Which of the following gives a precipitation when treated with NaOH Solution?

Analysis

The question asks us to identify which of the given substances will form a precipitate when reacted with Sodium Hydroxide (NaOH) solution. In chemistry, a precipitate is an insoluble solid that emerges from a liquid solution.

Concept

When NaOH is added to a solution, it provides hydroxide ions ($OH^-$). If these ions react with a metal cation to form an insoluble metal hydroxide, a precipitate will be observed.

• Aluminium ions ($Al^{3+}$) react with $OH^-$ to form Aluminium hydroxide, $Al(OH)_3$, which is a white gelatinous precipitate.
• Ammonium ions ($NH_4^+$) react with $OH^-$ to produce ammonia gas ($NH_3$) and water, not a precipitate.
• Sodium ions ($Na^+$) and Carbonate ions ($CO_3^{2-}$) are generally soluble in water, so they do not form precipitates with NaOH.

Step-by-Step Solution

1. Evaluate Option A ($AlCl_3$): When $AlCl_3$ reacts with $NaOH$, the following reaction occurs: $AlCl_3(aq) + 3NaOH(aq) \rightarrow Al(OH)_3(s) + 3NaCl(aq)$ $Al(OH)_3$ is an insoluble white precipitate. This fits the criteria.

2. Evaluate Option B ($NH_4Cl$): $NH_4Cl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l) + NH_3(g)$ This reaction produces ammonia gas (which has a pungent smell), but no precipitate is formed.

3. Evaluate Option C ($Na_2CO_3$): $Na_2CO_3$ is a sodium salt. Sodium salts are highly soluble in water. Adding $NaOH$ (which also contains sodium) will not result in any reaction or precipitation.

4. Evaluate Option D ($CH_3COONa$): This is Sodium ethanoate. Like $Na_2CO_3$, it is a soluble salt and will not form a precipitate when more sodium ions and hydroxide ions are added.

Conclusion

Only $AlCl_3$ produces an insoluble hydroxide precipitate upon the addition of $NaOH$.

Exam Tip

Remember the "Solubility Rules": Most metal hydroxides are insoluble in water (except for Group 1 metals like $Na^+$ and $K^+$). Since $Al^{3+}$ is not a Group 1 metal, its hydroxide is insoluble and will form a precipitate.