Krampf
Experiment of the Week - A Hot Change - 4/12/09
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Last
week (Experiment: A Cool Change), we took a brief
look at an endothermic process, seeing that
dissolving epsom salts in water lowered the temperature of the
water.
This week, we will examine a process that is exothermic, which means
that it gives off heat. To try this, you will need:
- a bowl
- a
thermometer
- 3%
hydrogen peroxide (from the grocery or pharmacy)
- yeast
- pencil and
paper
Pour
about 1/4 cup of hydrogen peroxide into the bowl. Find its
temperature
with the thermometer, and write it down, labeling it "Starting
temperature." Stick your finger into the liquid, and it should
feel
cool. Normally, you don't go around sticking your fingers into
chemistry experiments, but in this case, we know that hydrogen peroxide
is not harmful.
Sprinkle
about 1/4 of a teaspoon of yeast into the bowl, and give it a stir with
the thermometer. The mixture should start to foam, and its
temperature
should increase quite a bit. If you stick your finger into the
liquid,
it should be getting warm.
OK,
so why did it get so warm? Think back to last week's
experiment. We
said that whenever anything changed, energy is involved. Last
week,
the process of dissolving the epsom salts in water needed energy, so it
absorbed heat energy from the surrounding water, making it cooler.
This
time, we are looking at a different process. Hydrogen peroxide
has the
chemical formula H2O2, telling us that each molecule is made up of two
atoms of hydrogen and two atoms of oxygen. Looking at the
formula, it
looks very much like the formula for water, H2O, and it is. The
extra
oxygen atom is not very stable, so it is easy for it to be
released.
That is what formed the bubbles you saw. The gas inside was
oxygen.
(See Experiment Video
#60, Extra Oxygen to learn more.)
The
yeast contains an enzyme called catalase, cause the hydrogen peroxide
to break apart into water and oxygen gas. In the process, energy
that
was stored in the chemical bond is released as heat, causing the liquid
to get quite warm.
Any
time something changes; you need energy. Sometimes energy is
absorbed,
and sometimes it is given off, but energy is always involved in
change. That energy can be heat, as with these experiments, but
it can
also be other forms of energy, such as light, electricity, motion, etc.
Now
it is time for me to gain some energy and lose some at the same time,
by having a nice bowl of ice cream. The calories (stored chemical
energy) will warm me up, after the cold ice cream absorbs heat as I eat
it. What a wonder-filled way to study energy!