Reusable hand warmers are made using a supersaturated solution, typically of sodium acetate. This solution releases heat as it crystallizes. Let's break down the process:
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The Solution: The key ingredient is sodium acetate. It's dissolved in water at a high concentration and temperature, creating a supersaturated solution. This means the solution contains more dissolved sodium acetate than it normally would at room temperature.
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The Pouch: The supersaturated sodium acetate solution is sealed in a durable, often plastic, pouch.
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The Metal Disc: Inside the pouch, there's a small metal disc, usually made of stainless steel. This disc acts as a trigger for crystallization.
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Activation: Bending the metal disc creates a slight disturbance in the solution. This disturbance acts as a nucleation site, initiating the crystallization process.
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Crystallization and Heat Release: As the sodium acetate crystallizes, it releases heat in an exothermic reaction, warming the hand warmer. The solution solidifies and becomes cloudy.
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Recharging (Reusing): After the hand warmer has cooled and the sodium acetate has solidified, it can be "recharged" by boiling it. Boiling melts the sodium acetate crystals, returning them to their dissolved state and recreating the supersaturated solution. Once cooled, the hand warmer is ready to be used again. The boiling process essentially reverses the crystallization process.
Summary: Reusable hand warmers rely on the properties of a supersaturated sodium acetate solution. Bending the metal disc initiates crystallization, which releases heat. Boiling the solidified hand warmer returns the sodium acetate to its supersaturated state, allowing it to be reused.
