Exothermic and Endothermic Reactions: Hot and Cold Packs


A small amount of water is added to zip-loc baggies containing either calcium chloride dihydrate or ammonium chloride. The resultant solution becomes hot or cold respectively. The bags are passed around the room so that students can directly sense the temperature changes. 

Curriculum Notes 

This demonstration is usually used when discussing thermochemistry (exothermic and endothermic reactions) and enthalpies of solution. It is also a great introduction to the concept of entropy, spontaneity, and Gibbs free energy. The number of hot and cold packs used can be varied according to the size of the class. This demo takes a little less than ten minutes.

One day of lead time is required for this project.


The exothermic solution of calcium chloride dihydrate is intuitive to most students. The attractions of the calcium ion and the chloride ion for water molecules are greater than their attraction for each other, resulting in solution and the release of energy. The endothermic solution of ammonium chloride is less intuitive because entropy must be invoked. The ammonium ion and the chloride ion have greater attraction for each other than they have for water molecules, hence the process must absorb energy to proceed. What drives the process then? The dissolved state is much more disordered and random at the molecular level - the entropy is greater. The entropy increase is great enough to make the process proceed spontaneously, even at a net energy "cost." 


  • Sandwich-sized zip-loc baggies, half of which contain about a tablespoon of calcium chloride dihydrate, the other half containing the same volume of ammonium chloride.
  • A beaker of distilled water.
  • A tablespoon measure.


  • Measure a tablespoon of water into each baggie and seal it tightly.
  • Pass the baggies around the room.

Safety Precautions 

Concentrated calcium chloride solutions are very irritating to the skin and may cause burns. Ammonium chloride is a skin irritant. Be sure to close the baggies all the way and instruct students to be careful with them. If someone gets either solution on their skin, wash it off with copious amounts of water. If someone gets either solution in their eyes, rinse them out with copious amounts of water and seek medical attention.