That reminds me, I was going to put the teakettle on.
March 11, 2009 6:08 AM   Subscribe

Why is hot water more effective at cleaning than cold water?

I'm well aware of the power of using hot water, but it's never really occurred to me to wonder precisely why this is so. It occurred to me that hot water would be able to have more soap or whatever dissolved in it, but hot water even takes dried crusty stuff off plates more quickly, and is better for wiping off counters and tables. So what's the deal? What is it about the heat that makes it better for cleaning?
posted by Pope Guilty to Science & Nature (8 answers total) 3 users marked this as a favorite
Best answer: This link may provide some insight.
posted by Brettus at 6:09 AM on March 11, 2009

Response by poster: So basically hot water containing more energy makes the processes that make up "cleaning" more efficient and powerful. That is seriously neat, thank you.
posted by Pope Guilty at 6:14 AM on March 11, 2009

That article skirts somewhat around the obvious, Richard Feynman-y explanation: Heat and energy aren't mysterious additives. The molecules of hot water are literally moving around faster, with greater force, than those of cold water. They run into molecules of dirt or grease or whatever with that greater force, and, predictably, knock loose many more of them from whatever you're cleaning.

Sorry if that just seems redundant.
posted by Nomiconic at 7:16 AM on March 11, 2009 [1 favorite]

Response by poster: I had an idea something like that, but suspected that the scale on which such movements are occurring would be too small to be significant.
posted by Pope Guilty at 7:33 AM on March 11, 2009

Yep - I think it just comes down to that the properties of hot water break the bonds of the dirt, etc more easily than cold water. Of course, this is what soap does as well.
posted by jourman2 at 7:51 AM on March 11, 2009

Heat (almost?) always speeds up chemical reactions. It's the same reason you get a fever when you're sick, so that the biological processes that work to remove the disease work more quickly.
posted by delmoi at 8:49 AM on March 11, 2009

Heat (almost?) always speeds up chemical reactions. It's the same reason you get a fever when you're sick, so that the biological processes that work to remove the disease work more quickly.

Also the increased temperature can directly kill the infecting bacteria/viruses.
posted by jckll at 9:10 AM on March 11, 2009

The list in Brettus' link is probably the best summary of this so far:

1) Increased reaction rates: the molecules are moving faster, and so can wet surfaces more quickly, form (ionic and Van der Waals) bonds with the dirt molecules more readily and so forth. Chemical reactions, even if they result in a net stability increase, usually have some initial resistance to over come. Chemists call this the activation energy of a process. Often this is caused by molecules needing to be in the right alignment to react---just like Lego blocks, if they're not point at each other the right way, connections won't happen. Heat helps rush things over the initial speed bump.

2) Increased entropy: most greases dissolve in water not by chemical bond formation, but because doing so causes the system to become more disordered. This is the same phenomenon that ensures mixtures stay mixed, that clotheshangers and string always get tangled, etc... Entropy increase is the name for the concept that grease molecules get "tangled" into the water. Heat enhances this process-things can move around more, so they get tangled more quickly and more throughly.

3) Phase change: it's difficult and slow to dissolve/lift a solid. Waxes and greases are often in or near their melting ranges at room temperature. Adding heat causes them to melt and thus dissolve or react with the cleaner molecules more easily.

The 'catalyst' argument at the end of the article is essentially the same argument as #1). Heat helps cleaner molecules (soaps, detergents, biocides) react quicker too.
posted by bonehead at 9:28 AM on March 11, 2009

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