Just as when a salt crystal dissolves, each ion is thoroughly hydrated by being surrounded by three or four water molecules. Each proton is surrounded by water molecules with their negatively charged oxygens pointed toward it, and each fluoride ion is surrounded by other water molecules with hydrogens pointed at it (see right). Since water molecules are involved in the pulling apart or dissociation of HF, the preceding reaction really should be written

HF + (n + m) H₂0 ----> H+ (H₂0)n + F-(H₂0)m

The subscripts n and m represent the number of water molecules that can be accommodated around each ion. This is 3 or 4 for these small ions, but for larger ions, n and m can be 6 or even greater. This is another example of how a polar liquid such as water is a good solvent for other polar substances or for salts with charged ions. A more general term for hydration when the solvent is a liquid other than water, such as liquid ammonia, is solvation.