# Are there any substances that react very slowly with water to create heat?

I know there are things like the alkali metals that react very violently with water. I was wondering if there is anything that produces a slower more controlled reaction?

Also, is there anything that reacts with water to form heat that is safe for humans to touch?

I'm trying to think of ways to melt snow besides salt.

• Hi and welcome to chemistry.stackexchange.com. Feel free to take a tour of the site and visit the help center if you have any questions about it. I know that there are species that will react slow enough to liberate heat in a controlled and safe manner, but I think that none of those will come close to the cost-efficiency of salt (let alone top it). – Jan Oct 2 '15 at 23:23

Hand- and foot-warmers are made containing iron filings, with a bit of paper pulp and salt, and sometimes a sealed water packet. These rely on the (surprisingly rapid) rusting of iron in contact with air, catalyzed by the water and salt (the pulp is just to hold the water and keep it in contact with iron). This reaction is a fairly safe way to melt a bit of snow, with some caveats:

1. It's more expensive than using freezing point depression of salts such as $\ce{CaCl2}$, $\ce{Mg(C2H3O2)2}$, or $\ce{NaCl}$ (table salt). Even adulterated wine containing ethylene glycol has been used for runway deicing.

2. The rust will eventually leave brown streaks on any surface below the deicer, such as the walls of a building if used to melt snow on the roof.