Qualitative Analysis – SS3 Chemistry Lesson Note (Second Term)

Qualitative analysis is the identification of the cations (positive ions) and anions (negative ions) present in a substance.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define qualitative analysis, cation and anion.
2. identify common cations using colour, flame test and sodium hydroxide and ammonia solutions.
3. identify common anions using simple confirmatory tests.
4. test for gases such as ammonia, oxygen, hydrogen, carbon(IV) oxide and nitrogen(IV) oxide.

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos from the source

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Relevant salts
• Acids
• Bases
• Starch, etc.

What is qualitative analysis?

Qualitative analysis is the process of finding out which ions a substance contains, without measuring how much is present.

• Cation: a positively charged ion, mostly a metal ion, such as Ca²⁺, Pb²⁺, Al³⁺, Cu²⁺, Fe²⁺ and Fe³⁺.
• Anion: a negatively charged ion, such as Cl^–, SO₄^2-, CO₃^2- and NO₃^–.
• Reagent: a chemical added to a sample to produce a visible change.
• Precipitate: an insoluble solid that forms when two solutions react.

What are the stages of qualitative analysis?

1. Observe the colour and physical state of the sample.
2. Carry out a flame test.
3. Heat a small portion and note any change.
4. Dissolve the sample in water and add reagents to test for cations.
5. Carry out confirmatory tests for anions.

What are the rules in qualitative analysis?

• Dilution: do not dilute the test solution too much.
• Quantity: use only a small quantity of reagent.
• Addition: add the reagent drop by drop, then in excess.
• Apparatus: use clean test tubes.
• Records: write down every observation and inference.

What are the colours and physical states of some common substances?

The colour and state of a sample give the first clue to its identity.

What colour do cations give in solution?

• Cu²⁺: blue solution.
• Fe²⁺: pale green solution.
• Fe³⁺: yellow or brown solution.
• Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, NH₄⁺: colourless solutions.

What does the flame test show?

A flame test identifies some metal ions by the colour they give a non-luminous Bunsen flame.

What is the effect of heat on some solids?

Heating a solid can change its colour or release a gas, which helps to identify it.

• Zinc oxide: yellow when hot, white when cold.
• Lead(II) oxide: reddish-brown when hot, yellow when cold.
• Carbonates: most give off carbon(IV) oxide on heating.
• Ammonium salts: give off ammonia on heating with an alkali.

How do we test for cations?

Cations are tested by adding sodium hydroxide solution and ammonia solution, first a few drops, then in excess.

The cations tested are Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, Fe²⁺, Fe³⁺, Cu²⁺ and NH₄⁺.

How do we test for an alkaline gas?

1. Warm the sample with sodium hydroxide solution.
2. Hold moist red litmus paper at the mouth of the tube. It turns blue.
3. Bring a glass rod dipped in concentrated HCl near the gas. White fumes form.

• Conclusion: the gas is NH₃, so the sample contains NH₄⁺.

How do we test for anions?

Anions are identified by adding a specific reagent and observing a gas or a precipitate.

How do we test for common gases?

Each common gas has a simple test that identifies it.

Evaluation

1. State ten (10) examples of cations.
2. Give the colour of the following substances: distilled water, iron filings, manganese(IV) oxide and benzoic acid.
3. Give the common reagents used for the confirmatory test for cations.
4. State the colour of the solution when a sample that contains the following dissolves in water: (a) Fe²⁺ (b) Cu²⁺ (c) Fe³⁺
5. Describe how you would show that a salt contains NH₄⁺.

Assignment

Weekend assignment

1. All of the following give a white gelatinous precipitate in NaOH except: (a) Al³⁺ (b) Pb²⁺ (c) Cu²⁺ (d) Zn²⁺
2. One of the following gives a ‘pop’ sound when in contact with a lighted splint: (a) O₂ (b) H₂ (c) NH₃ (d) CO₂
3. An example of an ion that gives a gas during its confirmatory test is: (a) SO₃^2- (b) SO₄^2- (c) CO₃^2- (d) NH₄⁺
4. A deep green colour of the flame indicates the presence of: (a) sodium (b) calcium (c) copper (d) iron
5. The only alkaline gas that changes moist red litmus paper to blue is: (a) NH₃ (b) HCl (c) NaOH (d) NH₄OH

Theory

1. Explain the confirmatory test for the following cations: Ca²⁺, Al³⁺, Zn²⁺, Pb²⁺, Cu²⁺ and NH₄⁺.
2. State the flame test for the following: (a) calcium (b) copper (c) sodium (d) iron

Reading assignment

• School Chemistry by O.Y. Ababio, pages 165-183
• Practical Chemistry by R. I. Makanjuola, pages 31-36
• Practical Chemistry for Schools and Colleges by Godwin O. Ojokuku, pages 30-98