lesson
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If you dissolve table salt (NaCl) in a beaker of water, you might expect to find only Na+ and Cl− ions swimming around. But liquid water holds a secret: there is always a second pair of ions lurking in the background.
How do these extra ions get there even before you turn on an electric current or add another chemical?
Water Ionizes Itself
Pure water is not just neutral H2O molecules. A tiny fraction of water molecules spontaneously breaks apart in a process called self-ionization (when identical neutral molecules react to form charged ions).
In 1894, German chemist Friedrich Kohlrausch prepared ultra-pure water and proved it still conducted a minute electric current because of this reversible breakdown:
H2O(l)⇌H(aq)++OH(aq)−
Each split produces one positively charged hydrogen ion (H+) and one negatively charged hydroxide ion (OH−). The double arrow indicates a dynamic equilibrium where molecules split and recombine at the exact same rate.
📊Interactive diagram
Even though only about one in every 550 million water molecules is split at any given moment, those ions are always present in every aqueous solution (a solution where liquid water is the solvent).
So what happens to the ion count the instant we dissolve an ionic compound into this water?
The Four-Ion Inventory
When an ionic solid dissolves, water molecules pull the lattice apart into free-floating ions. Because the solvent water already supplies H+ and OH−, an aqueous salt solution contains at least four distinct ions.
To list the ions in any aqueous solution, separate the solute into its ions, then add the ions from water. We group them by charge into cations (positive ions) and anions (negative ions).