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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 Transitions: Vaporization and Condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize (convert into vapor). The process is called vaporization—a surface phenomenon. Vaporization occurs when the thermal motion of the molecules overcome the intermolecular forces, and the molecules (at the surface) escape into the gaseous state. When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase...

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).

Intermolecular Forces and Physical Properties

Intermolecular Forces and Physical Properties

States of Water

States of Water

Water exists in any one of the three classical states: solid (ice), liquid (water), and gas (steam or water vapor). The state of water depends on i) the intermolecular forces that draw molecules together and ii) the kinetic energy that leads to movements that pull them apart.
Water freezes when the intermolecular forces are greater than the kinetic energy. Unlike most other substances, water is less dense in its solid state than in its liquid state. This is because each water molecule can form...

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...

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Related Experiment Video

Updated: Aug 15, 2025

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

Published on: May 15, 2017

Liquid-Liquid Transition in Water from First Principles.

Thomas E Gartner¹, Pablo M Piaggi¹, Roberto Car^1,2,3,4

¹Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.

Physical Review Letters

|January 6, 2023

Summary

Scientists investigated if supercooled water can transition between high- and low-density liquid states. Using advanced simulations, they found definitive computational evidence for a liquid-liquid phase transition (LLT) in water from first principles.

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Area of Science:

Physical Chemistry
Computational Physics
Materials Science

Background:

A long-standing debate in water research concerns the potential for supercooled liquid water to undergo a liquid-liquid phase transition (LLT).
This transition involves a change between high-density liquid (HDL) and low-density liquid (LDL) states.
Understanding this phenomenon is crucial for various scientific disciplines.

Purpose of the Study:

To investigate the possibility of a liquid-liquid phase transition (LLT) in water using ab initio molecular simulations.
To provide definitive computational evidence for the existence of an LLT and its critical point.
To evaluate the LLT in a water model derived from first-principles calculations.

Main Methods:

Employed complementary molecular simulation techniques.
Utilized an ab initio neural network model of water.
The model was trained on density functional theory (DFT) calculations using the SCAN exchange-correlation functional.

Main Results:

Conclusively demonstrated the existence of a first-order liquid-liquid phase transition (LLT).
Identified an associated critical point for the LLT in the SCAN water model.
Provided the first definitive computational evidence for an LLT in water from first principles.

Conclusions:

The study confirms the occurrence of a liquid-liquid phase transition in the ab initio SCAN model of water.
This research offers robust computational support for the LLT hypothesis in supercooled water.
The findings represent a significant advancement in understanding the complex phase behavior of water.