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The nai melting point is the temperature at which a solid is able to melt. It is a useful technique to determine the purity of a chemical compound and can also be used to identify mixtures. The melting point of a pure substance is higher and has a narrower range than the melting point of an impure substance or mixture.
Generally, ionic compounds have higher melting points than covalent compounds, because of the electrostatic forces connecting the ions (the ion-ion interaction). These interactions are stronger than molecular-molecular or polar interactions that occur in covalent compounds.
A cation has a positive charge and an anion has a negative charge. In ionic compounds, the ions have a lattice structure of atoms and this causes the bonds between the ions to release quite a lot of energy when they melt.
This is what explains why barium oxide has a much higher melting point than sodium chloride, even though both have similar ionic distances and are ionized by the same electrolyte. The difference is more than 1000 degrees Celsius!
In ionic compounds, the attractive electrostatic forces between the ions increase with decreasing bond length and this increases the strength of the ionic bond. The short-range repulsive forces between the atoms in an ionic bond are also strong, but they do not increase as rapidly as the attractive electrostatic forces.
The ionic size of the ions affects the melting point, as smaller ions are more closely packed and therefore have greater attractions between them. Rubidium iodide, for example, has a lower melting point than sodium chloride because the ions are more tightly packed and therefore have less attractions between them.