INTRODUCTION TO QUALITATIVE ANALYSIS

Introduction to Qualitative Analysis

Test for carbons using H2S, NaOH and NH4OH, Confirmatory Test for the cat ions.

 

What is Qualitative Analysis?

Qualitative analysis involves examination of colour, flame test, effect of heat and confirmatory test for cat ion 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 small quantity of reagents
3. 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 indicate the presence of copper
2. Deep yellow colour indicate the presence of sodium
3. Brick red indicate calcium.

 

EVALUATION

1. State ten (10) examples of cations
2. Give the colour of the following substance.
3. Distilled water
4. Iron filling
5. Manganese(iv) oxide
6. 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 occur

In drop + warm             with libration of 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 confirmatory test for cations.
2. State the colour of the solution when the sample that contains the following dissolve in water:

1. Fe ²⁺ b. Cu ²⁺ c. Fe ³⁺

 

WEEKEND ASSIGNMENT

1. The following give 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 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 indicate 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 :

1. Calcium b. Copper c. sodium    d. iron

 

READING ASSIGNMENT

School Chemstry 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.

 

See also

METALS

PRELIMINARY PREPARATION

METALLIC BONDING

SIMPLE MOLECULES AND THEIR SHAPES

NUCLEAR REACTIONS