In chemistry, materials science, and physics, the liquidus is a line on a phase diagram above which a given substance is stable in the liquid phase. Most commonly, this line represents a transition temperature. The liquidus may be a straight line, or it may be curved, depending upon the substance. The liquidus is most often applied to binary systems such as solid solutions , including metal alloys.
The liquidus may be contrasted to the solidus. The liquidus and solidus do not necessarily align or overlap; if a gap exists between the liquidus and solidus, then within that gap, the substance is not stable as either a liquid or a solid.