Exothermic Rehydration of Gypsum
Rohm and Haas Company
and Space, General Science
completion of this lesson, the student will:
an experiment as a "Fair Test".
uncontrolled variables and the need to minimize them.
completion of this lesson, the student will:
that "setting" of plaster and cement is
a change, which involves a release of heat energy.
data collected experimentally to construct comparison
the graphs to make predictions about the outcome of
is widely distributed in the earth's crust. Only in
volcanic regions is gypsum completely absent. Gypsum
is useful as an industrial material because it readily
loses its water of hydration when it is heated and regains
it again as set, hardened plaster. Plaster and cement
have been used since ancient times, long before the
Greeks. Gypsum is a form of powdered calcium sulfate,
which incorporates two molecules of water as it, hardens
to make plasters. When the water is included into the
calcium sulfate molecule, heat is produced, and a new
molecule is formed. Addition of other minerals such
as limestone and feldspar or other materials such as
starch or acids can change the rate of the reaction.
Varying amounts of water available can also change the
reaction rate. Portland cement which is a powder made
from a heated mixture of limestone, clay and gypsum
also goes through an exothermic reaction as it rehydrates
during the hardening process.
reaction which produces heat is called Exothermic.
This release of heat may indicate either a physical
or chemical change. The container will feel warm. Scientists
try to design experiments which can compare quantitative
(using numbers) data. In this experiment you will compare
the amount of water you add to the original mixture
with the amount of heat produced.
lab groups of four students:
glasses for each person, aprons if possible
of Paris, 300grams/team
plastic pipettes or large drinking straws or plastic
baggies and tape( to be used to make a jacket for
the thermometers before you place them into the plaster)
wooden tongue depressors for stirring
foil or cardboard covers for cups
to be a "fair test" everything must stay constant
except the proportion of water to plaster, which you
will establish now.
Everyone must wear safety glasses.
Use the balance to obtain the tare mass of
the empty cup.
Add 60 grams of plaster to one cup and label it 60.
Then do the same for a 70g, 80g, and 90g cup
a jacket for each thermometer by cutting the bulb
and tip off of a plastic
pipette or adjusting the length of a drinking straw.
Another possibility is to make a
tubular plastic bag by cutting and taping a plastic
baggie. Important: the
thermometer must not be in direct contact with the
setting plaster or it will be
impossible to remove it later! All four thermometers
must have the same size and
type of jacket. Why?
Add 40 ml of water to each cup and stir to mix it
completely. Use a different stick for
each cup. Why is it important that each water sample
be the same temperature?
the cups on the table to remove air bubbles. Then
mark the exact height of the
plaster in the 60 cup. The volumes will not
be equal. It is necessary to remove some
of the mixtures in the other cups so that their volumes
are the same as the 60. Why?
a thermometer, PROTECTED BY ITS JACKET, into
each cup and record the
the top of the cups with foil or cardboard and support
the thermometers against a pile of books if necessary.
up a table in your notebook and record the temperature
in each cup every five minutes.
(Teacher: You may want to have two classes collect
and share this data. Keep
recording data for ninety minutes or until the cups
return to their original temperature.)
a graph from the data. Mark the horizontal (X) axis
with the five-minute time intervals you established
on the data table. Mark the vertical (Y) axis with
a temperature range which begins slightly below your
start temperature and continues up to slightly higher
than the highest temperature you reach. Use a separate
colored pencil to represent each cup.
remove the thermometers from the cups as soon as you
stop recording data. The thermometer should slide up
out of its jacket, which will remain fixed in the plaster.
If the thermometer does not come out easily, gently
try to rotate it slightly in its jacket. Pour a little
water down inside the jacket. Ask your teacher to remove
the thermometer if you have difficulty.
looking at the graph, you should see that there are
three distinct stages involved in the hardening of the
plaster. In the first stage calcium sulfate or gypsum
becomes covered with a gel as water combines with it.
In the second stage calcium sulfate gel becomes calcium
sulfate dihydrate crystal by a process called hydration.
The gel is replaced with interlocking needle shaped
crystals. The plaster sets due to the interlocking of
these crystals. In the last stage there is a deceleration
of chemical hydration, which gradually declines until
it is no longer detectable and the maximum number of
crystals has been formed.
long was the first stage for each cup?
cup produced the most heat?
mixture heated up the fastest? To find the rate of change
look at the steepness of the line on your graph. Scientists
like to use numbers to describe this steepness, called
the slope of the line. Beginning with the second
stage of the reaction, on the graph mark the lowest
and highest temperatures for each cup. For a particular
cup the rise is the difference between the highest
and lowest temperature. Calculate the run by
determining how many minutes it took to get from lowest
to highest or vice versa. The slope is the rise
divided by the run. The larger the slope, the
faster the rate.
mixture cooled fastest? Was the heating and cooling
rate for a given cup the same?
this experiment you kept the amount of water constant
and changed the amount of plaster. Would you get the
same result is you kept the plaster constant and changed
the amount of water? Water can absorb a lot of heat
before it changes temperature.
of Paris is a mixture of gypsum and limestone. Compare
the heat of hydration for Patching Plaster, Spackling
Compound, or Portland Cement to the Plaster of Paris.
To make it a fair test, be sure to use the same proportion
of water to powder for all of them.
your team is timing the reaction of the Plaster of Paris,
you might want to examine some other exothermic reactions.
a 50-ml beaker pour a dilute solution of copper sulfate
over a small wad of very fine steel wool. Record your
observations in your notebook. Observe the temperature
change with another thermometer. Record the mass before
several ice cubes with a small amount of water into
another beaker or cup and record the temperature as
the ice melts.
another cup add powdered detergent to water while recording
the temperature. This is even more dramatic if you put
a little red cabbage juice or some turmeric (a spice)
in the water before you add the detergent. The juice
or spice will change color, which indicates a change
exothermic reactions involve chemical changes, which
produce new products with new chemical properties, and
some exothermic reactions are evidence of a physical
change, which is easily reversible. Discuss which of
the changes you investigated today are physical and
which are chemical changes. Are some of them both?
This experiment is courtesy of