What is an exothermic reaction?
Learn about exothermic reactions with this simple investigation.
Setting: Indoors
Time: <30 minutes
Concepts: exothermic reactions; heat
Skills: observing, collecting data
Subject(s):
 Chemistry
 Physics
Ages:
 12-14
 15-17
Materials:
 Quick-rising dry yeast - 30 mL or 16 g
 Hydrogen peroxide solution - 60 mL (3% to 5%, found in most drug stores)
 Thermometer
 Small bowl
 Spoon
 Pencil and paper
 Measuring cup
 Measuring spoon
 Clock or stopwatch that indicates seconds
Safety First!
Always wear safety glasses when working with chemicals that could splash or spill
into your eyes.
What to do:
1. Record the room temperature and then place the thermometer in the bowl.
2. Predict what will happen to the temperature when the hydrogen peroxide and yeast are
combined.
3. Record your prediction on paper.
4. Pour 60 mL of hydrogen peroxide solution into the bowl and record the temperature.
5. Add 30 mL of yeast to the bowl and stir the mixture.
6. Record time and temperature readings every 30 seconds for a total of 4 minutes.
Photo/diagram/video –
https://www.youtube.com/watch?v=XKJ_OKV8yeE
What’s happening?
Hydrogen peroxide will naturally decompose into water and oxygen gas and a small amount of
heat energy. This decomposition reaction usually happens slowly. You can observe the speed
of the decomposition happening as bubbles form when the hydrogen peroxide is poured into a
container.
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What is an exothermic reaction?
When yeast is mixed in with the hydrogen peroxide, the yeast causes this decomposition
chemical reaction to occur much more quickly. The hydrogen peroxide changes into water and
oxygen gas much more quickly. The observed bubbles and foam is the oxygen gas being
formed very rapidly.
Because the reaction is occurring much more quickly than it normally would, the amount of heat
energy produced is much more noticeable. The increase in temperature can be determined by
examining the overall increase in temperature during the reaction. Any reaction that gives of
heat is called an exothermic reaction.
The yeast is not reacting with the hydrogen peroxide. Rather it is acting as a catalyst. A catalyst
is a substance that causes a chemical reaction to speed up but is not “used” up during the
reaction.
Why does it matter?
Exothermic reactions occur regularly and are important for everyday life. Specifically, all
combustion reactions (reactions with oxygen, such as burning fuel) are exothermic. Fire is one
of the most common and useful combustion reactions. Packaged heating pads for muscle
strains or for heating hands or feet in winter produce their heat from an exothermic reaction that
occurs when the chemicals inside the packet are combined by crushing the packet.
Investigate further!


A much more spectacular demonstration of this decomposition reaction can be seen by
adding a small squirt of liquid dishwashing detergent to the hydrogen peroxide before
adding the yeast. The dishwashing liquid will help trap the oxygen and will cause the
foaming action to be more dramatic. Warning: this can be messy! Add a couple of drops
of food coloring before adding the yeast for a more colourful result.
For an even more spectacular result, pour the hydrogen peroxide, soap and food
coloring mixture in a 2 litre pop bottle and mix. Dissolve the yeast in about 100 mL water
and then pour it into the bottle quickly. The resulting foam will be forced out through the
small opening of the bottle. Try repeating the experiment in different types of bottles to
observe the differences.
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