Which of the following is NOT classified as a strong base?

The classification of strong bases is based on their ability to completely dissociate in water, producing hydroxide ions. Potassium hydroxide (KOH), sodium hydroxide (NaOH), and lithium hydroxide (LiOH) are all alkali metal hydroxides, which are characteristic strong bases. These compounds dissociate completely in aqueous solutions to provide a high concentration of hydroxide ions, resulting in a significant increase in pH.

In contrast, ammonium hydroxide (NH4OH) is the aqueous solution of ammonia (NH3), which is a weak base. While it can accept protons and produce hydroxide ions, it does so only partially; hence it does not fully dissociate in solution. This limited dissociation is why NH4OH is not classified as a strong base, distinguishing it from the other choices listed, which are indeed strong bases. Thus, recognizing the complete versus partial dissociation of these substances is crucial in determining their classification as strong or weak bases.