onephase

Onephase, in scientific usage, denotes a system that exists in a single thermodynamic phase, such as a homogeneous liquid, solid, or gas. The term is used across thermodynamics, materials science, and chemical engineering to distinguish from multi-phase systems in which two or more phases coexist in equilibrium (for example, solid plus liquid, or immiscible liquid layers).

In phase diagrams, one-phase regions are areas where the material is thermodynamically stable as one phase. For water at room temperature and atmospheric pressure, the liquid is a single liquid phase; at conditions within the region, the system shows uniform density, composition, and properties, and there is no internal interface.

In practice, determining whether a system is one-phase can involve observing homogeneity, measuring property variations, and comparing to thermodynamic models. In experiments, absence of a visible interface or of phase separation suggests a one-phase region.

In mathematical modeling, one-phase models treat the domain as a single phase with spatially varying properties, contrasting with two-phase models that must track interfaces and relative motion between phases. One-phase models are common in fluid flow through porous media, heat conduction in homogeneous solids, or single-component diffusion.

Limitations: In real systems, conditions can cross into two-phase regions, causing phase transitions and latent heat effects. See also phase diagram, phase transition, and multiphase modeling.