Question

Provide explaination for the following:

a). Why is it that some combinations of ions do not seem to react while others result in a precipitate?

b). For an insoluble compound, how might you prevent formation of the precipitate?

c). What conditions can you manipulate to make the precipitate more soluble?

d). Does the size of the crystal affect the solublization process?

e). How do you accurately rule out the presence of alkali metals in a solution?

Answer 1

a) Some Inorganic compounds are soluble in water. So if ions present in the solution combine to give a soluble salt there will be no precipitate formation. As an example if there is Cl^- ion present in the sample along with Ag⁺ there will be precipitate formation. But in case of all other cations like Na⁺, K⁺, Ca²⁺ will not give precipitate as chlorides if these cations are soluble in water.

b) The precipitation of an insoluble salt can be prevented by adding a complexing agent. This complexing agent will complex with an ion (preferably the cation) and thus will reduce the effective concentration of the cation in the solution. Presence of ammonia in the solution prevents the precipitation of AgCl as NH₃ complexes Ag⁺ ion.

c) Usually solubility of any salt increases with the increase in temperature of the solution. So, if we increase the temperature of the solution the solubility of the salt will increase and a portion of the precipitate will get soluble.

d) The size of the crystal plays an important role in the solubilization process. As the solubilization process takes place on the surface of the crystal the smaller crystal will get dissolved first as the surface area increases with breaking of larger crystals in smaller crystals.

e) All alkali metals give specific flame colors. So, if the sample gives no color in flame test the presence of the alkali metals can be ruled out.