Gateway to Science presents
“Chemical Reactions”
Producing Changes in Heat Energy
All chemical reactions involve energy. Most chemical reactions involve heat energy. Some chemical reactions
produce excess energy that is absorbed by the surroundings causing the temperature to increase.
Hot Pack
An example of a reaction that produces a lot of extra heat energy is the burning of natural gas (or propane)
to heat your home. The chemical change that takes place produces excess energy, causing the temperature
to increase. Another example is the chemical change used in the product called “Hot Pack”. Hot Packs have
two different chemicals in separate compartments within a plastic bag. When you squeeze the plastic bag
the membrane separating the two chemicals breaks, allowing the chemicals to come in contact with each
other.
Materials:
Ziploc Bag (heavy duty) – Water – Calcium Chloride (sidewalk salt)
Experiment:
1. Keeping the chemicals separated on opposite sides of a Ziploc bag, add 1 tbsp. of water to one side
and 1 tsp. Calcium chloride (sidewalk salt) to the other side.
2. Seal bag tightly while squeezing out any excess air.
3. Allow the water and Calcium chloride to mix together. This should produce a very warm mixture or a
Hot Pack.
Try This: Add a little acid-base indicator (on back of this page) to the water before mixing it with the
calcium chloride. The calcium chloride makes the solution somewhat acidic resulting in a color change.
Cold Pack
Some chemical changes absorb excess heat energy from the surroundings causing the temperature to
decrease. Some items in your home that depend on these chemical changes are refrigerators, air
conditioners, and Cold Packs. Cold Packs are packaged in the same manner as Hot Packs. That is, two
different chemicals are packaged in separate compartments within a plastic bag. When the two chemicals
come in contact with each other, a chemical change takes place that requires heat energy, which is absorbed
from the surroundings causing a temperature to decrease.
Materials:
Ziploc Bag (freezer, quart size) – Water – Alum (used in pickling)
Experiment:
1. Keeping the chemicals separated on opposite sides of a Ziploc bag, add 1 tbsp. of water to one side
of the bag and 1 tbsp. of Alum to the other side of the bag.
2. Seal bag tightly while squeezing out any excess air.
3. Allow the water and Alum to mix together. The mixture should get quite cold. (Note: The actual
chemical used in Instant Cold Packs is Ammonium nitrate, but Alum is more readily available for this
experiment.)
Gateway to Science, 1810 Schafer Street, Suite 1, Bismarck, ND 58501-1218
(701) 258-1975
gscience@gscience.org
www.gscience.org