Stupid Question about Water

[Sela]

Recently I saw an instruction in an experiment that indicated to heat water beyond 100degrees celsius. Now, my background is pretty physics-poor, so I didn't want to press the point with the instructor, but my understanding was that at standard pressure (not temperature!) water cannot remain in the liquid phase beyond 100 degrees. That once water is 100degrees celsius it will cease to be liquid and enter the gas phase.

The way it was explained when I asked about it was that there were lots of other factors at play and one of the important ones was how humid the air was to begin with. That sounded sensible enough, but I still don't get how the >100degree liquid water could 'know' what the humidity was when reaching the evaporating point. In either case, I didn't want to belabor the point on a class I wasn't in.

So my question is: at a pressure of 1atm, can pure (or distilled) water be heated beyond 100degrees celsius, but still remain in the liquid phase? If so, how far beyond 100degrees can we go and why/what would we have to do?


Incidentally, I was curious so I tried to test it, but I wasn't sure that my thermometer was accurate as it might have been reading the ambient temperature from the hot plate beneath the beaker.


PS: If this is a *really* stupid question, feel free to just point me to where I should be reading more about this.

[General Trelane (Retired)]

What you're asking about is called superheating.

[aimlessgun]

Wow, that totally explains why sometimes when I microwave water for tea, it looks like it exploded out of the mug.

[Feil]

If you open the microwave and there is water all over, the cup has just boiled over in the boring way: bubbles jostled liquid water out of the cup. Superheating is possible in a microwave. The water will look like it hasn't boiled, but as soon as you break the surface (with a tea strainer, for instance) it will burst into a hard boil and spray boiling water all over and burn your hand unless you're expecting it.

Humidity doesn't make a difference, provided pressure is held constant. Increased humidity does increase the vapor pressure, but only because increased humidity increases atmospheric pressure. Varying humidity at constant pressure will affect the rate of evaporation (really the sum of evaporation and condensation, which occur simultaneously). But that is a different phenomenon from boiling.

My impression is that the instructor doesn't have the faintest idea what he's talking about.

If (a) the experimental procedure involves breaking the surface tension of the water in any way at any point after heating, or if any part of the procedure requires moving or jostling the container of heated water; or (b) if precautionary equipment and behavior for the possibility of the water suddenly and violently bursting into a hard boil is not mandatory, he's an ignorant purveyor of falsehoods at best and a danger to himself and his students at worst.

[Chardok]

I only ever experienced superheating in the microwave once. I was about 12 making a cup of instant coffee for my mom. I would first heat the water, then add the coffee. I grabbed the handle and began removing the cup from the microwave and as soon as it cleared the door (I guess I jostled it) about half the water exploded out of the cup. none of the water landed on me, somehow, but I just remember being stunned and thought to myself "The water boiled all at once! Cool!"