Yes, calcium remains soluble in ammonia solution, specifically in the context of precipitation reactions involving calcium ions. When aqueous ammonia is added to a solution containing calcium ions, calcium hydroxide, which might otherwise precipitate with a strong base, remains soluble.
Understanding Calcium Solubility in Ammonia
Aqueous ammonia (NH₃ dissolved in water) is a common laboratory reagent. It acts as a weak base due to its partial ionization:
NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
This reaction produces a low concentration of hydroxide ions (OH⁻) in the solution.
Why Calcium Hydroxide Remains Soluble
As highlighted in the provided reference:
- Aqueous ammonia is a weak alkali.
- It partially ionizes in aqueous solution.
- This ionization forms a low concentration of hydroxide ions.
- Since the concentration of hydroxide ions is low, calcium hydroxide remains soluble.
- Hence, no precipitate is formed when aqueous ammonia is added to a solution containing calcium ions.
In contrast, adding a strong base like sodium hydroxide (NaOH), which fully dissociates and produces a high concentration of OH⁻ ions, would likely cause calcium hydroxide to precipitate if the calcium ion concentration exceeds the solubility product of Ca(OH)₂.
Implications for Chemistry
This property is important in qualitative analysis and chemical separations. For instance:
- It allows for the selective precipitation of other metal hydroxides (like those of aluminum or iron) from solutions also containing calcium ions, as calcium will stay in solution.
- It helps distinguish between different metal ions based on their reactions with weak bases like ammonia versus strong bases.
In summary, while metallic calcium would react with water present in the ammonia solution, when discussing the solubility of calcium in solution in the context of base reactions, calcium ions effectively remain dissolved because the calcium hydroxide formed from the low concentration of hydroxide ions is soluble.
