Main note on Melting point

The temperature at which a pure, crystalline solid melts to become a liquid is known as its melting point. For small samples of a pure substance, the transition between solid and liquid is so sharp that melting points can be determined to 0.1oC. Solid oxygen, for example, has a melting point of -218.4oC.

The freezing point of liquids is the temperature during which they change from liquid to solid. In theory, a solid’s melting point should be the same as a liquid’s freezing point. 

Heating a solid over its melting point is difficult, if not impossible, because the heat that enters the solid above its melting point is used to turn the solid into a liquid. Some liquids, on the other hand, can be cooled to degrees below their freezing points without solidifying. The liquid is considered to be supercooled when this is done.

Melting point

The temperature at which the solid and liquid phases of a pure substance can coexist in equilibrium. The temperature of a solid rises when it is heated until it hits the melting point. More heat will then convert the solid to a liquid while maintaining the same temperature. Additional heat will boost the temperature of the liquid once all of the solid has melted. The melting point of crystalline solids is a useful metric for identifying pure compounds and elements. The majority of mixtures and amorphous materials melt at a wide range of temperatures. 

Measurement of melting point

One of the approaches to determine if a substance is pure is to determine its melting point. A pure substance typically has a melting range of one or two degrees (the range between the temperature at which the sample begins to melt and the temperature at which melting is complete). Because impurities tend to lower and broaden the melting range, the purified sample should have a higher and narrower melting range than the impure original.

Melting points can be determined using a variety of laboratory procedures. A Koffler bench that is a metal strip with a thermal gradient (between ambient temperature and300oC). Any substance could be placed on a part of the strip, which will reveal its thermal behavior at that temperature. Differential scanning calorimetry provides data on melting point as well as fusion enthalpy.

Mixed melting point

The inclusion of a foreign material reduces the melting point of a pure organic compound in the vast majority of cases. 

The technique of “mixed melting point” is used to identify chemical compounds. It’s especially useful for organic compounds, when a known-identity sample with a known melting point is mixed with an unknown purified sample to ascertain the melting point.

Mixed melting points are restricted in their utility because you need to know something about the chemical makeup of your unknown molecule and have a sample of the probable compound on hand.

Latent Heat

A substance’s energy received or released during a transition in its physical state (phase) without altering its temperature. The heat of fusion is the latent heat related to melting a solid or freezing a liquid; the heat of vaporization is the latent heat connected with vaporizing a liquid or solid or condensing a vapor. The quantity of heat (in joules or calories) per mole or unit mass of the substance experiencing a change of state is commonly expressed as latent heat.

When a kettle of water is continuously boiling, the temperature remains at 100oC (220oF) until the last drop evaporates, since all of the heat added to the liquid is absorbed as latent heat of vaporisation and carried away by the existing vapour molecules. Similarly, as ice melts, it maintains a temperature of 0 °C (32 °F), and the liquid water created by the latent heat of fusion maintains a temperature of 0 °C. Water has a heat of fusion of roughly 334 joules (79.7 calories) per gramme at 0 degrees Celsius, and a heat of vaporisation of approximately 2,230 joules (533 calories) per gramme at 100 degrees Celsius. Steam carries a lot of thermal energy that is released as it condenses because the heat of vaporisation is so high, making the water a great working fluid for heat engines.

Conclusion

A substance’s melting point is the temperature at which a liquid and solid phase can coexist in equilibrium, as well as the temperature during which matter transitions from solid to liquid form. Pure liquids and solutions are described as “pure.” Because the melting point is affected by pressure, it should be mentioned. Melting point tables are usually for standard pressures, such as 100 kPa or 1 atmosphere. The liquefaction point is the same as the melting point.