Question

Why do ionic compounds dissolve in water?

Answer 1

Ionic compounds dissolve in water because the water molecules hydrate the ions.

Explanation:

To dissolve an ionic compound, the water molecules must be able to stabilize the ions that result from breaking the ionic bond.

They do this by hydrating the ions.

Water is a polar molecule. It has a permanent dipole.

The `"O"` atom has a partial negative charge, and the `"H"` atoms have a partial positive charge.

When you place an ionic substance in water, the water molecules attract the positive and negative ions from the crystal.

The particles are then free to move around within the solution.

(from 2012books.lardbucket.org)

The positive ions have several water molecules around them, all with their `"O"` atoms close to the positive ion.

The negative ions have several water molecules around them, all with their `"H"` atoms close to the negative ion.

The "shell" of water molecules reduces the attractions between the ions. The ions are hydrated.

Here's a video that shows the process in action.

Answer 2

Ionic compounds dissolve in water due to the difference between its lattice energy and its hydration energy.

Explanation:

An ionic compound consists of two oppositely charged ions. Water, on the other hand is a polar solvent; the electronegativity difference between oxygen and hydrogen is high which is why water has a positive pole of `H` and a negative pole `O` (water is `H_2O`).

What happens is, when an ionic compound is put in water, the negative ion, or the anion, attracts the positive `H` pole around it, and the positive ion, or the cation, attracts the negative `O` pole around it. This results in the formation of a unique arrangement called the hydration.

Hydration releases energy, which is known as the hydration energy. Every ionic compound has an energy holding the lattice structure of the compound, known as lattice energy. If the hydration energy of an ionic compound exceeds its lattice energy, the lattice is broken and the ions in the compound separate, causing the compound to dissolve. If the hydration energy of the compound is lesser than the lattice energy, the compound will not dissolve.