Depending on temperature and pressure, substances move among the three phases: solid, liquid, and gas. A diagram showing this is called a phase diagram.
Basic Knowledge
A phase diagram has temperature on the horizontal axis and pressure on the vertical axis, dividing the stable regions of each phase with boundary lines.
① Fusion curve: boundary between solid and liquid
② Vapor-pressure curve: boundary between liquid and gas
③ Sublimation curve: boundary between solid and gas
The point where the three lines meet is the triple point (where all three phases coexist), and the endpoint of the vapor-pressure curve is the critical point (where liquid and gas can no longer be distinguished).
The triple point of water is 0.01℃, 611 Pa, and its critical point is 374℃, 22.1 MPa. Water has the unusual phase diagram in which the fusion curve slopes to the left, so pressure lowers its melting point (the reason ice floats on water).
Melting and solidification (solid↔liquid; heat of fusion ΔHfus)
Vaporization and condensation (liquid↔gas; heat of vaporization ΔHvap)
Sublimation and deposition (solid↔gas; CO2, I2, iodine)
Triple point (temperature and pressure at which three phases coexist)
Critical point (endpoint of the vapor-pressure curve)
Supercritical fluid (above the critical point, with no distinction between liquid and gas; supercritical CO2 is used to extract coffee)
Further Study (Principles and Applications)
A graph of temperature during a phase change has a horizontal segment where temperature remains constant during heating. The heat added here is used for the phase change as latent heat and does not contribute to a temperature increase. The heat of fusion of water is 6.0 kJ/mol and the heat of vaporization is 40.7 kJ/mol, so the latter is approximately seven times larger.
Supercritical CO2 has a density comparable to that of a liquid and diffusivity comparable to that of a gas. It is a green technology for extracting natural products without organic solvents (commercialized for decaffeinated coffee). It is also important in practice that distillation separation cannot be performed above the critical temperature.
- Three curves in a phase diagram = fusion, vapor-pressure, and sublimation curves
- Three phases coexist at the triple point
- Critical point ⇒ supercritical fluid
- Water's phase diagram has a “left-sloping” fusion curve (unusual)
- The heat of vaporization of water is approximately seven times its heat of fusion
- Temperature remains constant during latent heat transfer
- Dry ice (CO2) sublimates at ordinary pressure
Important Notes (Common Confusions and Frequent Mistakes)
① Do not confuse the triple point with the critical point. The triple point is where three phases coexist, while the critical point is where the distinction between liquid and gas disappears. ② Pressure lowers the melting point of water (the fusion curve slopes to the left), whereas pressure raises the melting point of most substances. ③ Distinguish latent heat (phase change) from sensible heat (temperature change). ④ At ordinary pressure, CO2 does not become liquid; it changes directly from solid to gas (sublimation).
Practice
- State the temperature and pressure of the triple point of water.
- Explain from the viewpoint of a phase diagram why dry ice (CO2) sublimates at ordinary temperature and pressure.
- Find the heat required to convert 36 g of ice at 0℃ completely into water vapor at 100℃ (heat of fusion 6.0 kJ/mol, heat of vaporization 40.7 kJ/mol, specific heat of water 75 J/(mol·K)).