Have you ever wondered how hand warmers and cold packs worked? The kind that can be stored at room temperature and then used when needed depend on exothermic and endothermic chemical reactions. An exothermic chemical reaction gives off heat into its surroundings. Conversely, an endothermic reaction absorbs heat from the surrounding environment.


Hand warmers are exothermic—that is, they give off heat. Simply tear open the outer pouch, remove the inner pouch exposing it to air and shake up the contents. After a few moments the exothermic reaction begins and you can stuff it into your jacket pocket to keep your hand toasty warm.

Cold packs are endothermic and absorb heat from the surrounding environment. Inside the cold pack are a lot of little beads made of a chemical compound called ammonium nitrate and a small sealed bag of water. To start the endothermic reaction, squeeze the cold pack bursting the small bag of water inside and shake it. The water will dissolve the ammonium nitrate beads and the pack will feel cold to the touch after a few moments.

Innovating Science has put together Exothermic/Endothermic reactions combination kit that can be used in the classroom to demonstrate exothermic and endothermic chemical reactions to students. You can check their catalog for current prices. The kit contains five bags of the endothermic reactions mixture (25g Ammonium Nitrate and 15g vermiculite) and five bags of the exothermic reactions mixture (25g Iron Metal Powder, 1g Sodium Chloride, 5g Calcium Chloride, 15g vermiculite). To start the reaction, simply add 10ml of distilled water and reseal the bag. Shake and squeeze the bag to mix the contents. You can have your student pass the packet around to feel the change in temperature.

Exothermic reactions packet, kit instructions, and commercial hand warmer. (Exothermic/Endothermic Combination kit reviewer’s sample courtesy Innovating Science)

Endothermic reactions packet, kit instructions, and commercial cold pack. (Exothermic/Endothermic Combination kit reviewer’s sample courtesy Innovating Science)

I cut open the commercial cold pack and removed the contents. The empty cold pack bag, the small bag of water, and the ammonium nitrate beads (stored in the BOD bottle) are picture below:

Perhaps a simple method of measuring the change in temperature of the endothermic reaction packet is just to place the bag on top of a thermometer.

I attached the thermometer to my ring stand and placed the bottom of the thermometer directly on top of the bag.

Using the exothermic reaction packet my initial temperature was 21 degrees C. Once I poured the water in (I used tap water—your mesurements may vary) and mixed the contents the temperature of the packet rose to 26 degrees C. At 5 minutes the temperature fell to 25 dgrees, then 24 degrees after ten minutes, and 23 degrees after fifteen minutes.

My initial temperature for the endothermic reaction packet was 22 degrees C. After pouring in the water (tap water again) and mixed the contents the temperature dropped to 15 dgrees. After five minutes the temperature fell to 4 degrees and held there until after ten minutes. After fifteen minutes the temperature began to rise slightly to 5 degrees.