While chemically pure materials have a single

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

, chemical mixtures often partially melt at the temperature known as the solidus (''T''_S or ''T''_sol), and fully melt at the higher liquidus temperature (''T''_L or ''T''_liq). The solidus is always less than or equal to the liquidus, but they need not coincide. If a gap exists between the solidus and liquidus it is called the freezing range, and within that gap, the substance consists of a mixture of solid and liquid phases (like a slurry). Such is the case, for example, with the

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

( forsterite- fayalite) system, which is common in Earth's mantle.

Definitions

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

, and

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, the liquidus temperature specifies the temperature above which a material is completely liquid, and the maximum temperature at which

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s can co-exist with the melt in thermodynamic equilibrium. The solidus is the locus of temperatures (a curve on a phase diagram) below which a given substance is completely

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

(crystallized). The solidus temperature specifies the temperature below which a material is completely solid, and the minimum temperature at which a melt can co-exist with

s in thermodynamic equilibrium. Liquidus and solidus are mostly used for impure substances (mixtures) such as

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

es, metal

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

s, ceramics, rocks, and

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s. Lines of liquidus and solidus appear in the phase diagrams of binary solid solutions, as well as in eutectic systems away from the invariant point.

When distinction is irrelevant

For pure elements or compounds, e.g. pure copper, pure water, etc. the liquidus and solidus are at the same temperature, and the term

may be used. There are also some mixtures which melt at a particular temperature, known as congruent melting. One example is eutectic mixture. In a eutectic system, there is particular mixing ratio where the solidus and liquidus temperatures coincide at a point known as the invariant point. At the invariant point, the mixture undergoes a eutectic reaction where both solids melt at the same temperature.

Modeling and measurement

There are several models used to predict liquidus and solidus curves for various systems. Detailed measurements of solidus and liquidus can be made using techniques such as differential scanning calorimetry and differential thermal analysis.

Effects

For impure substances, e.g.

honey Honey is a sweet and viscous substance made by several species of bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of pl ...

soft drink A soft drink (see #Terminology, § Terminology for other names) is a class of non-alcoholic drink, usually (but not necessarily) Carbonated water, carbonated, and typically including added Sweetness, sweetener. Flavors used to be Natural flav ...

, ice cream, etc. the melting point broadens into a melting interval. If the temperature is within the melting interval, one may see "slurries" at equilibrium, i.e. the slurry will neither fully solidify nor melt. This is why new snow of high purity on mountain peaks either melts or stays solid, while dirty snow on the ground in cities tends to become slushy at certain temperatures. Weld melt pools containing high levels of sulfur, either from melted impurities of the base metal or from the welding electrode, typically have very broad melting intervals, which leads to increased risk of hot cracking.

Behavior when cooling

Above the liquidus temperature, the material is homogeneous and liquid at equilibrium. As the system is cooled below the liquidus temperature, more and more

s will form in the melt if one waits a sufficiently long time, depending on the material. Alternately, homogeneous glasses can be obtained through sufficiently fast cooling, i.e., through kinetic inhibition of the crystallization process. The crystal phase that crystallizes first on cooling a substance to its liquidus temperature is termed ''primary crystalline phase'' or ''primary phase''. The composition range within which the primary phase remains constant is known as ''primary crystalline phase field''. The liquidus temperature is important in the glass industry because crystallization can cause severe problems during the glass melting and forming processes, and it also may lead to product failure.

References