What must an Arrhenius base contain?
An Arrhenius base must contain hydroxide ions.
An Arrhenius base is a substance that dissociates in water and increases the concentration of hydroxide ions. For example,
NaOH(aq) → Na⁺(aq) + OH⁻(aq)
To do this, Arrhenius believed the base must contain hydroxide (OH⁻) in the formula.
This makes the Arrhenius model limited, because it cannot explain the basic properties of aqueous solutions of ammonia (NH₃).
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
To get around this, Arrhenius believed that ammonia must be NH₄OH.
We now know that there is no such compound as NH₄OH. Instead, ammonia reacts reversibly with the water to form small amounts of hydroxide ions. Most of the ammonia in solution is still NH₃ molecules.