91Ó°ÊÓ

Ionic solids have high melting points because of the strong electrostatic forces between the positively charged cations and the negatively charged anions. To break these ionic bonds, a large amount of energy must be supplied, resulting in high melting points for ionic solids. However, when an ionic solid dissolves in water, the bonds between the ions can be broken without supplying a lot of energy. This is due to the interaction between ions and water molecules.

Water is a polar molecule, meaning that it has positively and negatively charged regions. When an ionic solid is placed in water, the negatively charged oxygen atoms in water molecules surround the positively charged ions, and the positively charged hydrogen atoms in water molecules surround the negatively charged ions. These interactions between the water molecules and the ions of an ionic solid are known as 'solvation' or 'hydration.'

When soluble ionic compounds dissolve in water, the overall process involves two energy changes. First, energy must be supplied to break the ionic bonds between the ions in the solid. This energy is called lattice energy. Secondly, energy is released when the ions are hydrated by the water molecules. This energy is called hydration energy.

The temperature remains almost constant during the dissolution of soluble ionic compounds in water because the energy required to break the interactions between the ions (lattice energy) is approximately balanced by the energy released when ions are hydrated (hydration energy). If the hydration energy is greater than the lattice energy, the dissolution process will be exothermic (releases heat), leading to an increase in temperature. Conversely, if the hydration energy is less than the lattice energy, the dissolution process will be endothermic (absorbs heat), leading to a decrease in temperature. In most cases, the dissolution of soluble ionic compounds in water results in only a small temperature change because the difference between the lattice and hydration energies is not significant. Therefore, even though ionic solids have high melting points, they can dissolve in water with little or no temperature change due to the balance of energy changes in the dissolution process.