The melting point of a substance is the temperature at which it changes from a solid to a liquid state. In general, substances with strong intermolecular forces have higher melting points than substances with weak intermolecular forces. Ionic compounds are solids composed of positive and negative ions that are held together by strong electrostatic forces of attraction. These electrostatic forces between the ions become stronger as the charges on the ions increase and as the size of the ions decreases. Therefore, solid ionic compounds generally have high melting points due to the strong electrostatic forces holding the ions in place. The strength of the ionic bonds depends on the charge of the ions and the distance between them. Ions with higher charges and smaller radii have stronger electrostatic attractions and, therefore, form ionic compounds with higher melting points. For example, sodium chloride (NaCl), an ionic compound with a relatively low melting point, melts at 801 °C and consists of sodium ions (Na+) and chloride ions (Cl-). In contrast, magnesium oxide (MgO), an ionic compound with a high melting point, melts at 2800 °C and contains magnesium ions (Mg2+) and oxide ions (O2-). The higher melting point of magnesium oxide is due to the higher charges on both the magnesium and oxygen ions.