SafekipediaMelting point
Adapted from Wikipedia · Adventurer experience
The melting point of a substance is the temperature at which it changes state from a solid to a liquid. At this temperature, the solid and liquid parts 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 a liquid turns into a solid, we call this the freezing point.
Examples
Further information: List of elements by melting point
For most materials, the temperature where they change from solid to liquid is about the same when they change back. 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.
| List of common chemicals | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chemical | Density (g/cm3) | Melt (K) | Boil (K) | |||||||||
| Water @STP | 1 | 273 | 373 | |||||||||
| Solder (Pb60Sn40) | 461 | |||||||||||
| Cocoa butter | 307.2 | - | ||||||||||
| Paraffin wax | 0.9 | 310 | 643 | |||||||||
| Hydrogen | 0.00008988 | 14.01 | 20.28 | |||||||||
| Helium | 0.0001785 | — | 4.22 | |||||||||
| Beryllium | 1.85 | 1,560 | 2,742 | |||||||||
| Carbon | 2.267 | — | 4,000 | |||||||||
| Nitrogen | 0.0012506 | 63.15 | 77.36 | |||||||||
| Oxygen | 0.001429 | 54.36 | 90.20 | |||||||||
| Sodium | 0.971 | 370.87 | 1,156 | |||||||||
| Magnesium | 1.738 | 923 | 1,363 | |||||||||
| Aluminium | 2.698 | 933.47 | 2,792 | |||||||||
| Sulfur | 2.067 | 388.36 | 717.87 | |||||||||
| Chlorine | 0.003214 | 171.6 | 239.11 | |||||||||
| Potassium | 0.862 | 336.53 | 1,032 | |||||||||
| Titanium | 4.54 | 1,941 | 3,560 | |||||||||
| Iron | 7.874 | 1,811 | 3,134 | |||||||||
| Nickel | 8.912 | 1,728 | 3,186 | |||||||||
| Copper | 8.96 | 1,357.77 | 2,835 | |||||||||
| Zinc | 7.134 | 692.88 | 1,180 | |||||||||
| Tin | 7.289 | 505.08 | 2,875 | |||||||||
| Gallium | 5.907 | 302.9146 | 2,673 | |||||||||
| Silver | 10.501 | 1,234.93 | 2,435 | |||||||||
| Cadmium | 8.69 | 594.22 | 1,040 | |||||||||
| Indium | 7.31 | 429.75 | 2,345 | |||||||||
| Iodine | 4.93 | 386.85 | 457.4 | |||||||||
| Tantalum | 16.654 | 3,290 | 5,731 | |||||||||
| Tungsten | 19.25 | 3,695 | 5,828 | |||||||||
| Platinum | 21.46 | 2,041.4 | 4,098 | |||||||||
| Gold | 19.282 | 1,337.33 | 3,129 | |||||||||
| Mercury | 13.5336 | 234.43 | 629.88 | |||||||||
| Lead | 11.342 | 600.61 | 2,022 | |||||||||
| Bismuth | 9.807 | 544.7 | 1,837 | |||||||||
Notes Z is the standard symbol for atomic number; C is the standard symbol for heat capacity; and χ is the standard symbol for electronegativity on the Pauling scale. Helium does not solidify at a pressure of one atmosphere. Helium can only solidify at pressures above 25 atmospheres, which corresponds to a melting point of absolute zero. Carbon does not melt at any temperature under standard pressure, instead it sublimes around 4,100 K | ||||||||||||
Melting point measurements
Main article: Melting-point apparatus
There are many ways scientists find the melting point of a substance. One common tool is a Kofler bench, 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 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.
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, but it helps scientists find melting points for very tough materials.
Thermodynamics
To make a solid turn into a liquid, we first need to heat it up to its melting point. Even after it reaches that temperature, we still need to add more heat for the melting to 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 and enthalpy of the material increase. This change happens at a specific temperature depending on the pressure.
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. Some special mixtures 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 usually does. This is because the added substance changes how the liquid works. People use this in real life. For example, they put salt on ice to stop it from freezing, or they mix special liquids with water in car radiators to keep the water from freezing when it is cold outside.
Carnelley's rule
In organic chemistry, Carnelley's rule says that substances with high molecular symmetry often have high melting points. This rule was made in 1882 by Thomas Carnelley.
High melting points happen when a lot of heat is needed to change the solid to a liquid. Symmetrical molecules fit together tightly in their solid form, which makes them harder to melt.
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. In 2011, a company shared data about the melting points of thousands of materials.
Melting point of the elements
The melting point is the temperature at which a solid turns into a liquid. At this temperature, the solid and liquid can exist together in balance. The melting point can change with pressure, but it is often given for a standard pressure of 1 atmosphere or 100 kPa.
Related articles
This article is a child-friendly adaptation of the Wikipedia article on Melting point, available under CC BY-SA 4.0.
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