Which of the following is NOT an alkali?

Analysis

The question asks us to identify which of the given substances is not an alkali. To answer this, we must distinguish between a "base" and an "alkali."

Concept Explanation

• Base: A base is a substance that reacts with an acid to produce salt and water only. Most metal oxides and hydroxides are bases.
• Alkali: An alkali is a soluble base. In other words, all alkalis are bases, but not all bases are alkalis. Alkalis dissolve in water to produce hydroxide ions ($OH^-$).

Let’s evaluate the options based on their solubility in water:

• NaOH (Sodium Hydroxide): Highly soluble in water. It is a strong alkali.
• Ca(OH)₂ (Calcium Hydroxide): Known as "limewater" when in solution. It is moderately soluble and acts as an alkali.
• NH₃ (Ammonia): When dissolved in water, it forms ammonium hydroxide ($NH_4OH$), which is a weak alkali.
• Mg(OH)₂ (Magnesium Hydroxide): This is a base, but it is insoluble (or very sparingly soluble) in water. Because it does not dissolve significantly to produce a high concentration of $OH^-$ ions in solution, it is classified as a base but not an alkali.

Step-by-Step Solution

1. Identify the definition: An alkali must be soluble in water.
2. Test the options: • NaOH: Soluble $\rightarrow$ Alkali. • Ca(OH)₂: Soluble $\rightarrow$ Alkali. • NH₃: Soluble $\rightarrow$ Alkali. • Mg(OH)₂: Insoluble in water $\rightarrow$ Base, but not an alkali.
3. Conclusion: Since $Mg(OH)_2$ does not dissolve in water to form an alkaline solution, it is the correct choice.

Exam Tip

Remember the mnemonic: "All alkalis are bases, but not all bases are alkalis." If you see a metal hydroxide that is insoluble (like $Mg(OH)_2$, $Fe(OH)_3$, or $Cu(OH)_2$), it is a base, but it is definitely not an alkali.