Dissociation
Definition and meaning of Dissociation in chemistry.
Dissociation is the reversible separation of a compound into two or more simpler ions, atoms, or molecules, most commonly observed when an ionic solid dissolves in a solvent or a molecule breaks apart upon heating.
In more detail
When an ionic solid dissolves in water, polar water molecules surround and pull apart the lattice into hydrated ions. Strong electrolytes (like most soluble salts and strong acids/bases) dissociate essentially completely, while weak electrolytes dissociate only partially, setting up a dynamic equilibrium described by a dissociation constant. Because the process is reversible, the reformed ions can reassociate, and the extent of dissociation depends on concentration, temperature, and solvent. Dissociation also occurs in the gas phase, where heat can split a molecule into smaller fragments, as in the thermal dissociation of N2O4 into NO2.
Key facts
| Field | General Chemistry |
|---|---|
| Example equation | NaCl(s) → Na⁺(aq) + Cl⁻(aq) |
| Key quantity | Degree of dissociation (α), fraction 0–1 |
| Governs | Equilibrium constant (Kd or Ka/Kb for acids/bases) |
When sodium chloride dissolves in water, it dissociates completely: NaCl(s) → Na⁺(aq) + Cl⁻(aq).
Frequently asked questions
Is dissociation the same as ionization?
Not exactly. Dissociation is the separation of an already-ionic substance (like NaCl) into its existing ions, while ionization is the creation of ions from a neutral molecule through reaction with a solvent, as when HCl gas reacts with water to form H3O+ and Cl−. In introductory chemistry the terms are often used interchangeably for acids and bases.
What determines whether dissociation is complete or partial?
It depends on whether the substance is a strong or weak electrolyte. Strong electrolytes, such as NaCl or HCl, dissociate essentially 100% in water, while weak electrolytes, such as acetic acid, dissociate only partially, reaching an equilibrium governed by a dissociation constant.