Which of the following is NOT an alkali?
NH3
Mg(OH)2
Ca(OH)2
NaOH
0
Answer: Mg(OH)2
Explanation: An alkali is a base that dissolves in water to produce hydroxide ions (OH−).
NaOH is highly soluble (strong alkali). Ca(OH)2 is sparingly soluble but still forms an alkaline solution. NH3 dissolves in water and generates OH− via \( \mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-} \), so aqueous ammonia is alkaline. Mg(OH)2 is practically insoluble in water, so it does not produce a significant amount of OH− in solution and is therefore not an alkali.
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