Question

Identify the salts P and Q from the observations given below: On performing the flame test salt P produces a lilac coloured flame and its solution gives a white precipitate with silver nitrate solution, which is soluble in Ammonium hydroxide solution.

Answer 1

Hint: We know that mostly chemists have to identify the composition of the unknown salts. There are various experiments that can be performed to identify the cation and anion present in the salt and this is known as salt analysis.

Complete answer:
The basic tests that are performed on a salt are: The appearance of the salt; Heating effect; Flame test; Solubility in water and the reaction of salt with various compounds. Ammonium when heated gives a colored flame whereas magnesium doesn't. Nitrate on heating gives out colored fumes or gas whereas carbonate on heating does not give any gas rather gives a precipitate.
On performing the flame test salt P produces a lilac coloured flame. This indicates potassium salt. Its solution gives a white precipitate with silver nitrate solution, which is soluble in Ammonium hydroxide solution. This indicates the presence of chloride ions.
 $ KCl+AgN{{O}_{3}}\to ~\underset{white~ppt}{\mathop{AgCl}}\,\downarrow +KN{{O}_{3}} $ & $ AgCl+2N{{H}_{4}}OH\to ~~\underset{soluble~complex}{\mathop{\left[ Ag{{\left( N{{H}_{3}} \right)}_{2}}Cl \right]}}\,+2{{H}_{2}}O $
Hence, the salt $ P $ is potassium chloride.

Note:
Remember that, we know that magnesium has a colorless flame, therefore it has to be a calcium compound. Now, it is also given in the question that the substance breaks down on heating giving oxygen and a brown gas. This brown gas shows that it is an oxide of nitrogen that is released and not carbonate.