Why is hot water more effective than cold when washing your hands, if the water isnt hot enough to kill bacteria?
In: Chemistry
For the purposes of killing bacteria, washing your hands with hot water isn’t more effective than washing them with cold water. The important variable is soap.
If you soak you hands atleast two minutes in a boiling water it should 100% kill of any bacteria.
I live in the northern part of the Netherlands. I always wondered why there is no hot water in the restrooms. Now I know. I thought it very strange when I moved here.
In addition to what others have said, using soap and the act of actually rubbing it into your hands is more important than temp. Hence, the 30 second “rule”.
Temperature is really just the amount of activity going on at a microscopic level. If its warmer that means all the molecules are bouncing around a lot faster and thus doing whatever it is they’re going to do faster. Because of this heat generally accelerates chemical reactions whether that’s water dissolving dirt or soap clinging to oil.
For the purposes of hygenic cleaning (killing germs, removing dead skin, cleaning a wound), temperature doesn’t matter and (in some scenarios eg washing off bodily fluids or with certain soaps.) cold water is actually preferable.
For the purposes of cosmetic cleaning (washing off stains, cleaning oily fingers, greasy marks), hot water can help soften long chain hydrocarbons like waxes, grease or oils and can help solubilise inks or other chemicals into the soap or water.
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Tl;dr (Better ELI5) is:
If you want to kill germs, temperature doesn’t matter. If you want to clean dirty hands, warm water can help.
In both cases, washing thoroughly (at least 15 seconds) with soap is the most important thing.
None of this matters if you’re that guy who puts soap on your hands, then immediately puts them under running water (washing off the soap) and rubs for like 5 seconds and calls it good. Wet hands, apply soap, use GOOD FRICTION (that’s what matters most) for *at least* 15 seconds being sure to get wrists and under rings, then rinse. Thanks for coming to my TED talk.
It’s not.
It is however, better for *cleaning* your hands. Solubility increases with temperature. Soap is already plenty soluble, but other things are not, so if you’ve got a lot of dirt, grime, etc on your hands, hotter water will help. It’ll also help soften and loosen dirt
Interesting comments so far.
I recall a recent study showing that cold water was as effective as hot water for cleaning hands.
I would have expected warmer water to better dissolve organic (carbon containing) compounds.
The study proved that the difference was insignificant; I guess I was wrong.
Most things are more soluble in warmer water. The stuff you want to remove from your hands dissolves in the water, and is also dissolved by the soap.
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