Why calcium ions do not form precipitate with aqueous ammonia| Qualitative analysis QA

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In this Chemistry learning video, we focus on why no precipitate is formed when aqueous ammonia is added to calcium salts.

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Qualitative Analysis is the process of identifying unknown substances in a solution.

Students are required to describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the following aqueous cations: aluminium (Al3+), ammonium (NH4+), calcium (Ca2+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+), lead(II) (Pb2+) and zinc (Zn2+) (formulae of complex ions are not required).

For easy memory, you may wish to first divide the cations into 2 groups – precipitate formed and no precipitate formed.

No Precipitate formed
If no precipitate is formed, we can confirm that the ion present is calcium ion.

Reason:

Calcium hydroxide is partially soluble/ sparingly soluble. This means that it will be soluble if the concentration of calcium and hydroxide ions are low. If the concentration of these ions are high, white precipitate will form.

Aqueous ammonia is a weak alkali. It partially ionizes in aqueous solution to form low concentration of hydroxide ions.

Since concentration of hydroxide ions is low, calcium hydroxide remains soluble. Hence, no precipitate is formed.

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