Which of the following is not an alkali?

Analyze the question: We need the substance that is not an alkali. An alkali is a base that dissolves in water to give hydroxide ions (OH⁻). So it must be both a base and soluble in water.

Concept:

• Bases: substances that neutralise acids.
• Alkalis: the soluble bases. All alkalis are bases, but not all bases are alkalis.
• Common alkalis: NaOH, KOH, Ca(OH)₂ (sparingly soluble, but it gives limewater), and NH₃ (aqueous ammonia, which forms NH₄⁺ and OH⁻).

Checking each option:

• A. Ca(OH)₂: Slightly soluble in water and ionises to give OH⁻ (limewater). It is an alkali.
• B. NH₃: Dissolves in water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. It is a weak alkali.
• C. NaOH: Very soluble and fully ionised. It is a strong alkali.
• D. Al(OH)₃: Almost insoluble in water, so it does not release enough OH⁻ to be an alkali. It is also amphoteric, reacting with both acids and strong bases. It is a base/amphoteric hydroxide, not an alkali.

Answer: D. Al(OH)₃.

Tip: Remember that the hydroxides of Group 1 metals (Na, K) and some of Group 2 (Ca, Ba) are alkalis, as is aqueous ammonia. Hydroxides of Al, Zn and Pb are amphoteric and insoluble, so they are not alkalis. If you see "not an alkali" in JAMB, look for Al(OH)₃, Zn(OH)₂, Fe(OH)₃ or Cu(OH)₂.