Question

Explain with examples the use of \(\mathrm{NaOH}\) and \(\mathrm{NH}_{4} \mathrm{OH}\) as reagents for the identification of cations in salts.

Short answer

Question: Explain the use of sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) as reagents for the identification of cations in salts. Answer: Sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) are alkaline reagents used in the identification of cations in salts due to their ability to form specific precipitates and color changes when reacting with different cations. Sodium hydroxide, a strong alkaline reagent, precipitates many metal ions as their hydroxides, with the color and solubility of these precipitates helping to identify the cation. Ammonium hydroxide is a weak alkaline reagent that forms more soluble precipitates, helping to differentiate some cations that form similar precipitates with NaOH. The reactions between these reagents and various cations make it easier to identify the cations present in a given salt.

Step-by-step solution

Introduction

Sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH) are both alkaline reagents that are widely used in the identification of cations in salts. When these reagents react with various cations, they form specific precipitates and color changes, making it easier to identify the cations present in a given salt.

Sodium hydroxide (NaOH) as a reagent

Sodium hydroxide is a strong alkaline reagent that can precipitate many metal ions in the form of their hydroxides. The color, appearance, and solubility of the precipitates can help identify the cations in salts. Here are some examples of how NaOH reacts with different cations: 1. With aluminum ions (Al³⁺): Al³⁺(aq) + 3NaOH(aq) → Al(OH)₃(s) + 3Na⁺(aq) Aluminium ions react with sodium hydroxide to form a white gelatinous precipitate of aluminum hydroxide (Al(OH)₃). 2. With calcium ions (Ca²⁺): Ca²⁺(aq) + 2NaOH(aq) → Ca(OH)₂(s) + 2Na⁺(aq) Calcium ions react with sodium hydroxide to form a white precipitate of calcium hydroxide (Ca(OH)₂). 3. With zinc ions (Zn²⁺): Zn²⁺(aq) + 2NaOH(aq) → Zn(OH)₂(s) + 2Na⁺(aq) Zinc ions react with sodium hydroxide to form a white precipitate of zinc hydroxide (Zn(OH)₂) that is soluble in excess NaOH.

Ammonium hydroxide (NH₄OH) as a reagent

Ammonium hydroxide is a weak alkaline reagent that can also precipitate metal ions as their hydroxides, but it often forms more soluble precipitates. This property can help differentiate some cations that form similar precipitates with sodium hydroxide. Here are some examples of how NH₄OH reacts with different cations: 1. With iron(III) ions (Fe³⁺): Fe³⁺(aq) + 3NH₄OH(aq) → Fe(OH)₃(s) + 3NH₄⁺(aq) Ferric ions react with ammonium hydroxide to form a reddish-brown precipitate of iron(III) hydroxide (Fe(OH)₃). 2. With copper ions (Cu²⁺): Cu²⁺(aq) + 2NH₄OH(aq) → Cu(OH)₂(s) + 2NH₄⁺(aq) Copper ions react with ammonium hydroxide to form a light blue precipitate of copper hydroxide (Cu(OH)₂), which becomes soluble and deep blue in excess NH₄OH due to the formation of a complex ion, [Cu(NH₃)₄]²⁺. 3. With silver ions (Ag⁺): Ag⁺(aq) + NH₄OH(aq) → AgOH(s) + NH₄⁺(aq) Silver ions react with ammonium hydroxide to form a light-brown precipitate of silver hydroxide (AgOH), which disproportionates to silver oxide (Ag₂O) and water. In conclusion, sodium hydroxide and ammonium hydroxide are essential reagents for the identification of cations in salts because they form specific precipitates and color changes with different cations, helping to differentiate between various cations present in salts.