http://en.wikipedia.org/wiki/EvaporationQuote:
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)
Login
Register
FAQ
Search
