Question

Why do ionic compounds have high melting points?

Answer 1

Hint: Melting point of any compound is dependent on packing of lattice points and three dimensional network of the compound. It is also dependent on charge and radius ratio of cation and anions as well.

Complete step-by-step solution:

Let us first understand the meaning of ionic compounds. An Ionic compound is composed of ions that are held together by electrostatic forces of attraction or bonds. These compounds are neutral overall, but consist of positively charged ions called cations and negatively charged ions called anions. For example ${ NH }_{ 4 }{ Cl }$ is an ionic compound composed of the cation ${ NH }_{ 4 }^{ + }$ and the anion ${ Cl }^{ - }$. Ionic compounds do not exist as molecules but as a crystalline solid that is a three dimensional structure composed of cations and anions.

Melting point of a substance is the temperature at which it melts i.e. changes its state from solid to liquid. At melting point, the solid and the liquid phase exist in equilibrium with each other. It is considered as a characteristic property of a substance as is dependent on pressure.

Ionic compounds have high melting and boiling points because of the electrostatic forces of attraction existing between the cations and the anions. When the charges on the ions are more and the distance between them is small, these forces become more pronounced. This is generally seen in the case of inorganic compounds consisting of simple ions such as ${NaCl}$ that has a high melting point and melts to give molten salt. But compounds with larger ions have melting points that are very close to room temperature and are therefore termed as ionic liquids. Ionic liquids have uneven charge distributions and bulky substituents that result in them having low melting points.

Therefore ionic compounds have high melting points.

Note: Not all ionic compounds show the properties that are generally shown by them. For example ionic liquids have very low melting points despite being ionic in nature. Similarly ${ AlCl }_{ 3 }$ shows molecule like structures when it melts.