinegar cruet. The vinegar is no longer
clear, but is a colorless liquid with tiny specks of brown floating
about in it. Tasting it, he thinks it must be dusty water. Salt,
pepper, mustard, onions, or anything he eats, is absolutely tasteless,
although some of the things _smell_ as strong as ever.
To tell the truth, I doubt if the man has a chance to do all of this
experimenting. For the salt in his blood turns to solid hard grains,
and the dissolved food in the blood turns to dustlike particles. His
blood flows through him, a muddy stream of sterile water. The cells of
his body get no food, and even before they miss the food, most of the
cells shrivel to drops of muddy water. The whole man collapses.
Plants are as badly off. The life-giving sap turns to water with
specks of the one-time nourishment floating uselessly through it. Most
plant cells, like the cells in the man, turn to water, with fibers
and dust flecks making it cloudy. Within a few seconds there is not
a living thing left in the world, and the saltless waves dash up on a
barren shore.
Probably we had better let the SOLUTION switch alone, after all.
Instead, here are a couple of experiments that will help to make clear
what happens when anything dissolves to make a _solution_.
EXPERIMENT 80. Fill a test tube one fourth full of cold water.
Slowly stir in salt until no more will dissolve. Add half a
teaspoonful more of salt than will dissolve. Dry the outside
of the test tube and heat the salty water over the Bunsen
burner. Will hot water dissolve things more readily or less
readily than cold? Why do you wash dishes in hot water?
EXPERIMENT 81. Fill a test tube one fourth full of any kind
of oil, and one fourth full of water. Hold your thumb over the
top of the test tube and shake it hard for a minute or two.
Now look at it. Pour it out, and shake some prepared cleanser
into the test tube, adding a little more water. Shake the test
tube thoroughly and rinse. Put it away clean.
When you shake the oil with the water, the oil breaks up into tiny
droplets. These droplets are so small that they reflect the light that
strikes them and so look white, or pale yellow. This milky mixture is
called an _emulsion_. Milk is an emulsion; there are tiny droplets of
butter fat and other substances scattered all through the milk. The
butter fat is _not_ dissolved in the rest of the milk, and the oil is
_not_ dissolved in the water. But the droplets may be so small that an
emulsion acts almost exactly like a solution.
[Illustration: FIG. 146. Will heating the water make more salt
dissolve?]
But when you shake or stir salt or sugar in water, the particles
divide up into smaller and smaller pieces, until probably each piece
is just a single molecule of the salt or sugar. And these molecules
get into the spaces between the water molecules and bounce around
among them. They therefore act like the water and let the light
through. This is a solution. The salt or sugar is _dissolved_ in the
water. Any liquid mixture which remains clear is a solution, no matter
what the color. Most red ink, most blueing, clear coffee, tea, and
ocean water are solutions. If a liquid is _clear_, no matter what
the color, you can be sure that whatever things may be in it are
dissolved.
[Illustration: FIG. 147. Will the volume be doubled when the alcohol
and water are poured together?]
EXPERIMENT 82. Pour alcohol into a test tube (square-bottomed
test tubes are best for this experiment), standing the tube
up beside a ruler. When the alcohol is just 1 inch high in the
tube, stop pouring. Put exactly the same amount of water
in another test tube of the same size. When you pour them
together, how many inches high do you think the mixture will
be? Pour the water into the alcohol, shake the mixture a
little, and measure to see how high it comes in the test tube.
Did you notice the warmth when you shook the tube?
If you use denatured alcohol, you are likely to have an
emulsion as a result of the mixing. The _alcohol_ part of the
denatured alcohol dissolves in the water well enough, but the
_denaturing_ substance _in_ the alcohol will not dissolve
in water; so it forms tiny droplets that make the mixture of
alcohol and water cloudy.
The purpose of this experiment is to show that the molecules of water
get into the spaces between the molecules of alcohol. It is as if you
were to add a pail of pebbles to a pail of apples. The pebbles would
fill in between the apples, and the mixture would not nearly fill two
pails.
The most important difference between a solution and an emulsion is
that the particles in an emulsion are very much larger than those in
a solution; but for practical purposes that often does not make
much difference. You dissolve a grease spot from your clothes with
gasoline; you make an emulsion when you take it off with soap and
water; but by either method you remove the spot. You dissolve part of
the coffee or tea in boiling water; you make an emulsion with cocoa;
but in both cases the flavor is distributed through the liquid. Milk
is an emulsion, vinegar is a solution; but in both, the particles
are so thoroughly mixed with the water that the flavor is the same
throughout. Therefore in working out inferences that are explained in
terms of solutions and emulsions, it is not especially important for
you to decide whether you have a solution or an emulsion if you know
that it is one or the other.
HOW PRECIOUS STONES ARE FORMED. Colored glass is made by dissolving
coloring matter in the glass while it is molten. Rubies, sapphires,
emeralds, topazes, and amethysts were colored in the same way, but by
nature. When the part of the earth where they are found was hot enough
to melt stone, the liquid ruby or sapphire or emerald, or whatever
the stone was to be, happened to be near some coloring matter that
dissolved in it and gave it color. Several of these stones are made
of exactly the same kind of material, but different kinds of coloring
matter dissolved in them when they were melted.
Many articles are much used chiefly because they are good
_emulsifiers_ or good _solvents_ (dissolve things well). Soap is a
first-rate emulsifier; water is the best solvent in the world; but it
will not dissolve oil and gummy things sufficiently to be of use when
we want them dissolved. Turpentine, alcohol, and gasoline find one of
their chief uses as solvents for gums and oils. Almost all cleaning is
simply a process of dissolving or emulsifying the dirt you want to get
rid of, and washing it away with the liquid. Do not forget that heat
helps to dissolve most things.
_APPLICATION 63._ Explain why clothes are washed in hot suds;
why sugar disappears in hot coffee or tea; why it does not
disappear as quickly in cold lemonade; why you cannot see
through milk as you can through water.
INFERENCE EXERCISE
Explain the following:
381. A kind of lamp bracket is made with a rubber cup. When
you press this cup against the wall or against a piece of
furniture and exhaust the air from the cup, the cup sticks
fast to the wall and supports the lamp bracket.
382. You can take a vaseline stain out with kerosene.
383. If the two poles of an electric battery are connected
with a copper wire, the battery soon becomes discharged.
384. Electric bells have iron bars wound around and around
with insulated copper wire.
385. Piano keys may be cleaned with alcohol.
386. Linemen working with live wires wear heavy rubber gloves.
387. Crayon will not write on the smooth, glazed parts of a
blackboard.
388. Varnish and shellac may be thinned with alcohol.
389. Filtering will take mud out of water, but it will not
remove salt.
390. Explain why only one wire is needed to telegraph between
two stations.