Which of the following hydroxides is not readily soluble in water?

Analyze the question: We need the hydroxide that is not readily soluble in water. This is about solubility rules for hydroxides.

Concept:

• Hydroxides of Group 1 metals (Li, Na, K, etc.) are very soluble in water. These are the alkali metal hydroxides.
• Ammonium hydroxide (NH₄OH, aqueous ammonia) is also very soluble, since ammonia gas dissolves readily in water.
• Hydroxides of Group 2 metals increase in solubility down the group: Mg(OH)₂ is almost insoluble, Ca(OH)₂ is only sparingly soluble, and Ba(OH)₂ is fairly soluble.
• Most other metal hydroxides (Fe, Cu, Zn, Al, Pb) are insoluble.

Solution:

• A. NH₄OH: readily soluble
• B. Ca(OH)₂: only sparingly soluble (about 1.5 g per litre at room temperature). This is why limewater is a dilute, clear solution, and why adding more solid Ca(OH)₂ gives a milky suspension.
• C. NaOH: very soluble (caustic soda dissolves readily and gives off heat)
• D. KOH: very soluble

So the hydroxide that is not readily soluble is Ca(OH)₂.

Answer: B

Exam tip: Remember the pattern "Group 1 hydroxides and NH₄OH dissolve easily; Ca(OH)₂ is sparingly soluble." Limewater, used to test for CO₂, is a very dilute solution of Ca(OH)₂, which reminds you that calcium hydroxide does not dissolve much. Also, when you see a choice of Na, K, NH₄ and Ca, the Ca compound is usually the odd one out for solubility.