13.5 Phase changes  (Page 3/15)

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 $(0\text{.}\text{01}\text{º}\text{C})$ , and is a more accurate calibration temperature than the melting point of water at 1.00 atm, or 273.15 K $(0\text{.}0\text{º}\text{C})$ . 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.

One example of equilibrium between liquid and gas is that of water and steam at $\text{100}\text{º}\text{C}$ 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 $\text{100}\text{º}\text{C}$ 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 $\text{100}\text{º}\text{C}$ 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 $\text{20}\text{.}0\text{º}\text{C}$ 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 $\text{20}\text{.}0\text{º}\text{C}$ , at a temperature below the boiling temperature.