What is the scientific reason behind hot water being able to wash dishes better than cold water?

And if this is the case, why do people brush their teeth with cold water... wouldnt hot water get them cleaner?

Don't know about other people but when I brush my teeth I use warm water; my teeth are sensitive. Bacteria grows better in warmth. When washing dishes, hot water is better because it handles oil better than cold water. Mom says so, although I see that could be fiction. I prefer warm water because otherwise my bones hurt. Either way you are to use dish washing liquid =P

I have wondered about this too, hot water seems like it would be better for brushing teeth, so I switched last year. Also, sometimes my teeth are too sensitive to the cold water.

Actually it's in the detergents they use. Today they have formulated cold water detergents for washing clothes, dishes and cars so you can go out and buy some today and save a few bucks on your hot water bill.

Heat increases the random molecular motion of whatever it is you're washing, so it helps things to dissovle faster.

Most of what you're washing is water soluable, or will become so with the aid of soap, so hot water does a better job of wiggling into the grime than cold does. Lipids-fats, oils, grease- become liquids at warm temperatures, so when you heat it up, they run off faster.

Here's a simple experience you can do with oil. Pour some on a pan, then try blasting it off with cold water. The cold water makes it thicken up and gel to the pan. Now use scalding hot water. The hot water increases the random molecular motion, making it thinner, and more of a liquid. Now it will just run off.

With teeth, you aren't washing anything off that's water soluable, nor will you be using any detergents that will make the gunk on your teeth water soluable, so water temp is inconsequential. Besides that, the tiny volume of water you enter into your mouth quickly becomes body temp, regardless of if it went in cold or hot.

Heat increases the random molecular motion of whatever it is you're washing

It is also for this reason that when you heat your tooth brush (via warm or hot water) the bristles become less like bristles and more like cooked angelhair pasta - not good for cleaning, whereas cold water helps keep the bristles stiff and hard, good for scraping off plaque and whatever else happens to be in there.

dishes and hot water: it's because soap acts better on the bacterial lipid bi-layer membranes in hot water than in cold (see Roman's experiment on oil above). In addition, many common bacterial and viral sources found on dishes live in a cooler temp. range, and will die under hot enough water - soap or no.


As for brushing teeth under cold water - it is because water hot enough to kill the bacteria in your mouth would also burn your mouth, so there is no advantage to hot. Cold water tastes better. :)