### Rusty mittens

Just for fun, and because it's winter, I thought I'd look at mitten heaters, specifically the non-reusable type that comes with a warning not to take the plastic off until you're ready to use them. You've probably seen them sold with outdoor recreation stuff, such as camping gear. They're little cloth pockets with a powder inside, and you can stuff them inside your shoes or mittens to help keep warm.

There are a few ingredients in that powder, but only one produces heat. I think the others moderate the speed of the reaction so it lasts about a half hour, instead of getting a lot hotter and only lasting a few minutes.

The source of heat is a multi-step, electrochemical reaction: rust.

More specifically, it's the reaction that turns metallic iron, Fe0, into iron oxide, Fe2O3.xH2O.

Overall, the reaction is:

$4\mathrm{Fe} + \mathrm{O}_2 + 4\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Fe}_2\mathrm{O}_3$

There is a whole bunch of solubilizing, and acid-base equilibria, and so on going on, but since I'm just talking about mitten heaters today, I'll let you follow the link if you're interested. The equation above is simplified to hide all that because I'm feeling lazy.

One thing you'll notice is that this reaction requires both water and oxygen. Metal left underwater in an area of low oxygen doesn't rust nearly as quickly, and metal kept dry also doesn't rust nearly as quickly.

In the case of mitten heaters, the iron filings get both from the atmosphere, even if it feels dry out. (Everybody's hands and feet sweat.) Enough oxygen gets in, even through heavy winter boots.

Of course, if you go some place without enough oxygen, as this pilot did, the oxygen "breathing" heaters don't work so well.