Qualitative analysis is used to identify anions, cations, compounds and etc. In this section, we only target inorganic ionic
compounds which are made of cations and anions. These compounds can be as a one chemical or as an one item in a mixture of
chemical compounds. Therefore we have to follow a specified
procedure to identify these compounds
one by one. There are many different experiments to identify different compounds in a sequence of steps. In qualitative analysis lesson,
we observe a physical change (such as a colour change), when a reaction is being occurred or after completed the reaction.
Simply, we can tell, qualitative analysis is identifying compounds, anions and cations by doing some experiments (reactions).
There are physical changes as mentioned and they are used to make decisions (to identify ions and compounds). We will discuss these physical changes with examples in this tutorial.
In the qualitative analysis of inorganic chemistry in grade 12, we have some cations (positively charged ions) to identify such as Cu2+, Fe2+ and more.
As cations, there are more anions we have to learn in qualitative analysis of inorganic chemistry section.
Now we are going to see few example which explains what is qualitative analysis and how it is done to identify cations and anions.
There are two brown colour solutions in two bottles. You have to identify these two solutions by doing some experiments. But you have informed that, these solutions contain ferric chloride (FeCl3) and ferric sulfate (Fe2(SO4)3). So your target is find out to which bottle contains which compound.
In this example, we use the concept formation a precipitate in one solution while other solution one remains as a solution.
To identify aqueous HCl and H2SO4 solutions, we use Ba(OH)2 strong base.
Add dilute HCl acid to solutions and heat the mixtures. One solution emits a gas. When metal sulfides are heated with dilute HCl, hydrogen sulfide gas is emitted. But metal sulfates do not emit gases with dilute acids.
Therefore we can identify sodium sulfide because it emits hydrogen sulfide gas with dilute HCl.
Note: Never try to identify a gas in qualitative analysis experiment by observing the odor because these gases can be extremely toxic. In this example too, Hydrogen sulfide is a highly toxic gas.
Solubility of compound is very important factor in qualitative analysis. We compare solubility of two compounds and see which has the best solubility in water to decide the unknown compound.Solubility of inorganic Compounds, s,p,d block elements - Tutorial
Above tutorial discuss how solubility changes with anions and cations.
We can identify solid sodium carbonate and solid magnesium carbonate from checking their solubility in water. Sodium carbonate is soluble in water. But magnesium carbonate is not soluble in water and forms white precipitate.