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Phase Diagrams

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. These diagrams indicate the physical states that exist under specific conditions of pressure and temperature and also provide the pressure dependence of the phase-transition temperatures (melting points, sublimation points, boiling points). Regions or areas labeled solid, liquid, and gas represent single phases, while lines or curves represent...

Phase Diagram

Phase Diagram

The phase of a given substance depends on the pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region provide considerable insights into the thermal properties of substances. Such plots are known as phase diagrams. For instance, in the phase diagram for water (Figure 1), the solid curve boundaries between the phases indicate phase transitions (i.e., temperatures and pressures at which the phases coexist).

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, and solubilities (miscibilities) of substances. Molar mass, molecular shape, and polarity affect the strength of different intermolecular forces, which influence the magnitude of physical properties across a family of molecules.
Temporary attractive forces like dispersion are present in all molecules, whether they are polar or nonpolar. They...

Phase Transitions: Sublimation and Deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and standard pressure, a piece of dry ice (solid CO2) sublimes, appearing to gradually disappear without ever forming any liquid. Snow and ice sublimate at temperatures below the melting point of water, a slow process that may be accelerated by winds and the reduced atmospheric pressures at high altitudes. When solid iodine is warmed, the solid sublimes...

Phase Transitions: Melting and Freezing

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy becomes large enough to partially overcome the forces holding the molecules or ions of the solid in their fixed positions, and the solid begins the process of transitioning to the liquid state or melting. At this point, the temperature of the solid stops rising, despite the continual input of heat, and it remains constant until all of the solid is...

Freezing Point Depression and Boiling Point Elevation

Freezing Point Depression and Boiling Point Elevation

Boiling Point Elevation
The boiling point of a liquid is the temperature at which its vapor pressure is equal to ambient atmospheric pressure. Since the vapor pressure of a solution is lowered due to the presence of nonvolatile solutes, it stands to reason that the solution’s boiling point will subsequently be increased. Vapor pressure increases with temperature, and so a solution will require a higher temperature than will pure solvent to achieve any given vapor pressure, including one...

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Related Experiment Video

Updated: Nov 26, 2025

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

Published on: January 24, 2014

Absolute chemical potentials for complex molecules in fluid phases: A centroid reference for predicting phase

Vikram Khanna¹, Michael F Doherty¹, Baron Peters²

¹Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, California 93106, USA.

The Journal of Chemical Physics

|December 9, 2020

Summary

This summary is machine-generated.

Predicting solid-fluid equilibria is simplified by bypassing direct phase transformation. This new simulation method uses distinct reference systems for solid and fluid phases, enabling accurate predictions of properties like sublimation vapor pressure.

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Last Updated: Nov 26, 2025

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

Published on: January 24, 2014

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Published on: April 8, 2020

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Published on: September 4, 2015

Area of Science:

Computational chemistry and physics
Thermodynamics and statistical mechanics

Background:

Solid-fluid phase equilibria are challenging to simulate due to the complex transformation of degrees of freedom.
Traditional methods require computationally intensive solid-to-fluid transformations.

Purpose of the Study:

To develop a novel simulation approach that bypasses the direct solid-to-fluid transformation step.
To accurately predict solid-fluid phase equilibria using reference systems.

Main Methods:

Utilizing separate chemical potentials for independent fluid and solid reference systems.
For the solid phase, transforming from an Einstein crystal to a fully interacting molecular crystal.
For the fluid phase, introducing a new 'centroid' reference system and transforming to gas phase molecules.

Main Results:

Successfully predicted the sublimation vapor pressure of succinic acid.
Demonstrated the feasibility of the new method for phase equilibria calculations.

Conclusions:

The proposed method offers a more efficient alternative for simulating solid-fluid phase equilibria.
This approach simplifies calculations by avoiding direct phase transformations and employing novel reference systems.