Why is fire hot?
What makes glowworms glow?
Why does cold quicklime boil when you pour cold water on it?
If no energy were released by chemical change, we should run down like
clocks, and could never be wound up again. We could breathe, but to do
so would do us no more good than it would if oxygen could not combine
with things. Oxidation would go on in our bodies, but it would neither
keep us warm nor help us to move. A few spasmodic jerks of our hearts,
a few gasps with our lungs, and they would stop, as the muscles would
have no energy to keep them going.
The sunlight _might_ continue to warm the earth, as we are not sure
that the sun gets any of its heat from chemical change. But fires,
while they would burn for an instant, would be absolutely cold; no
energy would be given out by the fuel combining with oxygen. But the
fires could not burn long, because there would be nothing to keep the
gases and fuel hot enough to make them combine with the oxygen.
Even during the instant that a fire lasted it would be invisible, for
it would give off no light if no energy were released by the chemical
change. Only electric lights and heaters would continue to work, and
even some of these would fail. The electric motors in submarines and
electric automobiles would instantly stop; battery flashlights would
go out as quickly as the fire; no doorbells would ring. In short, all
forms of electric batteries would stop sending currents of electricity
out through their wires, and everything depending upon batteries would
stop running.
A fire gives out heat and light; both are kinds of energy. And it is
the electric energy caused by the chemical change in batteries that
runs submarines, electric automobiles, flashlights, and doorbells.
Since burning (oxidation) is simply a form of chemical change, it is
not difficult to realize that chemical change releases energy.
WHY GLOWWORMS GLOW. When a glowworm glows at night, or when the head
of a match glows as you rub it on your wet hand in the dark, we
call the light _phosphorescence_. The name "phosphorus" means
light-bearing, and anything like the element phosphorus, that glows
without actively burning, is said to be phosphorescent. Match heads
have phosphorus in them. Phosphorescence is almost always caused
by chemical change. The energy released is a dim light, not heat or
electricity. Sometimes millions of microscopic sea animals make
the sea water in warm regions phosphorescent. They, like fireflies,
glowworms, and will-o'-the-wisps, have in them some substance that is
slowly changing chemically, and energy is released in the form of dim
light as the change takes place. Most luminous paint is phosphorescent
for the same reason,--there is a chemical change going on that
releases energy in the form of light.
When you poured the hydrochloric acid on the zinc to make hydrogen,
the flask became warm; the chemical change going on in the flask
released heat energy.
_APPLICATION 76._ Explain why pouring cold water on cold
quicklime makes the slaked lime that results boiling hot; why
a cat's eyes shine in the dark; why a piece of carbon and a
piece of zinc placed in a solution of sal ammoniac will make
electricity run through the wire that connects them; why fire
is hot.
INFERENCE EXERCISE
Explain the following:
481. A baking potato sometimes bursts in the oven.
482. Turpentine is used in mixing paint.
483. Sodium is a metal; chlorine is a poisonous gas; yet salt,
which is made up of these two, is a harmless food.
484. When bricklayers mix water with cement and lime, the
resulting mortar boils and steams.
485. Green plants will not grow in the dark.
486. Parts of the body are constantly uniting with oxygen.
This keeps the body warm.
487. Water will not always put out a kerosene fire.
488. Delicately colored fabrics should be hung in the shade to
dry.
489. A match glows when you rub it in the dark.
490. Candy hardens when it cools.