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    Exothermic Rehydration of Gypsum
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    This experiment is courtesy of 

    Exploring Exothermic Rehydration of Gypsum


    Sandra Sweeney
    Perkiomen Valley Schools
    Graterford, PA

    Kathleen Koziski
    Research Scientist
    Rohm and Haas Company


    Grade Level:

    6 through 9



    Earth and Space, General Science



    Upon completion of this lesson, the student will:

    1. Design an experiment as a "Fair Test".
    2. Identify quantitative variables.
    3. Recognize uncontrolled variables and the need to minimize them.



    Upon completion of this lesson, the student will:

    1. Demonstrate that "setting" of plaster and cement is a change, which involves a release of heat energy.
    2. Use data collected experimentally to construct comparison graphs.
    3. Analyze the graphs to make predictions about the outcome of future experiments.



    Gypsum 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.

    A 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.



    For lab groups of four students:

    • safety glasses for each person, aprons if possible
    • Plaster of Paris, 300grams/team
    • balance
    • Styrofoam cups 4/team
    • 4 thermometers
    • disposable 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)
    • wide wooden tongue depressors for stirring
    • graduated cylinder
    • ruler
    • aluminum foil or cardboard covers for cups
    • tap water
    • clock
    • colored pencils



    Remember, to be a "fair test" everything must stay constant except the proportion of water to plaster, which you will establish now.

    1. Everyone must wear safety glasses.
    2. Use the balance to obtain the tare mass of the empty cup.
    3. Add 60 grams of plaster to one cup and label it 60. Then do the same for a 70g, 80g, and 90g cup
    4. Prepare 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?
    5. 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?
    6. Tap 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?
    7. Place a thermometer, PROTECTED BY ITS JACKET, into each cup and record the initial temperature.
    8. Cover the top of the cups with foil or cardboard and support the thermometers against a pile of books if necessary.
    9. Set 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.)

    10. TIME

      CUP 60

      CUP 70

      CUP 80

      CUP 90









    11. Construct 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.

    Carefully 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.



    By 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.

    How long was the first stage for each cup?

    Which cup produced the most heat?

    Which 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.

    Which mixture cooled fastest? Was the heating and cooling rate for a given cup the same?

    In 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.

    Plaster 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.

    Going further:

    While your team is timing the reaction of the Plaster of Paris, you might want to examine some other exothermic reactions.

    In 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 and after.

    Place several ice cubes with a small amount of water into another beaker or cup and record the temperature as the ice melts.

    In 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 in pH.

    Some 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 

    My Dog Kelly

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    Last updated: June 2013
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