Introduction to Qualitative Analysis

 

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 | classnotes.ng

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) SO32- (b) SO42- (c) CO32- (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.

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

Pass WAEC, JAMB, POST-UTME & more in One Sitting for FREE!💃

Leave a Reply

Your email address will not be published.

Don`t copy text!