Qualitative Analysis of Inorganic Chemistry

Qualitative analysis is used to identify compounds. Those compounds are made of cations and anions. These compounds can be in a single mixture. 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 qualitative analysis, we observe a physical change, when a reaction is being occurred or after completed the reaction.

Examples of qualitative analysis

• Identify NaCl solution and ZnCl₂ solution
• Identify solid Na₂CO₂ from and MgCO₃ solution

Introduction to Qualitative Analysis

Simply, we can tell, qualitative analysis is identifying compounds, anions and cations by doing some experiments (reactions).

Physical changes can be seen in a reaction

Following changes are used when we identify a reaction. We compare differences of followings in two or more compounds when they react another compound.

• forming a colourful solution or colourless solution
• forming a precipitate - precipitate can be white or colourful
• emitting a gas or gases - emitting gas can be colourless or may have a colour

Cations and Anions

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.

Example Applications to Qualitative Analysis

Now we are going to see few example which explains what is qualitative analysis and how it is done to identify cations and anions.

Identify ferric chloride and ferric sulfate ( FeCl₃ and Fe₂(SO₄)₃ )

Question

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 you have to decide which bottle contains which compound.

Answer

• Ferric +3 cation is common for both FeCl₃ and Fe₂(SO₄)₃ compounds.
• So we have to know, we are going to test anions. (chloride and sulfate).
• Add BaCl₂ solution to both brown solution.
• In one solution, there is no difference after addition of BaCl₂.
• In other solution, a white precipitate is given.
• When we consider possible reactions, it's clear that BaSO₄ is the only precipitate which is deposited easily at bottom of the solution. That solution had the Fe₂(SO₄)₃.

In this example we use forming a precipitate in one solution while other solution one remains as a solution.

How do you identify aqueous dilute HCl acid and aqueous dilute H₂SO₄ acid solution?

To identify aqueous HCl and H₂SO₄ solutions, we use Ba(OH)₂ strong base.

Identify sodium sulfide and sodium sulfate solutions

Add dilute HCl 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 emitts hydrogen sulfide gas with dilute HCl.

Solubility of compound

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.

Identify solid sodium carbonate and solid magnesium carbonate

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.

Qualitative Analysis for cations

Qualitative Analysis for anions

Qualitative Analysis for gases

Problems