How is rock candy made?
Why is there sugar around the mouth of a syrup jug?
How are jewels formed in the earth?
You can learn how crystals are formed--and many gems and rock candy
and the sugar on a syrup jug are all crystals--by making some. Try
this experiment:
EXPERIMENT 83. Fill a test tube one fourth full of powdered
alum; cover the alum with boiling water; hold the tube over
a flame so that the mixture will boil gently; and slowly add
boiling-hot water until all of the alum is dissolved. Do not
add any more water than you have to, and keep stirring the
alum with a glass rod while you are adding the water. Pour
half of the solution into another test tube for the next
experiment. Hang a string in the first test tube so that
it touches the bottom of the tube. Set it aside to cool,
uncovered. The next day examine the string and the bottom of
the tube.
EXPERIMENT 84. While the solution of alum in the second test
tube (Experiment 83) is still hot, hold the tube in a pan of
cold water and shake or stir it until it cools. When white
specks appear in the clear solution, pour off as much of the
clear part of the liquid as you can; then pour a little of
the rest on a glass slide, and examine the specks under a
microscope.
[Illustration: FIG. 148. Alum crystals.]
In both of the above experiments, the hot water was able to dissolve
more of the alum than the cold water could possibly hold. So when the
water cooled it could no longer hold the alum in solution. Therefore
part of the alum turned to solid particles.
When the string was in the cooling liquid, it attracted the particles
of alum as they crystallized out of the solution. The force of
adhesion drew the near-by molecules to the string, then these drew the
next, and these drew more, and so on until the crystals were formed.
But when you kept stirring the liquid while it cooled, the crystals
never had time to grow large before they were jostled around to
some other part of the liquid or were broken by your stirring rod.
Therefore they were small instead of large. Stirring or shaking a
solution will always make crystals form more quickly, but it will also
make them smaller.
HOW ROCK CANDY IS MADE. Rock candy is made by hanging a string in a
strong sugar solution or syrup and letting the water evaporate slowly
until there is not enough water to hold all the sugar in solution.
Then the sugar crystals gather slowly around the string, forming the
large, clear pieces of rock candy. The sugar around the mouth of a
syrup jug is formed in the same way.
You always get crystallization when you make a liquid too cool to
hold the solid thing in solution, or when you evaporate so much of
the liquid that there is not enough left to keep the solid thing
dissolved.
When you make fudge, the sugar forms small crystals as the liquid
cools. When a boat has been on the ocean, salt crystals form on the
sails when the spray that has wet them evaporates.
But crystals may form also in the air. There is always some moisture
in the air, and when it becomes very cold, some of this moisture forms
crystals of ice. If they form up in the clouds, they fall as snow.
If they form around blades of grass or on the sidewalk, as the alum
crystals formed on the string, we have frost.
Still another place that crystals occur is in the earth. When the
rocks in the earth were hot enough to be melted and then began to
cool, certain substances in the rocks crystallized. Some of these
crystals that are especially hard and clear constitute precious and
semi-precious stones.
_APPLICATION 64._ Explain why you beat fudge as it cools; why
the paper around butter becomes encrusted with salt if it is
exposed to the air for some time.
INFERENCE EXERCISE
Explain the following:
391. Dynamos have _copper_ brushes to lead the current from
the coils of wire to the line wires.
392. A megaphone makes the voice carry farther than usual.
393. Copper wire is used to conduct electricity, although iron
wire costs much less.
394. A flute gives notes that differ in pitch according to the
stops that are opened.
395. There are usually solid pieces of sugar around the mouth
of a syrup jar.
396. You can beat eggs quickly with a Dover egg beater.
397. When ocean water stands in shallow open tanks for some
time, salt begins to form before the water has all evaporated.
398. In a coffee percolator the boiling water goes up through
a tube. As this water drips back through the ground coffee
beans, it becomes brown and flavored, and the coffee is made.
399. Kerosene will clean off the rim of soap and grease that
forms in bathtubs.
400. Beating cake frosting or candy causes it to sugar.