The ability of ions to dissolve in water depends a lot on how they interact with water molecules. Here are some important things to know:

1. Size and Charge of Ions: Smaller ions with a higher charge usually don’t dissolve well. For example, $Mg^{2+}$ doesn’t dissolve as easily as $Na^+$ because its stronger charge holds onto water molecules more tightly.

2. Lattice Energy: Ionic compounds have something called lattice energy. This is the energy needed to pull the ions apart. If it takes more energy to separate the ions than what is released when they mix with water, the compound will not dissolve.

3. Hydration Energy: This is the energy released when ions mix with water. When the hydration energy is high, ions can dissolve better. For instance, $K^+$ has a high hydration energy, which helps potassium salts dissolve easily.

4. Type of Ions: Alkali metal ions and ammonium ions ($NH_4^+$) usually dissolve well in water. In contrast, heavy metal ions often make compounds that don’t dissolve.

In summary, it’s all about finding the right balance between these factors!