Intermolecular Forces
Phase Diagrams
Phase Diagrams
Equilibrium can exist not only between the liquid and vapor phase of a substance but also between the solid and liquid phases, and the solid and gas phases of a substance.
A phase diagram is a graphical way to depict the effects of pressure and temperature on the phase of a substance:
The curves indicate the conditions of temperature and pressure under which equilibrium between different phases of a substance can exist
- The vapor pressure curve is the border between the liquid and gaseous states of the substance
- For a given temperature, it tells us the vapor pressure of the substance
- The vapor pressure curve ends at the critical point.
The temperature above which the gas cannot be liquefied no matter how much pressure is applied (the kinetic energy simply is too great for attractive forces to overcome, regardless of the applied pressure)
- The line between the gas and solid phase indicates the vapor pressure of the solid as it sublimes at different temperatures
- The line between the solid and liquid phases indicates the melting temperature of the solid as a function of pressure
- For most substances the solid is denser than the liquid
- An increase in pressure usually favors the more dense solid phase
- Usually higher temperatures are required to melt the solid phase at higher pressures
- The "triple point" is the particular condition of temperature and pressure where all three physical states are in equilibrium
- Regions not on a line represent conditions of temperature and pressure where only one particular phase is present
- Gases are most likely under conditions of high temperature
- Solids are most likely under conditions of high pressure
Phase Diagram for Water
- The frozen state of water (ice) is actually less dense than the liquid state, thus, the liquid state is more compact than the solid state
- Increasing pressure, which will favor compactness of the molecules, will thus favor the liquid state
Increasing pressure will thus lower the temperature at which the solid will melt
- The melting curve slopes to the left, unlike most compounds
- At 100 °C the vapor pressure of water is 760 torr or 1 atm, thus at this temperature water will boil if it is at 1 atm of pressure
- At pressures below 4.58 torr, water will be present as either a gas or solid, there can be no liquid phase
1996 Michael Blaber