Melting point

A melting point is the specific temperature where a substance changes state from a solid to a liquid. This transition is a fundamental property used to identify materials and check their purity. At this exact temperature, the solid and liquid phases exist in a state called equilibrium. This means the two forms can exist together at the same time. While we often talk about melting, the reverse process of a liquid turning into a solid is called the freezing point or crystallization point. For most materials, these two temperatures are nearly identical.

The process of melting requires more than just reaching a specific temperature. To melt a solid, you must first add heat to raise its temperature to the melting point. Once that point is reached, you must continue adding heat to complete the phase change. This extra energy is known as the heat of fusion, which is a type of latent heat. From a scientific perspective, melting occurs when the Gibbs free energy of the liquid becomes lower than the Gibbs free energy of the solid. During this transition, the enthalpy and entropy of the material both increase.

Temperature and pressure are closely linked during this change. The melting point of a substance depends on the amount of pressure applied to it. This relationship is described by the Clausius–Clapeyron relation. In most cases, increasing the pressure raises the melting point. This happens because most solids are denser than their liquid forms. However, water is a notable exception to this rule. For water, the melting point actually decreases as pressure increases. Other substances, such as silicon, germanium, gallium, and bismuth, also show this unusual behavior.

Scientists use various specialized tools to measure these temperatures accurately. A Kofler bench is a metal strip with a temperature gradient ranging from room temperature to 300 °C. By placing a sample on the strip, researchers can observe its thermal behavior at different points. Another common tool is the automatic digital melting point meter. For extremely hot materials, known as refractory materials, scientists use a black body furnace. They may also use an optical pyrometer to measure the light intensity of a hot object. This device matches the light from the sample to a calibrated source to find the temperature.

Materials vary wildly in their heat resistance. Tungsten is a metal with a very high melting point of 3,695 °C. This makes it an excellent choice for use as electrical filaments in incandescent lamps. Carbon is another interesting case. At normal atmospheric pressure, carbon does not melt but instead undergoes sublimation at about 3,975 K. A liquid phase of carbon only exists at pressures above 10 MPa. Even more extreme is hafnium carbonitride (HfCN). This refractory compound has the highest known melting point of any substance. It is the only confirmed material with a melting point above 4,000 °C at ambient pressure.

Measuring melting points also helps determine how pure a substance is. A pure substance will melt at a single, sharp temperature. If a substance is impure or is a mixture, its melting point will be lower. Mixtures also melt over a wider temperature range called the pasty range. The temperature where melting begins is the solidus, and the temperature where it ends is the liquidus. Some special mixtures, called eutectics, behave like single phases. They melt sharply at a constant temperature to form a liquid of the same composition.

Not all materials follow the same rules of solid and liquid transitions. Crystalline solids have clear melting points, but glasses do not. Instead of melting sharply, glasses undergo a smooth glass transition into a viscous liquid. They gradually soften as they are heated. At the other end of the temperature scale, helium is very unusual. Helium will not freeze at normal pressure even at temperatures near absolute zero. To make helium freeze, you must apply a pressure of more than twenty times normal atmospheric pressure.