Qualitative Analysis of Inorganic Chemistry to Identify Cations, Anions, Compounds

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.

Content

Examples of usage of qualitative analysis in chemistry
Introduction to Qualitative Analysis
- Observations of physical changes in a reaction
- Identify cations and anions in qualitative analysis
Examples to qualitative analysis experiments
- Identify ferric chloride and ferric sulfate (FeCl₃ and Fe₂(SO₄)₃)
- identify aqueous dilute HCl acid and aqueous dilute H₂SO₄ acid solution?
- Identify sodium sulfide and sodium sulfate solutions
Solubility of compound
Qualitative analysis for cations
Qualitative Analysis for anions
Identify gases observing physical and chemical properties of gases
Qualitative analysis question and answers

Examples of usage of qualitative analysis in chemistry

Identify aqueous Sodium chloride (NaCl) solution and aqueous Zinc chloride (ZnCl₂) solution: In this example, we need to prove the presence of sodium ion and zinc ion separately in two solutions.
Identify solid Sodium carbonate (Na₂CO₂) from and solid Sodium chloride (NaCl). In this example, we need to identify presence of anions; carbonate and chloride.

Introduction to Qualitative Analysis

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

Observations of physical changes in a reaction

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.

Formation of colourful or colourless solution
Formation of precipitate - These precipitate can be white or other colour
Emitting a gas or gases - Emitting of gas can be observed. These gases can have a colour or may be colourless.

Identify cations and anions in qualitative analysis

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.

Examples 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.

Question 1

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

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.

Answer

qualatative analysis example 1 - FeCl3 and Fe2(SO4)3

Ferric +3 cation is common for both FeCl₃ and Fe₂(SO₄)₃ compounds.
So we have to test only anions. (chloride and sulfate).
Add aqueous Barium chloride (BaCl₂) solution to both brown solution. Aqueous Barium chloride solution is colourless.
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 should had the Fe₂(SO₄)₃.

In this example, we use the concept formation a precipitate in one solution while other solution one remains as a solution.

Question 2

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.

Question 3

Identify sodium sulfide and sodium sulfate solutions

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

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.