Exothermic and Endothermic Reactions: Hot Packs and Cold Packs

By — John Wiley & Sons, Inc.
Updated on Dec 13, 2010

All reactions, whether chemical or physical, involve energy. Exothermic reactions release more energy than they absorb. These reactions can often be identified because they cause an increase in temperature. Endothermic reactions absorb more energy than they release, so they cause temperatures to drop. In this activity you will compare an endothermic reaction to an exothermic reaction.


Safety goggles; Resealable plastic bag of calcium chloride; Resealable plastic bag of ammonium nitrate; Water; Graduated cylinder

Prepare a bag of calcium chloride by placing 3 tablespoons of calcium chloride powder in a small plastic bag and sealing it.  Label the bag as calcium chloride.  Also, prepare a bag of ammonium nitrate by placing 3 tablespoons of ammonium nitrate in a small plastic bag, sealing it, and labeling each of these bags as ammonium nitrate.


  1. Put on your safety goggles.
  2. Open the bag containing powdered calcium chloride. Using the graduated cylinder, carefully add 65ml of water to the bag.
  3. Seal the bag. Use your hands to knead the water and calcium chloride until all of the solid dissolves.
  4. Can you feel any difference in temperature on the outside of the bag as the dissolving takes place? Set this bag to the side.
  5. Open the bag containing solid ammonium nitrate. Carefully add 65ml of water to the bag.
  6. Repeat steps 3 and 4 with this bag.

Follow-Up Questions

  1. Which bag would make the better hot pack? Which bag would make the better cold pack?
  2. In which bag did an endothermic reaction occur? In which bag did an exothermic reaction occur?
  3. Do you know for sure whether these reactions were physical or chemical? Why?


  1. Calcium chloride makes a good hot pack, and ammonium nitrate makes a good cold pack.
  2. Calcium chloride produced an exothermic reaction, and ammonium nitrate produced an endothermic reaction.
  3. You can't tell just from the results of this experiment, because all physical and chemical reactions involve energy.


Repeat steps 1 through 3 substituting three tablespoons of potassium bromide for the calcium chloride. Would this bag make a hot pack or a cold pack? Was the reaction of potassium bromide with water endothermic or exothermic?

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