Melting points vary from substance to substance. Ice melts at 0 degrees C, but gold melts at 1,063 degrees C and oxygen melts at a frigid minus 218.79 degrees C, according to the Department of Physics and Astronomy of Georgia State University. Moreover, a given substance does not always melt at the same temperature. These phenomena result from various factors that affect the melting point.
Ionic compounds, such as NaCl, do not exist in the form of discrete molecules, but as positive and negative ions attracted to each other by electrostatic charges. Because of the strength of this electrostatic attraction, ionic compounds often form crystal lattices in which each ion is surrounded by ions of the opposite charge. Breaking up an ionic crystal lattice requires considerable energy, especially if the positive and negative ions occur in a stable symmetrical array. As a result, ionic compounds have high melting points. For example, the melting point of NaCl is 801 degrees C, according to the University of Babylon.
Covalent compounds consist of molecules rather than ions. Since the attraction between molecules is weaker than is the attraction between ions, covalent compounds usually have a lower melting point than do ionic compounds. Moreover, some covalent compounds have polar molecules in which one end is more electronegative than is the other. Because of the moderate electrostatic forces that holds them together, such polar compounds have a higher melting point than do compounds with nonpolar molecules, which cohere solely as a result of a weak intermolecular force called van der Waals attraction. The greater the polarity of a compound, the higher its melting point will be.
Shape of Molecules
Molecular shape influences the melting point of a substance. If the shape of the molecules allows them to be packed together in a compact fashion, the substance has a higher melting point than does a similar compound with oddly shaped molecules that do not pack well. For this reason, symmetrical molecules give neopentane a higher melting point than that of isopentane, the molecules of which do not pack well, according to the University of Iowa.
Size of Molecule
Molecular size affects the melting point. When other factors, such as polarity, are equal, smaller molecules melt at lower temperature than do larger molecules. For example, ethyl alcohol liquefies at cryogenic temperatures, while the large cellulose molecule remains solid at temperatures well above the boiling point of water. Diamond also has a high melting point because many carbon atoms unite together to form one large molecule. However, in the case of ionic compounds, those with smaller ions have a higher melting point than that of compounds with larger ions because smaller ions can get closer together and form more stable bonds.
Melting Point Variations
Other factors can alter the melting point of a given substance. If a solute, such as salt, is added to water, the solution freezes at a lower temperature. Pressure also affects the melting point. It raises the melting point of most substances, but lowers the melting point of water, according to the School of Physics of the University of New South Wales.