Introduction to Qualitative Analysis

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Welcome to class! 

In today’s class, we will be talking about the introduction to qualitative analysis. Enjoy the class!

Introduction to Qualitative Analysis

Qualitative analysis involves the examination of colour, flame test, the effect of heat and confirmatory test for cation and anions.

Cations are metallic ions e,g. Ca²⁺, Pb²⁺, Al³⁺, Cu²⁺, Fe²⁺ , Fe³⁺, etc

Rules in qualitative analysis

1. Your test solution should not be diluted too much
2. Use only a small quantity of reagents

Examination of colour and physical state of specimen

Substance                            colour                         physical state

1. Sulphur                      yellow                             solid
2. Copper(ii)oxide          black                               solid
3. Iodine                        dark brown                      solid
4. Nitrogen iv oxide      reddish-brown                 gas
5. Mercury                     white                                liquid

Flame test

1. Deep green colour of flame indicates the presence of copper
2. Deep yellow colour indicates the presence of sodium
3. Brick red indicates calcium.

Evaluation

1. State ten (10) examples of cations
2. Give the colour of the following substance.

• Distilled water
• Iron filling
• Manganese(iv) oxide
• Benzoic acid.

Test for cations

The Cations are Ca²⁺, Zn²⁺, Al³⁺, Pb²⁺, Fe²⁺. Fe³⁺, Cu²⁺, NH4 ⁺

Test for Ca²⁺

                     Test                       Observation                                   Inferences

• Sample + H₂O                     dissolve to give a                            soluble
• Colourless solution
• + NaOH in drop                  white powdery precipitate
• in excess                               insoluble in excess                          Ca²⁺ present
• soln + NH₃ soln                    No noticeable reaction                   Ca²⁺ present

Test for Zn²⁺

• Sample + H₂O                     dissolve in water to                        soluble
• Give a colourless solution
• Soln  + NaOH in                  white gelatinous precipitate        Zn²⁺, pb²⁺,
• Drop in excess                     soluble in excess                          Al³⁺ may be present
• Soln  +  NH₃ soln                  white gelatinous precipitate

In drop

In excess                               ppt soluble in excess                     Zn²⁺ present

Test for Al³⁺

• Sample +H₂O                      dissolve in water to give               soluble

a colourless solution

• soln + NaOH

in drop                                  white gelatinous precipitate     Zn²⁺,pb²⁺ Al³⁺ may be

in excess                               soluble in excess                             present

• Soln + NH₃OH                      white gelatinous ppt

In drops

In excess                               insoluble in excess                          Al³⁺ or pb²⁺

Test for Pb²⁺

Sample + H₂O                     dissolve in water to give soluble

Colourless solution

• Soln + NaOH

In drop                                  White gelatinous ppt                     Zn²⁺, Pb²⁺, Al³⁺

In excess                               soluble in excess                             may be present

Soln+ NH₃ soln

In drop                                  white gelatinous ppt                     Al³⁺ or Pb²⁺

In excess                               insoluble in excess                          may be present

• Soln + dil HCl                         white ppt                                        Pb²⁺ present

Ppt + heat                            ppt dissolve when hot and

Reappear when cool                     Pb²⁺ present

• Soln + K₂CrO₄                      yellow precipitate                           Pb²⁺ present

Test for Fe²⁺

Test                        Observation                             Inferences

• Sample + H₂O         dissolve in water                             soluble
• Soln +NaOH soln

In drops                     dirty green gelatinous ppt           Fe²⁺ present

In excess                   insoluble in excess

• Soln +NH₃ in             Dirty green  gelatinous

Drop                          precipitate                                       Fe²⁺ present

In excess                   insoluble in excess

Test for CU²⁺

Test                       Observation                             Inference

• Sample +H₂O          dissolve in water                             soluble
• Soln +NaOH in

Drops                         Blue gelatinous ppt

In excess                   insoluble in excess                          Cu²⁺ present

• Soln+NH₃ in drop    pale blue gelatinous ppt

In excess                   soluble in excess to give a

Deep blue solution                        Cu²⁺ present

Test for NH₄⁺

Test                       Observation                                       Inferences

• Sample + H₂O         dissolve in water to give a

Colourless solution                                     soluble

• Soln +NaOH             No ppt, but effervescence occurs

In drop + warm      with libration of a colourless gas with choking smell

Gas + moist              It turns moist red litmus paper                 Alkaline gas

Litmus paper            blue.

Gas  + con HCl       It gives white fumes with con HCl          NH₃ gas from NH₄⁺

General evaluation

1. Give the common reagents used for the confirmatory test for cations.
2. State the colour of the solution when the sample that contains the following dissolve in water: a. Fe ²⁺ b. Cu ²⁺ c. Fe ³⁺

Weekend assignment

1. The following gives white gelatinous precipitate in NaOH except? (a) Al ₃₊ (b) pb₂₊ (c) CU ₂₊ (d) Zn₂₊
2. One of the following gives ‘pop’ sound when contact with a lighted splint. (a) O₂ (b) H₂ (c ) NH₃ (d) CO₂
3. Example of cations that gives gas during its confirmatory test is (a) SO₃^2- (b) SO₄^2- (c) CO₃^2- (d) NH₄⁺
4. Deep green colour of 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
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.

 

In our next class, we will be talking about Test for Anions, Identification of Gases.  We hope you enjoyed the class.

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