Previous discussions have focused on phase changes at constant pressure, specifically atmospheric pressure (1 atmosphere).
Atmospheric pressure is defined as pressure at sea level on Earth.
Variability of Pressure
Pressure varies depending on location (e.g., different planets, different atmospheric conditions).
Phase diagrams are used to understand how states of matter change with temperature and pressure.
Phase Diagram for Water
Axes:
Y-axis: Pressure
X-axis: Temperature
States of Matter:
Solid, Liquid, Gas
Example:
At 0°C and 1 atmosphere, water is at the boundary between solid and liquid.
Higher pressure decreases melting temperature.
Lower pressure increases freezing point.
Boiling and Freezing Points
At 1 atmosphere:
Water freezes at 0°C.
Water boils at 100°C.
Lower pressure (e.g., in Denver or on Mount Everest) lowers boiling point and increases freezing point.
Higher pressure raises boiling point and lowers freezing point.
Pressure and Phase Changes
High pressure makes it harder for molecules to escape as gas.
Low pressure facilitates transitions from liquid to gas or solid to gas.
Example: On the Moon, low pressure allows ice to sublimate directly into gas.
Phase Diagram for Carbon Dioxide
At 1 atmosphere:
Solid CO2 sublimates to gas above -78.5°C.
Liquid CO2 requires pressure above 5 atmospheres.
Sublimation: Transition from solid to gas without becoming liquid.
Unique Points on Phase Diagrams
Triple Point:
All three states (solid, liquid, gas) exist in equilibrium.
Water: 0.611 kPa and slightly above 0°C.
Critical Point:
Above this point, substance becomes a supercritical fluid.
Supercritical fluids have properties of both liquid and gas.
Application of Phase Diagrams
Phase diagrams can predict behavior under varying pressures and temperatures.
Understanding supercritical fluids and their applications, such as solvents.
Conclusion
Phase diagrams are essential for understanding and predicting the behavior of substances under different temperature and pressure conditions. They highlight how substances transition between different states and offer insight into unique phenomena such as sublimation and supercritical fluids.