February 2018:
Materials:
Two different sized ziplock bags; Water; Calcium Chloride ice-melt pellets
Experiment:
In the large bag, put the ice-melt pellets in so that it is about one-fourth full. Fill the small bag about half full with water and make sure that it is sealed well. Place the water bag inside the large ziplock bag and remove as much air as possible and seal the large bag. When you are ready to use the hand warmer, squeeze the water bag till it breaks open or open it up so that it can mix with the ice-melt. Make sure that the large bag is sealed so it doesn’t leak all over. The heating will begin immediately and last for 20-60 minute
What’s happening?
Different hand warmers work in different ways. The one that you are making works because some salts (like calcium chloride – CaCl2), when dissolving in water give off heat while others take in heat and get cold. These are called exothermic and endothermic processes, respectively. The calcium ions and the chloride ions are stuck together in the crystal; when you add the water to the salt, the bonds are broken and that will require heat. However, as the Ca-Cl bonds are broken, new bonds are being made between the water and the calcium ions and water and chloride ions. The making of these bonds releases heat – a lot more heat is released here than the heat that was required to break the calcium and chloride bonds. Therefore, the overall process generates heat that you can feel through the ziplock bag.
References:
https://www.popsci.com/diy/article/2012-02/5-minute-project-hand-warmers#page-3
http://www.diynetwork.com/how-to/make-and-decorate/crafts/how-to-make-pocket-hand-warmers
To view all past “ChemShorts for Kids”, go to:
http://chicagoacs.org/articles/article_category/1
- Paul Brandt
