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.

An exothermic reaction

One way to describe chemical reactions or physical processes is based on what they do with heat – do they produce or absorb it?

A process is endothermic if it absorbs heat from its surroundings. A familiar example is the melting of ice at room temperature – ice absorbs heat from the surroundings and melts as a result.

By contrast, a process is exothermic if it releases heat to its surroundings. The combustion of petrol is an exothermic reaction that emits a large quantity of heat to the surroundings.

Heat packs use exothermic processes. However, unlike the combustion of........

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