Intermolecular Phase Diagrams

• Solids: In solids, particles are tightly packed together. They have a fixed shape and volume, and cannot be easily compressed.

• Liquids: In liquids, particles are spaced a bit further apart. Liquids also have a fixed volume, but they take the shape of their container. They are also not compressible.

• Gases: In gases, particles are very far apart. Gases have no fixed shape or volume and can be compressed.

Phase Diagrams

Phase diagrams show the state (solid, liquid, or gas) of a substance at various pressures and temperatures.

AB Line

• The AB line represents the boundary between liquid and vapor (gas).
• It starts at the triple point (A), where all three states can coexist in equilibrium, and ends at the critical point (B), where liquid and gas become indistinguishable (known as a supercritical fluid).
• Any point along this line indicates the boiling point of the substance at that specific pressure.

AD Line

• The AD line marks the boundary between liquid and solid.
• The melting point of the substance at different pressures can be found along this line.

AC Line

• Below point A, the substance cannot exist as a liquid.
• The AC line represents the sublimation points of the substance at different pressures, where solid turns directly into gas.

Important Terms

• Isobaric transitions occur at constant pressure.
• Isothermal transitions occur at constant temperature.
• Normal conditions refer to circumstances at standard atmospheric pressure.

Example: Carbon Dioxide

Carbon dioxide cannot exist as a liquid unless the pressure is above 5.11 atm; below that, it sublimates directly from solid to gas at normal pressures.