Cool Science HandWarmers

Sometimes during these cold winter months you need a little help to stay warm. This is especially true if you are spending long periods of time standing or sitting outside, without moving around much to help you stay warm. Fortunately, about 90 years ago, a Japanese inventor named Niichi Matoba found a solution by inventing the first portable hand warmer. Since Matoba's time there have been many different types of hand warmers that have come on the market, all of which involve chemistry.

The two different types of hand warmers shown in the video are the most common ones you will find in stores today. The first type of hand warmer is a single-use, disposable version that typically releases heat for 1 to 10 hours. Inside the package there is a combination of ingredients (cellulose, iron, water, activated carbon, vermiculite and salt). When these ingredients are mixed together and exposed to air, it begins a chemical reaction called oxidation. The iron loses electrons to oxygen molecules and in the process releases energy as heat. This heat will last as long as the materials inside continue reacting. It takes about 30 minutes for all of the ingredients to start reacting and for the hand warmer to reach its full warmth.

They are great for emergencies because they can be stored for long periods of time without the ingredients inside expiring (so make sure you put some of these in your car emergency kit!). Leaving the package out in the open allows the heat to dissipate faster, so these hand warmers work best when put into your coat pocket or the toe of your socks. The reaction can be paused by placing the hand warmer into an air tight bag, pushing out all of the air and sealing it. The hand warmer can be used over and over until the full duration of its warmth has been reached. As soon as the active ingredients are used up, it has to be thrown away, the only real downside with this type of warmer.

Another version of hand warmer available on the market is the re-usable gel-type. This type heats up when the supersaturated solution within it begins to crystallize. A supersaturated solution is just a solution that has been forced to dissolve more solids. When you dissolve sugar in water, eventually you will reach a point where you cannot dissolve any more sugar. But, if you heat up the solution, more sugar will dissolve. This supersaturated solution is very unstable and can crystallize (turn into a solid) very easily. In the demonstration in this video, I pour a supersaturated solution of sodium acetate (a salt) onto a small piece of the solidified salt. This immediately begins the crystallization process.

The same kind of thing is happening inside the hand warmer that is used in the video. The blue, gel-like substance is a supersaturated solution of sodium acetate. When the flexible metal activator is bent, a few tiny pieces of metal break off. These tiny pieces act as a place for crystals to start growing (often called a nesting site). Once the process of crystallization begins, it won't stop until all of the solution has turned solid. During this process, a large amount of heat is released; up to 50°C for about 30-40 minutes. To reuse the warmer, it just needs to be reheated in boiling water for a few minutes. The sodium acetate re-dissolves and turns back into a gel-like liquid. These warmers can be reused dozens of times!

So, whether you use the reusable type or the disposable, both hand warmers involve an interesting chemical reaction called an exothermic reaction. An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. In this case, we have harnessed the heat of this reaction to help us stay warm on those bitterly cold winter days!



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