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All three curves on the phase diagram meet at a single point, the triple point    , where all three phases exist in equilibrium. For water, the triple point occurs at 273.16 K ( 0 . 01 º C ) size 12{ \( 0 "." "01"°C \) } {} , and is a more accurate calibration temperature than the melting point of water at 1.00 atm, or 273.15 K ( 0 . 0 º C ) size 12{ \( 0 "." 0°C \) } {} . See [link] for the triple point values of other substances.

Equilibrium

Liquid and gas phases are in equilibrium at the boiling temperature. (See [link] .) If a substance is in a closed container at the boiling point, then the liquid is boiling and the gas is condensing at the same rate without net change in their relative amount. Molecules in the liquid escape as a gas at the same rate at which gas molecules stick to the liquid, or form droplets and become part of the liquid phase. The combination of temperature and pressure has to be “just right”; if the temperature and pressure are increased, equilibrium is maintained by the same increase of boiling and condensation rates.

Figure a shows a closed system containing a liquid and a gas. A thermometer with one end in the liquid indicates an unspecified temperature, and a pressure gauge indicates an unspecified pressure. A vector from the liquid to the gas represents the rate of vaporization, and a vector from the gas into the liquid represents the rate of condensation. The two vectors are equal in length, illustrating that the two rates are equal. Figure b is essentially the same as figure a, except that the pressure, temperature, and rates of condensation and vaporization are all greater than in figure a. The rates of vaporization and condensation in figure b are equal to each other, even though they are greater than the rates in figure a.
Equilibrium between liquid and gas at two different boiling points inside a closed container. (a) The rates of boiling and condensation are equal at this combination of temperature and pressure, so the liquid and gas phases are in equilibrium. (b) At a higher temperature, the boiling rate is faster and the rates at which molecules leave the liquid and enter the gas are also faster. Because there are more molecules in the gas, the gas pressure is higher and the rate at which gas molecules condense and enter the liquid is faster. As a result the gas and liquid are in equilibrium at this higher temperature.
Triple point temperatures and pressures
Substance Temperature Pressure
K size 12{K} {} º C size 12{°C} {} Pa size 12{"Pa"} {} atm size 12{"atm"} {}
Water 273.16 0.01 6 . 10 × 10 2 size 12{6 "." "10"×"10" rSup { size 8{2} } } {} 0.00600
Carbon dioxide 216.55 −56.60 5 . 16 × 10 5 size 12{5 "." "16" times "10" rSup { size 8{5} } } {} 5.11
Sulfur dioxide 197.68 −75.47 1 . 67 × 10 3 size 12{1 "." "67"×"10" rSup { size 8{3} } } {} 0.0167
Ammonia 195.40 −77.75 6 . 06 × 10 3 size 12{6 "." "06"×"10" rSup { size 8{3} } } {} 0.0600
Nitrogen 63.18 −210.0 1 . 25 × 10 4 size 12{1 "." "25"×"10" rSup { size 8{4} } } {} 0.124
Oxygen 54.36 −218.8 1 . 52 × 10 2 size 12{1 "." "52" times "10" rSup { size 8{2} } } {} 0.00151
Hydrogen 13.84 −259.3 7 . 04 × 10 3 size 12{7 "." "04"×"10" rSup { size 8{3} } } {} 0.0697

One example of equilibrium between liquid and gas is that of water and steam at 100 º C size 12{"100"°C} {} and 1.00 atm. This temperature is the boiling point at that pressure, so they should exist in equilibrium. Why does an open pot of water at 100 º C size 12{"100"°C} {} boil completely away? The gas surrounding an open pot is not pure water: it is mixed with air. If pure water and steam are in a closed container at 100 º C size 12{"100"°C} {} and 1.00 atm, they would coexist—but with air over the pot, there are fewer water molecules to condense, and water boils. What about water at 20 . 0 º C size 12{"20" "." 0°C} {} and 1.00 atm? This temperature and pressure correspond to the liquid region, yet an open glass of water at this temperature will completely evaporate. Again, the gas around it is air and not pure water vapor, so that the reduced evaporation rate is greater than the condensation rate of water from dry air. If the glass is sealed, then the liquid phase remains. We call the gas phase a vapor    when it exists, as it does for water at 20 . 0 º C size 12{"20" "." 0°C} {} , at a temperature below the boiling temperature.

Explain why a cup of water (or soda) with ice cubes stays at 0 º C size 12{0°C} {} , even on a hot summer day.

The ice and liquid water are in thermal equilibrium, so that the temperature stays at the freezing temperature as long as ice remains in the liquid. (Once all of the ice melts, the water temperature will start to rise.)

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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