How Do Hand Warmers Work

The Types of Hand Warmers

There are two main types of hand warmers; each one uses a different method to create heat. However, both of them rely on chemical reactions.  One gives long-lasting results that can last up to ten hours; the other only last for short periods of up to two hours but is reusable.  The most common is the Air activated hand warmer, which is what people buy at the store in the hot hands etc.  So let’s start by explaining the most popular type first.

How do Air Activated Warmers Work?

The air-activated warmers are self-explanatory.  They work as soon as air hits them.  The process is significantly more complicated than that. Every hand warmer comes sealed in a packet that contains a variety of substances: iron, cellulose or sawdust, water, vermiculite, activated carbon, and salt.

Once you unzip the packet, oxygen and air flows into the contents and begin oxidizing iron and turning it into rust.  The salt speeds up the oxidization, and the carbon distributes the heat evenly.  The only reason for the sawdust is the make the packet have content, so it takes up a little space

The actual process is an electrochemical process where the iron gives up electrons, and the oxygen gains electrons.  This process on a microscopic level is what generates the heat.  The final rusting process leaves the iron taking up more space than the iron occupied initially; in other words, the iron increased in size. When the process is over, the packets are unusable and need to put in the trash.

They work best when in confined spaces like pockets.  However, the process can generate immense heat.  It is best if you have a layer in between your skin and the hand warmer.  Extra caution should be displayed with children.

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​Super Saturated Solution Warmers

The second most popular type of hand warmer is the supersaturated solution warmer.  The supersaturated solution hand warmers come in a sealed container and contain water, salt, and a metal (usually stainless steel) disc or strip.  These hand warmers rely on a crystallization process to release heat.  The most common chemical is a solution of sodium acetate.  These hand warmers are also known as hot ice.

The user activates the hand warmer by flexing or snapping the piece of metal.  The metal acts as a nucleation surface for crystal growth.  The sodium acetate starts to crystallize, and this creates heat up to 130 degrees.

Unlike the transfer of electrons up above, it is the bonding of crystalline structures that creates heat. As the bonding occurs, an exothermic reaction takes place, which releases the heat.  Once the process is completed, there is no more heat created.  This means that there is a short heat-producing element for these types of hand warmers.  However, you can place them in boiling water, let them cool and reuse them again.

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