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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

See also  METALS AND THEIR COMPOUND

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

See also  NUCLEAR REACTIONS

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+

  1. One of the following gives ‘pop’ sound when contact with lighted splint. (a) O2 (b) H2 (c ) NH3

(d) CO2

  1. Example of cations that gives gas during its confirmatory test is (a) SO32- (b) SO42- (c) CO32-

(d) NH4+

  1. Deep green colour of flame indicate the presence of (a) sodium (b) calcium (c) copper (d) iron
  2. 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

 

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