Melting point is the temperature at which a solid turns into a liquid. In theory, the melting point of a solid is the same as the freezing point of the liquid – the point at which it turns into a solid. For example, ice is a solid form of water that melts at 0 degrees Celsius/32 degrees Fahrenheit and changes to its liquid form. Water freezes at the same temperature and turns into ice. It's difficult to heat solids to temperatures above their melting points, so finding the melting point is a good way to identify a substance.
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Molecular composition, force of attraction and the presence of impurities can all affect the melting point of substances.
Composition of Molecules
When molecules are tightly packed together, a substance has a higher melting point than a substance with molecules that do not pack well. For example, symmetrical neopentane molecules have a higher melting point than isopentane, in which molecules do not pack well. Molecular size also affects the melting point. When other factors are equal, smaller molecules melt at lower temperatures than larger molecules. For example, the melting point of ethanol is -114.1 degrees Celsius/-173.4 degrees Fahrenheit, while the melting point of the larger ethyl cellulose molecule is 151 degrees Celsius/303.8 degrees Fahrenheit.
Macromolecules have giant structures made up of many nonmetal atoms joined to adjacent atoms by covalent bonds. Substances with giant covalent structures, such as diamond, graphite and silica, have extremely high melting points because several strong covalent bonds must be broken before they can melt.
Force of Attraction
A strong attraction between molecules results in a higher melting point. In general, ionic compounds have high melting points because the electrostatic forces connecting the ions – the ion-ion interaction – are strong. In organic compounds, the presence of polarity, especially hydrogen bonding, usually leads to a higher melting point. The melting points of polar substances are higher than the melting points of nonpolar substances with similar sizes. For example, the melting point of iodine monochloride, which is polar, is 27 degrees Celsius/80.6 degrees Fahrenheit, while the melting point of bromine, a nonpolar substance, is -7.2 degrees Celsius/19.04 degrees Fahrenheit.
Presence of Impurities
Impure solids melt at lower temperatures and may also melt over a wider temperature range, known as melting point depression. The melting point range for pure solids is narrow, usually only 1 to 2 degrees Celsius, known as a sharp melting point. Impurities cause structural defects that make the intermolecular interactions between the molecules easier to overcome. A sharp melting point is often evidence that a sample is fairly pure, and a wide melting range is evidence that it is not pure. For example, a pure organic crystal has uniform molecules, perfectly packed together. However, the crystals are impure when they occur in a mixture of two different organic molecules because they don't fit together well. It takes more heat to melt the pure structure.