What’s Hot and What’s Not?
Introduction: This year’s contest explores the world of chemistry by studying the energy changes when
a solid dissolves in a substance or when two substances react.
Physical changes involve no permanent changes in the materials which are mixed. Dissolving is a
physical change only because by removing the water from the solution, the original substance can be
obtained, and it will be unchanged. For example, when a teaspoon of sugar is stirred into a cup of hot
coffee, the sugar seems to disappear; but it is still present as sugar and can be detected by the sweet
taste of the mixture. Changes in energy can be detected by measuring the change in temperature
during the dissolving process. If the temperature of the dissolving liquid goes down, the process is
called an ENDOTHERMIC process. Heat is removed from the liquid surroundings and the temperature
decreases. ( Endo = take in). If the temperature of the dissolving liquid goes up, the process is called an
EXOTHERMIC process. Heat is given off to the liquid surroundings and the temperature increases. ( Exo =
give off).
Chemical changes also can result in temperature changes for the substances involved. So there are both
endothermic and exothermic chemical changes as well. However, during a chemical change there is
evidence of a change in the substances. Besides a change in temperature, there will always be new
substances produced. Many times these substances are in a different form from those which reacted.
New substances such as gases, liquids or solids may form. There may be a change in color. In some
cases, unlike physical changes, it is difficult to reverse the process to get the original substance(s) back.
Some examples of chemical change which occur around the house include baking a cake, cooking an
egg, and toasting bread. In the case of baking a cake, the bubbles which are formed causing the cake to
rise are due to carbon dioxide gas being formed from a chemical reaction with the substances within the
cake. In cooking an egg, the proteins are changed resulting in the slimy slippery egg white being
transformed into a solid white substance. In toasting bread, the outside of the bread is burned in a
chemical reaction giving a bit of carbon on the outside surface.
In this investigation, common household substances will be mixed in a systematic order and the
temperature measured. The system will be observed for evidence of chemical change. The students
will need to be able to define the following terms: solvent, solute, concentrated, dilute
Experimental Design:
The students will be expected to design an investigation to determine whether a change is a physical
change or chemical change, and whether a change is endothermic or exothermic.
Materials:
Thermometers
Graduated cylinders, 100 mL (Teacher can calibrate a plastic cup to allow measurements to proceed
faster)
Balance, electronic preferred
Styrofoam cups
Plastic or paper cups to serve as weighing dishes
Plastic spoons
Large, plastic margarine containers
Paper towels
Charts to record data
Household substances: vinegar, baking soda (NaHCO3), washing soda (Na2CO3), yeast- bread machine or
little packets, lemon juice (large bottle), Epsom salts** (MgSO4-7H2O), both hydrated and dried, citric
acid (powdered), 3% hydrogen peroxide (H2O2), tap water at room temperature. ** Teacher preparation
required for dried Epsom salts: Place approximately 100 g of the Epsom salts as purchased in the
bottom of an aluminum pie pan and heat for about 30 minutes to dry it out. Depending on how many
students and trials you intend to perform, you may need several pans of the dried material. Allow to
cool and store in an air-tight container.
Procedure:
1. Carefully measure 100 mL of each liquid and place in a clean dry Styrofoam cup. Place the cup in
the margarine container to catch possible spills/run overs.
2. Measure and record the amount of each substance to be tested. For baking soda, washing soda
and Epsom salts the mass should be around 10 grams.
3. For the yeast, only about 1.0 grams is required.
4. For the citric acid, a solution should be prepared prior to adding the solid. Place 3.00 g of citric
acid in the 100.0 mL of water; then add the solid.
5. Measure and record the temperature of the liquid in the cup.
6. Pour the measured solid into the liquid, and stir.
7. Record the highest or lowest temperature observed.
8. Repeat if necessary.
Liquid
Volume
Solid
Water
100 mL
Baking soda
Vinegar
100 mL
Baking soda
Lemon Juice
100 mL
Baking soda
Water
100 mL
Vinegar
100 mL
Lemon Juice
100 mL
Water
100 mL
Water
100 mL
Citric acid
solution
Citric acid
solution
Hydrogen
peroxide
Water
100 mL
Washing
soda
Washing
soda
Washing
soda
Epsom salts
(dried)
Epsom salts
(not dried)
Baking soda
100 mL
100 mL
Washing
soda
yeast
100 mL
Sugar
Vinegar
100 mL
Lemon Juice
100 mL
Vinegar
100 mL
Epsom salts
(dried)
Epsom salts
(dried)
Sugar
Mass
Initial
Final
Temperature
Temperature Temperature Change
Analyze the data you obtained, and construct a table listing the changes which were ENDOTHERMIC,
which were EXOTHERMIC.
In a different table, list the chemical changes and physical changes and an explanation of how that was
determined.
Supplies for Science Bowl 2017
Don’t use the Borax
Set up for the experiment: Place Styrofoam cup inside another container like a
margarine container to catch spills and run-overs.
Epsom salts (Magnesium sulfate, hydrated) must be heated in a low oven (275F)
for at least 30 minutes to drive off the waters of hydration. Do this by placing the
Epsom salts in a disposable aluminum container (pie plate). Put enough to cover
the bottom of the pan. When dried and cooled, store in an air tight container
(labeled) until ready to use.