Hot water = condensation?

March 2, 2009 8:23 PM

Why when you pour hot water into a mug, water appears below your mug?
posted by slepore09 to Science & Nature (12 answers total)

Where does the water appear? Is the mug recently washed? Is the surface below it damp?
posted by Science! at 8:31 PM on March 2, 2009

Theory: you spilled.
posted by grobstein at 8:32 PM on March 2, 2009

How soon does it appear? Could it be condensation?
posted by fructose at 8:36 PM on March 2, 2009

possibilities:

a. Water sloshed.

b. Mug has hole(s)

c. Water was already there.

d. Trick of the fairies. (listen for high-pitched laughter)
posted by longsleeves at 8:36 PM on March 2, 2009

Further questions: what temperature is the air? What temperature is the cup?
posted by turgid dahlia at 8:38 PM on March 2, 2009

Best answer: Here's what I'm going to go with, either the mug or the counter is very slightly damp from being washed or humidity or a spill of some kind. The hot water heats up the mug significantly hotter than the surface it's resting on, and the small lip around the bottom of the mug prevents a lot of heat from transferring to the counter. The hot mug causes steam to develop by heating water from the mug, or maybe the counter, which then condenses on the relatively cooler counter surface, this is aided by the partially different air pressure created in the "bubble" between the mug's lower rim and the counter.
posted by Science! at 8:39 PM on March 2, 2009

My bet is it is condensation.
posted by Black_Umbrella at 8:40 PM on March 2, 2009

I think Science! has the correct explanation. Toast does this too a lot of times.
posted by jessamyn at 8:40 PM on March 2, 2009

1) Vapour from hot water travels up the inside of the cup, and over the lip.
2) As the water fills the cavity, the early vapour has hit the outside of the top of the mug, which is still cool enough to cause condensation.
3) Condensation slides down outside of mug, wetting the bench.
4) Profit!
posted by pompomtom at 8:40 PM on March 2, 2009 [2 favorites]

More data, please. Dry the bottom and sides of the cup with a dry paper towel. Ditto the counter. Wrap paper towel completely around side of cup to prevent drips running down. Add hot water to cup. Do you still find water under the cup?
posted by exphysicist345 at 8:58 PM on March 2, 2009

Damn, Jessamyn beat me to toast prints.
posted by 517 at 9:10 PM on March 2, 2009

I just did a bit of experiment because I've been curious about this myself. I had always suspected there was some wacky micro-weather system hidden under the cup that would make it rain under there and disappear as soon as I looked.

So the experiment was to take two ceramic mugs, one that had just been washed, one that had been in the cupboard for a week, filled them with near boiling water (well, made buckwheat tea actually) and let them sit in my wooden table for 5 minutes. The washed one had condensation under it, the week-long dry one didn't.

So Science! (as well as regular old science) indeed did have the best answer. Very cool! I've wondered about this forever and never thought about the water that remained on/absorbed into the cups.

(And ew... toast condensation is poisonous!)
posted by Ookseer at 11:26 PM on March 2, 2009 [4 favorites]