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Evaporative equilibrium
Vapor pressure of water vs. temperature. 760 Torr = 1 atm.

If evaporation takes place in an enclosed area, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium,[1] the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius-Clapeyron relation:

\ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right)

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.

The ability for a molecule of a liquid to evaporate is based largely on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.

Author:	Amanar [ Mon Oct 29, 2012 9:10 am ]
Post subject:	Coffee Physics
So, I've been thinking about coffee this morning, specifically keeping it hot. It seems like everywhere I read there is the same advice: "Don't keep coffee warm on a hot plate. Use a vacuum thermos instead." I can't find any good explanation for why though. Most people suggest that the hot plate on a coffee maker will continue to "cook" the coffee or something, making it stale and bitter. But I don't understand how a thermos could possibly be better. It seems to me that 170 degrees is 170 degrees, how can it matter where the heat is coming from? How can coffee being kept at 170 from a hot plate be any different than coffee kept at 170 by insulation? They're both the same temperature. If the coffee is being "cooked" on the hot plate, it should be being cooked in the vacuum thermos too. The only difference I can see is, the coffee on the hot plate would still be open to air, so would continue to evaporate and become more concentrated... But wouldn't adding hot water to the pot occasionally make up for that? That seems a lot easier than transferring it to another container. Anyway, just one of my random thoughts this morning.

Author:	Amanar [ Mon Oct 29, 2012 9:14 am ]
Post subject:
Oh, and then there's the issue of reheating in the microwave, which most people seem to think is the "worst thing ever." But if heat is what makes coffee go stale or whatever, wouldn't it be best to let the coffee cool after brewing, and then reheat it to the proper temp later?

Author:	Hopwin [ Mon Oct 29, 2012 9:30 am ]
Post subject:	Re: Coffee Physics
Amanar wrote: The only difference I can see is, the coffee on the hot plate would still be open to air, so would continue to evaporate and become more concentrated... But wouldn't adding hot water to the pot occasionally make up for that? That seems a lot easier than transferring it to another container. ^ Exactly what you outlined. You can add water periodically, but I would imagine it is a ***** to gauge exactly how much. If you are talking a whole pot the evaporation would be a fairly low amount I think this advise is more toward people who buy a mug hot plate (pictured below).

Author:	Numbuk [ Mon Oct 29, 2012 10:47 am ]
Post subject:	Re: Coffee Physics
A vacuum sealed thermos is suggested simply because the transference of ambient hot/cold energy is slowest in them. We bought a couple a few years back at target. My wife put an ice cube in one and it had not fully melted 12 hours later. So cold things stay cold for much longer, hot thing stay hot for much longer. Just don't use them for soda pop. I've gone through two of them because of the pressure buildup warps the lid. But, my drinks were always cold, hours later.

Author:	Corolinth [ Mon Oct 29, 2012 1:08 pm ]
Post subject:
A vacuum thermos prevents any coffee from escaping, causing evaporation to stop once there's an equilibrium state. Furthermore, a hot plate continues to add energy to the system. In a vacuum thermos, chemical reactions can only take place using the thermal energy already present. To put it into perspective, water at 212F doesn't automatically boil. That's just the temperature where boiling can occur. You still need more energy to carry out the liquid to gas conversion. The same thing is going on in your coffee. If you leave it on the hot plate, it gets that extra energy to make stuff happen. Poured into a thermos, your coffee reaches a steady state for whatever temperature it's at, and then just sits there.

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Author:	Amanar [ Tue Oct 30, 2012 8:42 am ]
Post subject:
Well let's assume you start out with coffee that's at 170 degrees. In one scenario it's in a vacuum thermos and losing negligible heat to the surroundings. So the energy is being held constant. In the other scenario, you have coffee in the pot on the heating element. Let's say it's taking in 50 watts of energy, while at the same time radiating 50 watts of energy to the surroundings, so it's also staying at the same temperature. In both situations you have no net change in energy, so what does it matter?

Author:	TheRiov [ Tue Oct 30, 2012 8:58 am ]
Post subject:
http://en.wikipedia.org/wiki/Evaporation Quote: Evaporative equilibrium Vapor pressure of water vs. temperature. 760 Torr = 1 atm. If evaporation takes place in an enclosed area, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium,[1] the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius-Clapeyron relation: \ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right) where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil. The ability for a molecule of a liquid to evaporate is based largely on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization. tl;dr because the thermos is a closed system, any water molecules that vaporize go into the air in the container. Pressure increases. As pressure & density increase, they become more likely to return to liquid state. Eventually an equilibrium is reached where the rate of vaporization = rate of condensation. On the other hand, in an open system (pot open to the air) the density of the water in the air doesn't change that much, and the molecules just escape (and are far more likely to condense back out when they've cooled such as by mixing with the atmosphere further away from the heat source, or on a cool surface)

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