Melting point

The melting point of a substance is the temperature at which it changes state from a solid to a liquid. At this special temperature, the solid and liquid parts of the substance can exist together in balance, called equilibrium.

This temperature depends on the pressure around the substance, but it is usually talked about at a common pressure, like 1 atmosphere or 100 kPa. When we talk about the reverse process, where a liquid turns into a solid, we call this the freezing point. Because liquids can sometimes stay liquid below their normal freezing temperature, the freezing point might seem lower than it really is. In practice, scientists often find this point by watching when ice disappears, which is the same as finding the melting point.

Examples

Further information: List of elements by melting point

For most materials, the temperature at which they turn from solid to liquid and back again is about the same. For example, mercury turns to liquid and freezes at 234.32 kelvins (−38.83 °C; −37.89 °F). However, some materials change at different temperatures depending on whether they are turning to liquid or freezing. Agar, for example, melts at 85 °C (185 °F; 358 K) but freezes at 31 °C (88 °F; 304 K).

The melting point of ice at normal air pressure is very close to 0 °C (32 °F; 273 K), which is called the ice point. The metal with the highest melting point is tungsten, at 3,414 °C (6,177 °F; 3,687 K). This makes tungsten great for use in electrical parts inside old-style lamps. Helium does not freeze at normal pressure even when it is very, very cold.

Melting point measurements

Main article: Melting-point apparatus

There are many ways scientists use to find out the melting point of a substance. One common tool is a Kofler bench, which is a metal strip that changes temperature along its length. By placing a tiny bit of a substance on the strip, you can watch how it behaves as the temperature changes. Another method called differential scanning calorimetry can tell us both the melting point and how much energy is needed for the substance to change from solid to liquid.

A simple melting point apparatus uses a container of oil with a window to see inside. Tiny pieces of a solid are placed in a thin glass tube and put into the oil. As the oil is heated, you can watch the solid turn to liquid at a certain temperature. Some modern tools can do this automatically. In factories, like oil refineries, they can test the freezing point of fuels while the process is running, making it easier to test often.

Techniques for refractory materials

For materials that melt at very high temperatures, special tools are needed. One way is to heat the material in a special furnace and measure its temperature with a device called a pyrometer. This method needs careful calibration and can be tricky, but it helps scientists find melting points even for very tough materials.

Thermodynamics

To make a solid turn into a liquid, we first need to heat it up to its melting point. But even after it reaches that temperature, we still need to add more heat for the melting to actually happen. This extra heat is called the heat of fusion, and it is a type of latent heat.

When a substance reaches its melting point, important things happen inside it. The energy needed for it to change from solid to liquid makes the entropy (a measure of disorder) and enthalpy (a measure of heat content) of the material increase. This change happens at a specific temperature depending on the pressure. For most materials, if they are denser as a solid than as a liquid, their melting point goes up when pressure increases. Water is an exception — its melting point changes in the opposite way when pressure changes. Very high pressures can cause big changes in melting points. For example, silicon melts at 1415 °C under normal pressure, but at very high pressures, this drops to 1000 °C.

Melting points help scientists understand substances and check how pure they are. Pure substances melt at a higher, more specific temperature than mixtures. Mixtures start melting at a lower temperature and over a wider range. Some special mixtures, called eutectics, melt at one sharp temperature like single substances. Unlike regular solids, glasses don’t have a sharp melting point. Instead, they slowly soften when heated.

Freezing-point depression

Main articles: Freezing-point depression and Supercooling

When you add something to a liquid, it can make the liquid freeze at a lower temperature than it normally would. This happens because the added substance changes how the liquid behaves. People use this idea in real life, like when they put salt on ice to keep it from freezing, or when they mix special liquids with water in car radiators to stop the water from freezing in cold weather.

Carnelley's rule

In organic chemistry, Carnelley's rule tells us that substances with high molecular symmetry often have high melting points. This rule was made in 1882 by Thomas Carnelley after he looked at 15,000 chemical compounds. For example, among three similar compounds with the formula C₅H₁₂, the melting point gets higher as the structure becomes more symmetrical.

High melting points happen when the heat needed to change the solid to liquid is high, or when the change in disorder during melting is low, or both. Symmetrical molecules fit together tightly in their solid form, which makes it harder to melt them.

Predicting the melting point of substances (Lindemann's criterion)

In 1910, a scientist named Frederick Lindemann tried to guess when solid materials would start to melt. He noticed that as things get hotter, the tiny parts inside them shake more. Melting starts when these parts shake so much that they start to overlap.

Lindemann said that melting happens when the shaking gets bigger than a certain amount. Scientists use special formulas to estimate this temperature, but they need to know things like how heavy the parts are and how close together they sit. There are many ways to do these calculations, and scientists keep finding new ones. In 2011, a company shared data about the melting points of thousands of materials, which helped scientists make better guesses.

Melting point of the elements

The melting point is the temperature at which a solid changes into a liquid. At this special temperature, the solid and liquid can exist together in balance. The melting point can change depending on the pressure, but it is often given for a standard pressure of 1 atmosphere or 100 kPa.