Final answer:
The compound that is NOT a strong base among the options is NH₄OH, which is a weak base because it does not fully dissociate in water, unlike KOH, LiOH, and Ca(OH)₂ which are strong bases.
Step-by-step explanation:
The compound that is NOT a strong base among the provided options is NH₄OH (ammonium hydroxide). Strong bases are characterized by their complete dissociation in aqueous solutions into their respective ions. Hydroxides of alkali metals like KOH (potassium hydroxide) and LiOH (lithium hydroxide) as well as heavy alkaline earth metals such as Ca(OH)₂ (calcium hydroxide) are considered strong bases because they fully ionize in water. In contrast, NH₄OH is a weak base, as it does not dissociate completely in water.
For example, KOH and LiOH completely dissociate in water to form K+ and OH- ions and Li+ and OH- ions, respectively. Ca(OH)₂ also dissociates completely into Ca²+ and 2 OH- ions. However, ammonium hydroxide only partially dissociates, which makes it a weak base. It's important to note that the term 'strong' and 'weak' here refer to the degree of dissociation in water, not the concentration of the solution.