How Do Instant Heat Packs Work? A Chemist Explains

During these winter days, you might be fortunate enough to be skiing, or unfortunate enough to have an outdoor job. In both cases, you need to keep toasty, and sometimes wrapping yourself up in warm clothes doesn’t quite do the trick.

So what better way than an instant heat pack to warm up those fingers and toes? It can feel like magic — you pull a hand warmer out of a plastic packet, put it in your pocket, and feel it slowly warm up. Hours later, it’s still radiating a gentle heat.

But how do these heat packs work? Why do they last so long, and why are some reusable and some not? To understand this, let’s dive into some basic chemistry.

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