Demo: Energy in Hot and Cold Packs
FOR THE TEACHER
Summary
In this demonstration, students will observe temperature changes in chemical hot and cold packs and
discuss processes of endothermic and exothermic changes. They will also see that common household
products can be used to make a hot and cold pack.
Grade Level
Middle and high school
AP Chemistry Curriculum Framework
This demonstration supports the following unit, topic, and learning objective:
 Unit 6: Thermodynamics
o Topic 6.1: Endothermic and Exothermic Processes
 ENE-2.A: Explain the relationship between experimental observations and
energy changes associated with a chemical or physical transformation.
The extension lesson additionally supports the following unit, topic, and learning objective:
 Unit 6: Thermodynamics
o Topic 6.4: Heat Capacity and Calorimetry
 ENE-2.D: Calculate the heat q absorbed or released by a system undergoing
heating/ cooling based on the amount of the substance, the heat capacity, and the
change in temperature.
NGSS Alignment
This demonstration will help prepare your students to meet the performance expectations in the
following standards:
 HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal
energy when two components of different temperature are combined within a closed system
results in a more uniform energy distribution among the components in the system (second law
of thermodynamics).

Science and Engineering Practices: Plan and conduct an investigation individually and
collaboratively to produce data to serve as the basis for evidence, and in the design: decide on
types, how much, and accuracy of data needed to produce reliable measurements and consider
limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the
design accordingly.

Crosscutting Concepts:
o Systems and System Models: When investigating or describing a system, the
boundaries and initial conditions of the system need to be defined and their inputs and
outputs analyzed and described using models.
Objectives
By the end of this demonstration, students should be able to
 Better understand what happens in terms of energy when a substance dissolves.


Define endothermic and exothermic processes.
Classify a change as either endothermic or exothermic.
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Chemistry Topics
This demonstration supports students’ understanding of
 Energy/Thermodynamics
 Exothermic/endothermic processes
 Energy flow between a chemical system and the surroundings
Time
Teacher Preparation: 15 minutes
Lesson: 20 minutes
Materials
 Chemical Hot Pack (2)
 Chemical Cold Pack (2)
 Scissors
 Ice Melt with calcium chloride as the
main ingredient (25g)
 Salt Substitute with potassium chloride
as the main ingredient (50g)
 Snack size Ziploc bag (2)
 Quart size Ziploc bag (2)
 Water (tap water is fine)
 Electronic balance
 Scoopula
 Graduated cylinder (100ml)
 Temperature probe or thermometer
(optional)
 Insulated cups (4)
Safety
 Always wear safety goggles when handling chemicals in the lab.
 Students should wear proper safety gear during chemistry demonstrations. Safety goggles and
lab apron are required.
 Students should wash their hands thoroughly before leaving the lab if they have handled any
materials.
 Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
 Potassium Chloride SDS
 Calcium Chloride SDS
Teacher Notes
 You will only need two hot packs and two cold packs for this demonstration. Budget permitting,
you may want to get more so that the students can easily pass them around.
 The hot packs and cold packs can contain a variety of substances. Choose one that has a packet
of water and a solid chemical that is activated by squeezing the pack to break the water pouch so
that it mixes with the water.
 Teaching Points:
o For Hot Pack: When the hot pack is activated the dissolving process produces heat,
making it an exothermic process. When students touch the activated packet ask them
how the energy is flowing (from pack to hand, which is why hand feels warm).
o For Cold Pack: When the cold pack is activated the dissolving process absorbs heat,
making it an endothermic process. When students touch the activated packet ask them
how the energy is flowing (from hand to pack, which is why hand feels cool).
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


Before beginning the lesson, carefully cut open one of the hot and one of the cold packs. These
will be passed around for students to observe the contents
during the lesson. Carefully place them in a small plastic
container with a lid so that you can store them for future
lessons.
You only need to make one homemade hot pack and cold
pack. Budget permitting, you may want to get enough
materials for each student, or pair of students, to make a hot
pack and a cold pack.
For a more advance lesson you can collect data while
assembling the chemical hot and cold pack and use it to
calculate the enthalpy of solution for each of the chemicals used.
Lesson
1. Define or review the terms exothermic and endothermic process with the students.
2. Activate the hot pack and pass it around for students to touch and ask them to define how the
energy is flowing in terms of the pack and their hands. Ask students which type of process
(exothermic or endothermic) is happening in the pouch.
3. Show the students the opened hot pack so that they can observe the contents.
4. Activate the cold pack and pass it around for students to touch and ask them to define how the
energy is flowing in terms of the pack and their hands. Ask students which type of process
(exothermic or endothermic) is happening in the pouch.
5. Show the students the opened cold pack so that they can observe the contents.
6. Show the students how they can make a chemical hot pack at
home:
a. Put 200ml of tap water into a snack size Ziploc bag. Seal
the bag, pressing out a much of the air as possible.
b. Mass approximately 25g of Ice Melt and add it to the
quart size Ziploc bag.
c. Put the sealed bag of water into the quart Ziploc bag with
the Ice Melt.
d. Carefully seal the quart bag.
e. To activate the bag press down on the inner bag of water until it pops open. Shake the
contents and feel the bag.
7. Show the students how they can make a chemical cold pack at home:
a. Put 200ml of tap water into a snack size Ziploc bag. Seal
the bag, pressing out a much of the air as possible.
b. Mass approximately 50g of Salt Substitute and add it to
the quart size Ziploc bag.
c. Put the sealed bag of water into the quart Ziploc bag with
the Salt Substitute.
d. Carefully seal the quart bag.
e. To activate the bag press down on the inner bag of water
until it pops open. Shake the contents and feel the bag.
8. Disposal: Dispose of the resulting solutions in accordance with the Safety Data Sheet (SDS).
Extension Lesson
1. Collect data while making the hot and/or cold packs to allow students to do calculations.
2. Pour the water from the bag into two, stacked insulated cups. Record mass of water.
3. Use a temperature probe or thermometer to record the initial temperature of the water.
4. Record mass of the solid chemical and pour it into the insulated cup of water, stirring with the
temperature probe.
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5. Record the highest or lowest temperature reached.
6. Assume that the heat capacity of the solution is 4.18 J/g °C
7. For mass to mole conversions, assume that the Ice Melt is completely calcium chloride and the
Salt Substitute is completely potassium chloride.
8. Calculate the heat absorbed by the resulting solution and the enthalpy of solution for the solid
using the following equation: q = mCΔT .
q = heat (J)
m = total mass (g) of water and solid
C = heat capacity (J/g °C)
ΔT = temperature change (°C)
9. Divide the value of q for the dissolving process by the moles of solid involved to get the enthalpy
of solution. Note: qsolution and ΔHdissolving have opposite signs.
a. For an exothermic dissolving process the value of heat (q) for the solution will be positive
and the enthalpy of solution for the chemical will be negative.
b. For an endothermic dissolving process the value of heat (q) for the solution will be
negative and the enthalpy of solution for the chemical will be positive.
10. Common misconception: For physical changes, a temperature increase indicates an exothermic
change and a temperature decrease indicates an endothermic change. Many students try to
infer the same for chemical changes. Focus students on the flow of energy between the reaction
system and the surroundings for chemical changes.
a. When a reaction is exothermic it produces energy it releases it to the surroundings and
the surroundings will feel heat from the addition of energy.
b. When a reaction is endothermic it absorbs energy from the surrounds and the
surroundings will feel cold from the loss of energy.
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