The melting point of a compound is the temperature at which the solid phase transitions into the liquid phase at a standard pressure of 1 atmosphere. The melting point of a compound is a physical property, like solubility, density, color, and electronegativity that can be used to identify a compound. Determining the exact temperature at which a compound begins to melt is a challenging task; because of this, the melting point of compounds is reported as a range. The lower limit of the melting point range is the temperature at which the first drops of liquid are observed. The upper limit of the range is the temperature at which all of the solid phase has transitioned to the liquid phase. Reference guides with accepted values exist in the literature, which are used to identify compounds.
One major factor that impacts the melting point of the compound is the type of intermolecular forces that exist within the compound. Intermolecular forces are either attractive or repulsive between the molecules of a compound. In the solid phase, the molecules of a compound will form an organized lattice structure as the molecules are packed close together. There are three major types of intermolecular forces:
Each type of intermolecular force has a different strength of attraction. Therefore, compounds containing hydrogen bonds require more energy to break the attraction between molecules than a nonpolar compound that only has London dispersion forces. Thus, the presence of hydrogen bonds increases the melting point of a compound.
Reported literature values of melting points assume that you have a pure sample of the compound in question. Often in the lab or in unknown samples, the samples being tested are not pure compounds. Impurities cause the observed melting point of a mixture to be lower than the actual melting temperature of the pure compound. The observable range is greater than that of the pure substance.
In a pure compound, the solid is composed of a uniform and ordered structure and requires a certain amount of temperature to break the structure apart for the compound to transition into the liquid phase. In a mixture containing impurities, the solid phase is composed of a disorganized structure. This requires much less energy to transition into the liquid phase, thus lowering the melting point. This phenomenon is known as melting point depression. The more impurities in the sample, the broader the melting point range, and the lower the melting temperature.
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