I have a question about the change of state (liquid to solid); in this case water. At this time of year, when the air temp is just above freezing, frequently I will go out to my car in the morning, and there will be either water drops or a mist on the windshield. If I turn on the wipers, it will instanly flash to ice! How does this happen-is the water in a supercooled state, or does the increase in surface area cool the water to the freezing point?
I would venture to guess, egkelly, that the outside temperature is very near water’s triple point and that explains the phenomenum. Don’t explain me to explain triple point exactly, but it’s a temperature at which a substance can exist as a gas, solid and liquid.
The Coyote gnaws …
but he does not swallow.
My guess is that the increase in surface area increases the rate of evaporative cooling, cooling the film of water and freezing it.
jrf
Sorry, but nope. The triple point of water is 0.0098 degrees C and 0.0060 atmospheres. Whereas the temp is possible, the pressure is just a bit too low to be possible (on the ground where his/her car is).
Yep, that’s it.
JonF is probably correct.
H[sub]2[/sub]O can exist at 32[sup]o[/sup]F as either a liquid or a solid. As liquid H[sub]2[/sub]O at 32[sup]o[/sup]F loses energy, it turns into ice. Wiping the water drops that were probably right at this temp over the surface of your cold windshield caused just enough energy to be removed for the water to turn to ice, with little or no temperature change.
What would Brian Boitano do / If he was here right now /
He’d make a plan and he’d follow through / That’s what Brian Boitano would do.
Originally posted by AWB:
Well… kinda sorta. Or, that is… maybe.
The windshield wipers actually added energy to the near-freezing water, but only a tiny amount. The water droplets (pre-wipers) were already at or near the point of freezing; the windshield glass not covered by drops was probably already below that point. The action of the wipers both increased the surface area and decreased the depth of the water on the glass, allowing heat to escape from the water to the surrounding air and the cold glass very quickly, resulting in the ‘flash-freezing.’ In all likelihood, had egkelly waited a few minutes before venturing outside, the water drops would have been frozen.
I don’t know why fortune smiles on some and lets the rest go free…
T