Question

When we write `NaCl(s) stackrel(H_2O)rarr Na^+ + Cl^-`, what do we mean by `Na^+(aq)`, and `Cl^(-)(aq)`?

Answer 1

When salt dissolves in water we commonly write:

`NaCl(s) rarr Na^(+)(aq) + Cl^(-)(aq)`

Explanation:

What do we mean by `(aq)`?

In fact, we mean the `"solvated"` or `"aquated"` ion. That is `Na^(+)(aq)` is a sodium ion, surrounded by or aquated by up to 6 water molecules: i.e. `[Na(OH_2)_6]^+`, an ion-dipole complex. We could similarly represent the gegenion, `Cl^(-)(aq)`, though here the partial positive charge of the hydrogen in water would solvate the anion.

And note that this dissolution represents an unequivocal example of chemical change in that strong (electrostatic) bonds have been broken, and new bonds have been formed.