An aqueous solution conducts electric current efficiently when it contains mobile ions. A substance that produces ions when dissolved in water is an electrolyte.

Soluble ionic compounds usually dissociate into their constituent ions. For example,

$$\mathrm{CaCl_2(s)\rightarrow Ca^{2+}(aq)+2Cl^-(aq)}.$$

The formula determines the ion ratio. If a $0.150,\mathrm M$ calcium chloride solution is treated as completely dissociated, then

$$[\mathrm{Ca^{2+}}]=0.150,\mathrm M$$

and

$$[\mathrm{Cl^-}]=2(0.150)=0.300,\mathrm M.$$

Square brackets such as $[\mathrm{Cl^-}]$ denote molar concentration of that species.

A strong electrolyte produces ions essentially completely under the stated conditions. A weak electrolyte produces only a fraction of the possible ions, so substantial neutral solute remains present. A nonelectrolyte can dissolve while remaining mainly as neutral molecules and therefore contributes little ionic conductivity.

These categories describe ion production in solution, not simply whether a substance dissolves. Sugar dissolves readily in water but remains molecular and is a nonelectrolyte. A sparingly soluble ionic solid may produce ions in the small amount that dissolves, yet its overall ion concentration can remain low because little solid enters solution.

For weak electrolytes, the fraction ionized depends on chemical equilibrium and conditions. Quantitative prediction of that fraction belongs to later equilibrium and acid-base chemistry; the present distinction is whether dissolution creates mobile charged species.