Form 4 Chemistry – QUALITATIVE ANALYSIS

What is qualitative analysis?

     Qualitative analysis is the analysis which deals with the determination or identification of the components of the unknown given chemical solid substance. The unknown chemical solid substance actually we talk about the solid salt which normally we are given but the name is unknown. The solid salt is placed into the watch-glass, then you are required to identify “cation” and “Anion” then chemical formula of the salt

     SOLID SAMPLE [unknown salt]

This is the solid salt which contains the ions which you are required to identify.

Consider the solid salt in the watch glass below;

  

ION(S)

This is the positively or negatively charged particle formed by the Electron loss or gain by atom/element

 

TYPES OF IONS                  

There are two types

Cation e.g. NH₄⁺, Na⁺, K⁺, Mg²⁺, Pb²⁺ etc

Anion e.g SO₄^2-, HSO₄^–, CO₃^2-, HCO₃^–, NO₃^–, CL^– etc

NOTE;

CATION; is the positively charged particle formed by the Electron loss by an atom/Element. Example;

NH₄⁺, Ca²⁺, Zn²⁺, Pb²⁺, Mg²⁺, K⁺ etc

ANION; is the negatively charged particle formed by the Electron gain by an atom/Element. Example;

SO₄^2-, HSO₄^– – Bisulphate/ Hydrogen sulphate

Cl^–, NO₃^–, HCO₃^– – Bicarbonate/ Hydrogen carbonate

CO₃^2-, C₂O₄^2-, S₂O₃^2-, SO₃^– etc

 
 

                                           QUALITATIVE GUIDE SHEET (QDS)

-This is the sheet which contains the information used or guiding in order to answer the Qualitative analysis questions. Normally in the Examinations room you are provided with this sheet. Consider how the sheet appears and it includes .experiment, observation and Inference.

 
 

                                                    EXPERIMENT

This is the practically done action by the experimenter.

An Experimenter can

   (i)Take the salt sample from Watch glass then;-

Observe its colour

Salt solid sample observed on the hand by observer, the colour of the sample must be stated.

E.g. Blue- Cu²⁺, red- Cu⁺, green- Fe²⁺, Fe³⁺ – brown/yellow, White- NH₄⁺, Na⁺, Zn²⁺, Pb²⁺, K⁺ etc

Check the texture of the salt.

Smell the salt or its odor

Dissolve it into the distilled water into the test then observe if soluble or not soluble (insoluble).

OBSERVATION

   This is what an experimenter has seen due to the Experiment which he/she had performed.

The following are the common or possible observation which an experimenter can observe

1.  Powdered/crystalline solid sample
   
2. The colour E.g. Green, yellow, white.
   
3. Evolution of the gas e.g. O₂, NH₃, NO₂,SO₂
   
4. Formation of the ppt( precipitation)/(precipitate)
   
5. The colour of Residue formed after heating the solid sample.
   
6. The smell of the solid sample E.g. Urine (NH₃)smells irritating chocking smell
   
7. Solubility of the salt if soluble/insoluble.
   

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Note;

           On doing the solubility leave the salt to stay for certain period of time, because some of the salt takes the time to dissolve completely. Don’t harry up to state that the salt is Insoluble.

                    INFERENCE (conclusion)

             This is the stage which an experimenter is required to take the conclusion above the result observed after doing an experiment.

 
 

Conclusion; cation NH₄⁺ Anion –NO₃^– formula of TT –NH₄NO₃

 
 

                                              TYPES OF QUALITATIVE ANALYSIS

     There are;-

               i./Guided Qualitative Analysis

               Ii./Systematic Qualitative Analysis (unguided)

GUIDED QUALITATIVE ANALYSIS

           -Is the type of Qualitative which the experiments to be done by an experimenter is guided or instructed or limited.

        POSSIBLE QUESTION

The salt sample WW contains one cation and Anion in the tabular form of Experiment, Observation and Inference Identify cation and Anion in the salt solid sample WW.

 
 

 
 

Conclusion Cation – Anion- Formula of WW

 
 

                               SYSTEMATIC QUALITATIVE ANALYSIS

-Is the type of the qualitative analysis which no instructions is given but an experimenter should follow the procedures/stages which can led to identification of solid sample

                           POSSIBLE QUESTION

The salt sample WW contains one cation and Anion in the tabular form below identifies cation, Anion and formula of WW.

 
 

SOLN/ ANS:

Conclusion: Cation NH₄⁺ Anion SO₄^2- formula (NH₄)₂SO₄

 
 

                         STAGES OF SYSTEMATIC QUALITATIVE ANALYSIS

They include;-

             -Preliminary test

             -Test in solution

             -Confirmatory test

            PRELIMINARY TEST

 These are the tests which are done at the first stage such that preliminary information of the solid sample can be obtained;-

These Experiments includes;

     1. Appearance of the sample (solid)

Here we talk about colour, Texture of sample,

     2. Solubility of the sample (solid)

     3. Smell of the solid sample

     4. Action of heat on solid sample

     5. Action of dil. HCl (Hydrochloric Acid) or concentrated sulphuric Acid (con.H₂SO₄)on the solid sample into test tube(TT)

     6. Flame test of solid sample

     7. Residue test of the solid sample

 
 

                             THE REQUIREMENTS;

a. Clean test tubes

b. Test tube holder

   

C. Heat source e.g. Bunsen burner

d. Distilled Water E.g.

 

e.) Acid E.g.

                             APPEARANCE OF SOLID SAMPLE

   Rewind or Recall from periodic table.

  

POSSIBLE QUESTION

        
 

          
 

           SOLUBILITY OF SOLID SAMPLE

Take solid sample put small (little) amount into clean test tube then Add distilled Water into the solid sample observe it if soluble or insoluble. Be careful on observing

             E.g. salt -AA or CC or TT etc

  

Note;

Transition metals [Cu²⁺,Cu⁺, Fe³⁺] they tend to display [show] their colour if their salt dissolved into Water provided that they are soluble-salt.

 
 

         COLOR OF SOLUTION FORMED ON DISSOLVING SALT SAMPLE IN WATER.

-Consider the table below for more illustration;

   E.g. SALT SAMPLE

         POSSIBLE QUESTION

Fill (i) and (ii)

Answer

1. Blue solution was formed
   
2. Solubility of a sample
   

    
    

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                           SMELL OF THE SOLID SAMPLE

Take the solid sample then smell it. Normally Ammonium salt -NH₄⁺ Even at the room-temperature they tend to give out the Urine smell (smell of Ammonia gas –NH₃ or When they are Heated (∆). But other salts are (odorless) E.g. Don’t give out the smell , salt of Na⁺,Cu²⁺, Zn²⁺,Pb²⁺,Ba²⁺,Mg²⁺,K⁺ .etc.

 
 

                        TABLE OF RESULTS

 
 

POSSIBLE QUESTION

          Fill (i), (ii), (b)

Ans   (i) NH₄⁺ confirmed       (ii) no specific smell detected         (b) Smell of the sample

 
 

NATURE OF GASES, SMELL, ACTION OF LITMUS, COLOR, SOLUBILITY

• Some of the gases are Acidic, Basic (Alkaline), colorless, colored, soluble, insoluble have smell (odor), odorless.
  

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Note: We can try to know the nature of the gas by dissolving it into Water then resulting solution may be Acidic, Basic, or Neutral. E.g.

1. NH₃ + H₂O →NH₄OH  – Base/Alkaline
   
2. CO₂ + H₂O→H₂CO_3 – Acid
   
3. NO₂ + H₂O→HNO_3  –  Acid
   
4. Cl₂ + H₂O→ HClO + HCl  –   Acid
   
5. SO₃ +H₂O→H₂SO_4   –  Acid
   
6. SO₂ + H₂O→H₂SO_3  –   Acid
   

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                   ACTION OF HEAT ON SOLID SAMPLE IN THE DRY AND CLEAN TEST TUBE

To the clean, dry test tube put the salt sample then heat it for few minutes and observe the changes which occur some of the salt are stable(under composes) but other decomposes(stable) During heating process you can observe the colour if gas Evolved, smell of the Gas Evolved. Normally each radical releases

                   Example

       SALT RADICAL + HEAT →         GAS EVOLVED        +          RESIDUE

 

You have been given with salt sample AA, PP, QQ, NN, ZZ, and TT.

                          
 

POSSIBLE QUESTION

You have been given with salt sample ZZ, QQ, Which was the piece of the paper isolated in the Examination room by form IV students at Mtazamo secondary school.

                                                                                 
 

 
 

a. Fill the inference column

b. Given cation – Anion and formula of salt ZZ including cation -Anion -in QQ

c. Name gas FF, PP, Residue HH, GG.

                         
 

                     
 

                  b. Cation – Zn²⁺, Anion-No₃^– formula Zn (NO₃)₂ OF ZZ cation -Pb²⁺ Anion – CO₃^2-/HCO₃^–

                  c.   FF- Nitrogen dioxide – NO₂

                       PP-Carbon dioxide – CO₂

                       HH – Lead oxide (PbO),

                       GG- zinc oxide (ZnO)    

 
 

             HOW TO IDENTIFY THE GAS EVOLVED DURING EXPERIMENT [Heating using Acid]

           The following methods can be used;-

1. Through smelling
   
2. By using the damp litmus paper
   
3. By observing the colour of gas
   
4. Experiments
   

   
    

    
    

   

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                 FLAME TEST [Give the cation E .g Pb2+ Cu2+,Na+etc

→The Nichrome wire or glass rod or Test tube then dip into the test-tube containing dil. HCl the reason; Is to provide White vapour on the flame on heating the salt. Take the Wet nichrome or glass rod or test tube dip into the salt sample, so the salt sample should stuck on the Wire. Transfer the salt on the flame burn it While observing the colour given by the flame.

 
 

 
 

OBSERVATIONS;

Example Blue- Green flame – Cu²⁺, Green flame-Fe²⁺,yellow spark-Fe³⁺,Golden yellow flame-Na⁺, Lilac/Purple flame-K⁺, Blue flame – Pb²⁺,Brickred flame- Ca²⁺, No specific flame Zn²⁺,Al³⁺, Mg²⁺.

 
 

                                             PROCEDURES OF THE FLAME TEST

Refer the diagram below      

        

Consider the diagrams below showing how to perform Flame test

                                               

 
 

                                  
 

        ACTION OF DILUTE HCl, HNO₃ OR CONCENTRATED SULPHURIC ACID [con.H2SO4] ON SOLID SALT IN DRY AND CLEAN TEST TUBE

       Take the solid salt put into the clean and dry test tube, and then Add either dilute HCl or con.H₂SO₄ or dil.HNO₃ then observe what happened. Hydrochloric acid –HCl, Nitric Acid – HNO3 and

H₂SO₄have the tendency to break down radical such as CO₃^2- OR HCO₃^– then form CO₂– carbon dioxide which give off Effervescence. But on additional of con.H₂SO₄or HNO₃ (dil) or dil. HCl to the salt which have no Co₃^2-/HCO₃^– No Co2. Evolved probably other gas

 

PROCEDURES DURING ACTION OF ACID ON SOLID SAMPLE [SALT] BY USING ACID

[Dil. HCL, H₂SO₄, dil.HNO₃]

           You have been given with salt JJ, KK, and LL . examples ACIDS ADDED TO SOLID SALT

   
 

RESIDUE TEST [Identify cation Example Zn²⁺, Cu²⁺ etc.

Take the given salt sample put it into the test tube then heat until no changes occurs. The test tube should be clean and dry on doing the residue test. Note that some of the salt doesn’t decompose.

                                         

• These salt – CO₃^2- of group I (alkaline metal) are stable on heating so they don’t decompose at all
  

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• These salt of group II (Alkaline metal) their Sulphates – SO₄^2- are very stable
  

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Illustration

         OBSERVATION;

1. Blue- Residue turn black- CuO cation- Cu²⁺
   
2. No colour E.g. MgO , Al₂O₃ etc
   

    
    

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                             PROCEDURES FOR THE RESIDUE TEST

You have been given with salt sample solid WW, MM and TT.

  
 

 
 

POSSIBLE QUESTION

               Solid PP was White on heating gave yellow ─ Residue When hot on cooling become White PP was;-

1. ZnCO₃
   
2. ZnO
   
3. ZnCl₂
   

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TEST IN THE SOLUTION

               This is the second stage of the Qualitative analysis which the test is done in the solution form.

Example –Take the salt given then put iit into the beaker or Test tube then add distilled water in order to obtain the solution.

  

  
 

Note; AA – Known as mother solution (M.s) or stock solution/original solution

Then you can divide the solution into the different solutions E.g. First solution, second solution Third solution, then you can add given reagent such as NaOH, NH₄OH etc

 

 
 

 

          NaOH +CuCl₂→CU (OH) ₂ + Nacl

• NH₄OH + Cuso4→ Deep – Blue ppt
  

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          NH₄OH + Cuso₄→ Cu (OH )₂ + (NH₄)₂SO₄

• NaOH + solution → Green ppt -Fe²⁺
  

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          NaOH + Fe (NO₃)₂ → Fe (OH)₂ + HNO₃

 
 

OBSERVATION;

       Brown ppt -Fe³, Na⁺, Ca²⁺, NH₄⁺, K⁺    White ppt

 
 

                     PROCEDURES DURING THE TEST IN SOLUTION

         You have been given with the salt RR, FF.

Take the salt dissolve into the given distilled water in order to obtain the solution. This solution can be divided into the different solution named first, second, third etc.

The following solution, solution (reagent) can be Added to the prepared solution;-

1. Sodium hydroxide – NaOH
   
2. Ammonium hydroxide / Aqueous solution – NH₄OH
   
3. Potassium hexaferrocynide
   

   

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THE MAIN AIM OF THE ADDITIONAL OF SODIUM HYDROXIDE ─NaOH or NH₄OH TO THE ORIGINAL SOLUTION (O.S) or STOCK SOLUTION (S.S) OR MOTHER SOLUTION (M.S)

       The aim is to identify the cation in the solution given, and then observe what occurred into the solution you make.

 
 

                                                  CONFIRMATORY TEST

This is the third stage and lastly stage of the Qualitative analysis which can be used to determine or identify the specific cation or Anion present in the unknown given salt. Each cation or Anion has the specific confirmatory reagent used to identify them.

                                             E.g. IDENTIFICATION OF ANION  

 
 

  

(i)In-A-Solution

(ii)In-D-Solution

                                           COMMON SALTS IN THE QUALITATIVE ANALYSIS

You must be familiar with the following salts

(a)Ammonium salts

Example;

1. Ammonium chloride – NH₄Cl
   
2. Ammonium Nitrate – NH₄NO₃
   
3. Ammonium sulphate – (NH₄)₂SO₄ etc
   

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(b) Iron salts

Example;

1. Iron II chloride – FeCl₂
   
2. Iron III chloride – FeCl₃
   
3. Iron II sulphate – FeSO₄
   

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(c) Calcium salts

Example;

1. Calcium chloride -CaCl₂
   
2. Calcium carbonate – CaCO₃
   

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(d)Sodium salts

Example;

1. Sodium carbonate – Na₂CO₃
   
2. Sodium bicarbonate  – NaHCO₃
   
3. Sodium sulphate – Na₂SO₄
   
4. Sodium Nitrate – NaNO3
   
5. Sodium chloride – NaCl
   

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(e)Zinc salts
Example;

1. Zinc carbonate – ZnCO₃
   
2. Zinc sulphate – ZnSO₄
   

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(f)Copper salts

Example;

• Copper II sulphate -CuSO₄
  
• Copper II chloride -CuCl₂
  
• Copper II Nitrate -Cu(NO₃)₂
  
• Copper I sulphate – Cu₂SO₄
  

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(g) Lead salts

• Lead chloride – PbCl₂
  
• Lead Nitrate – Pb(NO₃)₂
  
• Lead sulphate – PbSO₄
  

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 (h) Potassium salts

• Potassium chloride – KCl
  
• Potassium carbonate – K₂CO₃
  

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          NOTE

         You should be able to;-

-Use the qualitative Analysis sheet

-Know the Appearance of the salt

-Know the flame test

-Identify the cation, Anion

-Know how to write down the chemical formula of the compound.

-Write down the balanced chemical Equation Which take place or occur during practical

-Prepare the solution of the salt etc