Qualitative inorganic chemistry for A-level students

Qualitative inorganic chemistry for A-level students

It involves use of observations and chemicals to identify in ions in a given compound usually identified by a letter such as K, P, Q and so on.

The trick is simple when you know the following basic information

Definitions

Cation is positively charged ion e.g. Pb²⁺, Zn²⁺, Cu²⁺ etc.

Anion is a negatively charged ion such SO₄^2-, NO₃^–, Cl^–, CO₃^2- etc.

Syllabus

Knowing which ions you are expected to identify, helps you to focus your attention in a practical exams.

A sample compound in exam is expected to contain one or more of the

Cations:                Zn²⁺, Pb²⁺, Al³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Co²⁺, Mn²⁺, Ni²⁺, Cr³⁺, Ba²⁺, Ca²⁺, Mg²⁺ or NH₄⁺

Anions:                 SO₄2^–, Cl-, CO₃^2-, NO₃^–, HCO₃^–, C₂O₄^2-, CH₃COO^–, I^–, or  Br^–.

Reagents

It should be noted that there are reagents that test for cations and reagents that test for anion. It follows that deductions must correspond to these reagents. When a reagent that test for cation is used, do not include anions in the deductions and the vice versa. Only in the deductions boxes of “comment” “Heat” and/or “dissolve”  can contain  a MIXTURE of probable cations and anions (for the reasons to be explained later), elsewhere all deduction boxes should contain either probable cations or anions ONLY

The following reagents are only used to test for cations only; using these reagents and include any anion in the deduction box, make everything in the deductions box wrong since it will guess work.  The concentration in brackets may be used.

• Dilute sodium hydroxide solution (4g in 100cm³)
• Dilute ammonia solution (10 cm³of concentrated ammonia in 100cm³)
• Dilute hydrochloric acid solution (10cm³of concentrated HCl in 100cm³)
• Potassium iodide solution (4g in 100cm³)
• Sodium carbonate solution (4g in 100cm³)
• Potassium hexacyanoferrate (II) solution (4g in 100cm³)
• Potassium hexacyanoferrate (III) solution (4g in 100cm³)
• Dimethyl glyoxime (dissolve 1g in 20cm³ of ethanol, add water to make 100cm³)
• Potassium thiocyanate (4g in 100cm³)
• Ammonium carbonate (4g in 100cm³)
• Potassium chromate (4g in 100cm³)
• Ammonium thiocyanate (4g in 100cm³)

The following reagents are used to test anions ONLY: using these reagents and include any cation in the deduction box, make everything in the deductions box wrong since it will guess work.

• Lime water (dissolve excess Ca(OH)₂ powder and filter)
• Barium chloride/nitrate solution (2g in 100cm³)
• Silver nitrate solution (1g in 100cm³ protect the solution from light/use opaque container)
• Lead nitrate/ethanoate solution (4g in 100cm³)
• Litmus paper

Others qualitative reagent include

• Copper turning
• Hydrogen peroxide (4cm³ of hydrogen peroxide 20% to make 100cm³)
• Dilute Nitric acid solution (5cm³ of conc. HNO3 to 100 cm³ of solution)
• Dilute sulphuric acid (5cm³ of conc. HNO3 to 100 cm³ of solution)
• Ethanoic acid (1cm³ into 100cm³)

 

 

Implication of test procedures

1. Comment

When required to comment on a compound you are expected to

• Tell the color the compound and this relates cations present i.e.

Color of common compound	Cations that show the colors
Green	Ni2+, Cr3+, Fe2+, Cu2+
Blue/Purple	Cu2+, Ni2+, Cr3+
White/ colorless in solution	Pb2+, Zn2+ Al3+, NH4+, Ba2+, Ca2+, Mg2+
Brown/orange/yellow/pink	Fe3+, Co2+, Mn2+, Cr6+
Black/Purple	Cu2+, Cr3+, Mn7+, Mn4+

 

• Texture: this relates to anions, a compound can be
• Crystalline indication presence of SO₄^2-, NO₃^–, or Cl^–

Or

• Powder indicating presence of CO₃^2-, oxide

Example the following are expected deduction for the observation

Test	Observation	Deductions
Comment on the sample Q   Or   Or	Green crystals	Ni2+, Cr3+, Fe2+, SO42-, NO3–, or Cl–
	White powder	Pb2+, Zn2+ Al3+, NH4+, Ba2+, Ca2+, Mg2+ CO32-, O2-/oxide
	A mixture of blue crystal and white powder	Cu2+ SO42-, NO3–, or Cl– Pb2+, Zn2+ Al3+, NH4+, Ba2+, Ca2+, Mg2+ CO32-, O2-/oxide

 

2. Heat

On heating a compound the following observations (if any) occur in the order

Test	Observation	Deductions
Heat a spatula endful of Q	–          A liquid forms on the cool part of the test tube turn anhydrous copper Sulphate powder blue or anhydrous cobalt sulhate blue (use one of them not both to avoid wastage) –          A gas ·         That turns damp blue litmus paper red and  forms white precipitate with lime water Or ·         That turns damp blue litmus paper red and  bleach a litmus paper dipped in KMnO4 solution Or ·         Brown fume turn damp blue litmus paper red –          Comment on the color of the residue (the deduction of the residue is made with reference to the initial of the compound as follows)	–          Hydrated compound         SO2; SO42- suspected     CO2; CO32-, HCO3– suspected   NO2, NO3– confirmed

Color of compound	Color of the residue	Suspected ions
Green	Yellow/brown	Fe2+ (being oxidized to brown Fe3+), or, Cr3+ (being oxidized to yellow Cr6+)
Green	Black	Cu2+(CuCO3 is green but is converted to black CuO on heating)
White	Yellow/brown	Zn2+ or Pb2+(lead or zinc compound being decomposed to Oxide)
White	white	Al3+, Ba2+, Ca2+, Mg2+

 

3. Dissolving in water (solubility relates to anions)

Soluble compound/solution usually contain: SO₄^2-, Cl^–, and NO₃^–,

Insoluble compound/residue contain CO₃^2-

Note that a filtrate is a solution that goes through a filter paper and a residue is a solid that remains on a filter paper.

 

Example

Test	Observation	Deductions
Dissolve a spatula endful of compound Q and filter	A green filtrate   White residue	Solution: Ni2+, Cr3+, Fe2+, SO42-, NO3–, or Cl–   Residue: CO32-, Pb2+, Zn2+ Al3+, NH4+, Ba2+, Ca2+, Mg2+

 

4. Testing ions in solution

• When testing for cations: the test reagents are used in order Sodium hydroxide solution; ammonia solution and finally a confirmatory test. A confirmatory test is a specific test for a cation in question. From the observation of sodium hydroxide and ammonia solution; the cations in the deduction box are suspected. When a confirmatory reagent is used, the word “confirmed” is required in the deduction box to show that you are sure of the cation in the compound.
• When testing anions: the test reagents are used in order lead nitrate solution and then a confirmatory reagent.

Note that : both SO₄^2- and Cl^– form  white ppt. with lead nitrate; the pp. PbCl₂ is soluble on heating while that of PbSO₄ is insoluble on hating

 

Confirmatory test/ reagent

SO₄^2- form a white ppt. which barium chloride/nitrate solution insoluble in nitric acid

Cl^– form white ppt. insoluble in nitric acid solution soluble in excess ammonia.

CO₃^2-  carbonate ions are determined in the residue; they dissolve in an acid with effervescence giving of a gas that forms white ppt. with lime water.

 

Summary for chemical identification of Cations

The summary below shows reactions that can be used to identify cations that show close remembrance when reacted with bench reagents.

 

Group 1

This contains Zn²⁺, Pb²⁺ and Al³⁺

Compounds of these cations are usually white in color, and dissolve in water to give colorless solution. The main difference of this group with other white salts is that, they form gelatinous precipitates with sodium hydroxide soluble in excess. The various observations given by these cations with the various bench reagents are given the table below

Test/reagent	Observations
	Zn2+	Pb2+	Al3+
Color of the Common compounds	White	White	White
Color of the Common compounds	White	White	White
Color of solution for common compounds	Colorless	Colorless	Colorless
Sodium hydroxide added drop wise until in excess.	White gelatinous ppt.	White gelatinous ppt.	White gelatinous ppt.
Ammonia solution added drop wise until in excess.	* White gelatinous ppt. Soluble in excess	White gelatinous ppt. insoluble in excess	White gelatinous ppt. insoluble in excess
Sodium carbonate (Na2CO3) solution added drop wise until in excess and warm	White gelatinous ppt. insoluble in excess	White gelatinous ppt. insoluble in excess	*White gelatinous ppt. with effervescence, soluble excess or warming
Addition of a few drops of potassium iodide (KI)	No observable change	*Yellow ppt. formed	No observable change

 

 

Group 2

The group contains Fe²⁺, Ni²⁺, and Cr³⁺ ions. The color of their salts is green.

Some of their distinguishing reactions are shown in the table below:

Table of test and observations

Test/reagent	Fe2+	Ni2+	Cr3+
Color of the Common compounds	Green	Green	Green/purple
Color of solution for common compounds	Green	Green	Green/purple
Sodium hydroxide added drop wise until in excess.	Dirty green gelatinous ppt. insoluble in excess	Grey/green ppt. insoluble in excess	* Grey/green ppt. soluble in excess
Ammonia solution added drop wise until in excess.	Dirty green gelatinous ppt. insoluble in excess	* Grey/green gelatinous ppt. Soluble in excess to give deep blue	* White gelatinous ppt. Insoluble in excess
Sodium carbonate (Na2CO3) solution added drop wise until in excess and warm	Green gelatinous ppt. insoluble in excess	White gelatinous ppt. insoluble in excess	*White gelatinous ppt. with effervescence, soluble excess or warming

 

Group 3

This contain only copper (II) ions

Note that Cu²⁺ may be considered with Group 2 ions because some of its compounds such a carbonate is green. But most of its compounds are blue. It reacts with some the bench reagents as follows:

Test/reagent	Cu2+
Color of the Common compounds and solution	Green/blue
Sodium hydroxide added drop wise until in excess.	A light blue ppt. Insoluble in excess
Ammonia solution added drop wise until in excess	* A blue gelatinous ppt. Soluble in excess to give deep blue solution
A few drops Sodium carbonate (Na2CO3) solution and warm	Light blue gelatinous ppt., insoluble in excess
Addition of Potassium hexacyanoferrate II	A bloody red of brown ppt.
Addition of potassium iodide to the solution	* A white ppt with a yellow solution 2Cu2+ (aq)  + 4I–(aq)  →   2CuI (s)  + I2 (aq)

 

Group 4

This contains Fe³⁺, Co²⁺ and Mn²⁺

Compound of these cations are usually pink/brown / yellow in color, and dissolve in water to give yellow, brown or pink solution.

Table of test and observations

Test/reagent	Fe3+	Co2+	Mn2+
Color of the Common compounds and solution	Brown/yellow	Brown/pink	Brown/pink
Sodium hydroxide added drop wise until in excess	Dirty brown gelatinous ppt. insoluble in excess	Blue ppt. insoluble in excess, turn pink on standing.	* White ppt. insoluble in excess, turns brown on standing
Ammonia solution drop wise	Dirty brown gelatinous ppt. insoluble in excess	Blue ppt. insoluble in excess, turn pink on standing.	* White ppt. insoluble in excess, turns brown on standing
* A few drops of hexacyanoferrate (II)	*Dark blue ppt	No observable change	No observable change
To 1 ml of solution add a few crystals of KSCN followed by amyl alcohol	No observable change	*Blue color in organic layer	No observable change
To 1 ml add equal amount of conc. HNO3 followed by a few crystals of sodium bismuthate	No observable change	No observable change	A temporary purple coloration that turns brown ppt. (NB the test is positive in absence of Cl–)

 

Group 5

This group contains Mg²⁺, Ca²⁺, and Ba²⁺. Compound of the ions are white and dissolve in water to give colorless solution. Unlike the white compound in-group 1, these form curdy /powder like precipitate with sodium hydroxide solutions which are insoluble in excess.  Because alkaline earth metal behave very similarly to each other in aqueous solutions, it is very difficult to distinguish them and especially to separate them.  The different reactions used to distinguish between then are shown in the table below:

Test	Mg2+	Ca2+	Ba2+
To 1 ml of solution add dilute sodium hydroxide drop wise until in excess	White curdy ppt. Insoluble in excess	White curdy ppt. Insoluble in excess	White curdy ppt. Insoluble in excess
To 1 ml of solution add Freshly prepared ammonia solution drop wise until in excess, followed with ammonium chloride solution	White gelatinous ppt. Soluble in ammonium chloride solution	No Observable change	No Observable change * With an aged solution a precipitate may occur due to formation of calcium carbonate
Ammonium carbonate followed by a few drops of ammonium chloride	White ppt. Soluble in ammonium chloride	White ppt. insoluble in ammonium chloride	White ppt. insoluble in ammonium chloride
Potassium chromate followed with dilute acetic acid (30%)	No ppt. formed	No ppt. formed	*A yellow ppt insoluble in acetic acid
Dilute sulphuric acid followed by conc. H2SO4 and boil	No ppt. formed	White ppt. Soluble in hot concentrate sulphuric acid	White ppt. Insoluble
Saturated solution of calcium sulphate	No ppt. formed	No ppt. formed	*White ppt
Ammonium oxalate solution followed by drop acetic acid	No precipitate formed	White ppt insoluble in acetic acid	White ppt soluble in acetic acid
To 1 ml add a spatula endful of ammonium chloride followed by 1 ml Na2HPO4 solution	No Precipitate formed	White ppt formed	White ppt formed

The following salts and concentrations can be used to study the reactions of the cations

Ion	Salt	g in 100cm3 of solution enough for 15 students
Pb2+	Pb(NO3)2	4
Zn2+	ZnSO4	4
Al3+	Al2(SO)3	4
Fe2+	FeSO4,  (NH4)2Fe(SO4)2	4
Fe3+	Fe2(SO4)3	4
Cu2+	CuSO4	4
Ni2+	NiSO4	4
Cr3+	KCr(SO4)2	4
Co2+	CoSO4	4
Mn2+	MnSO4	4
Ca2+	CaCl2	4
Mg2+	MgSO4	4 mix and filter, use the filtrate
Ba2+	BaCl2, Ba(NO3)2	4 mix and filter, use the filtrate

 

The following solutions can be used to study the reactions of the anions

Ion	Solution
SO42-	Dilute sulphuric acid
NO3–	Dilute nitric acid
Cl–	Dilute hydrochloric acid
CO32-	ZnCO3 salt

 

How to promote learning of qualitative inorganic chemistry

1. Create a relationship with laboratory personnel. It is you to lose when the laboratory personnel hates your class.
2. Share this knowledge with the laboratory personnel, s/he may be ignorant
3. Volunteer to help the laboratory personnel with washing equipment, delivering and removing equipment to and from the laboratory after the practical lesson. Knowing that laboratory work is tedious.
4. Interact the laboratory equipment freely with no fear after all it your parents to refund the broken apparatus
5. Do not panic, otherwise will contaminate the solution by mixing up droppers from solution to solution leading erroneous observation.
6. If you are a teacher it advisable to carry out any practical before giving it your student however familiar a practical may be.
7. Before you hand your answer sheet ensure that your observation and deduction are coherent/logical. For instance when Pb²⁺ ions are confirmed with a yellow ppt. using potassium iodide solution, the a white ppt soluble in excess dilute sodium hydroxide solution should have been observed (and Pb²⁺, Al³⁺, Zn²⁺ deduced), and a white ppt. insoluble in excess with ammonia should have been observed (and Pb²⁺, Al³⁺ deduced).

Reasons for improper/missed observations

1. Improper preparation of reagents
2. Contamination especially during the practical lessons when students mix up droppers. In exams it is advisable to start with the qualitative numbers before the reagents are contaminated with reckless students. If a practical is done in shifts fresh reagents should be provided for each shift.
3. Using a different reagent in panic instead of that mentioned in the test.

Example of experimental results

 

Example 1

You are provided with a substance Q [Fe(NH₄)₂(SO₄)₂] that contains two cations and one anion. You are required to identify the cations and anion present in Q. Carry out the following tests and record your observation and deduction in the spaces provided. Identify any gases evolved.

Tests	observation	Deductions
a) Comment on the appearance of Q	Green crystal	Fe2+, Ni2+ Cr3+ (green color) SO42-, Cl–, NO3– (crystalline)
b) Strongly heat one spatula end full of Q in a dry test tube	A liquid that turned the color of anhydrous copper Sulphate blue A gas that turned damp red litmus Paper blue A gas decolorized KMnO4 solution     A brown residue	Hydrated compound   NH3, NH4+ confirmed   SO2, SO42- suspected (Note that SO2 is given off after NH3 the reason why a red litmus turns blue then red) Fe2+ suspected oxidized to Fe3+
c) Dissolve one spatula end full of Q in about 5 ml of water, shake to dissolve and divide the solution into five portions	A green solution	Fe2+, Ni2+ Cr3+ (green color) SO42-, Cl– (soluble ions)
i) To the first portion add sodium hydroxide solution drop wise until in excess. Warm the resultant mixture	A green ppt. insoluble in excess Turns brown on standing A gas that turned damp red litmus blue	Fe2+   NH3, NH4+ confirmed
ii) To the second portion add ammonia solution drop wise until in excess	A green ppt. insoluble in excess Turns brown on standing	Fe2+
iii) Use the third portion to carry out a test of your choice to confirm the second cation	A deep blue solution on addition of Potassium hexacyanofferate III	Fe2+ confirmed
iv) To the fourth portion add 2-3 drops of lead nitrate solution and boil the resultant mixture	A white ppt. insoluble on boiling	SO42-, Cl– suspected
Use the fifth portion to carry out a test of your choice to confirm the anion	Additional of barium chloride solution followed by dilute nitric acid, a white ppt. formed	SO42- Confirmed present

 

Identify the        Cations: Fe²⁺ and NH₄⁺

Anion:    SO₄^2-

 

Example 2

You are provided with a substance P ( CuSO₄ + NaCl in ratio 2:1, add little to make a damp green solid) that contains one cation and two anions. You are required to identify the cation and anions present in A2. Carry out the following tests and record your observation and deduction in the spaces provided. Identify any gases evolved.

Tests	Observations	Deductions
(a) Place 2-3 Spatula end full of P in a boiling tube and add to it about 5 cm3 of distilled water. Then add excess silver nitrate solution and filter. Keep both the residue and the filtrate.	Blue filtrate   White residue	Cu2+   Cl–
(b) Divide the filtrate into 4 portions (i) To the first portion add sodium hydroxide solution drop wise until in excess. Warm the resultant mixture	Blue ppt. turn black on warming	CuO formed, Cu2+ suspected
(ii) To the second portion add ammonia solution drop wise until in excess	A blue ppt. dissolved in excess ammonia to form a deep blue solution	Cu2+ confirmed
(iii) To the third portion add 2-3 drops of lead nitrate solution then few dilute nitric acid. Warm the resultant mixture	A white ppt. insoluble in nitric acid	SO42- suspected (NB. Cl– were removed by silver nitrate)
(iv) Use the fourth portion to carry out a test of your choice to confirm the first anion in the filtrate	Addition of barium chloride solution followed by dilute nitric acid, a white ppt. formed	SO42- confirmed
(c)    Add water to the residue several times, decant to wash it To the cleaned residue, add dilute nitric acid drop wise followed by dilute ammonia until in excess	The  residue is insoluble in nitric acid but soluble in ammonia	Cl– confirmed (NB. The residue was AgCl)

Identify the        Cations: Cu²⁺

Anions:    SO₄^2- and Cl^–

 

Example 4

You are provided with a substance K (MnSO₄ + ZnCO₃) that contains two cation and two anions. You are required to identify the cation and anions present in K. Carry out the following tests and record your observations and deductions in the spaces provided. Identify any gases evolved.

Tests	Observations	Deductions
(a) Heat a spatula end-full of K first gently and then very strongly until there is no further change	A liquid that turned anhydrous copper Sulphate blue A gas that turned lime water milk Brown residue	Hydrated compounds   CO2 given off, CO32- Pb2+, Zn2+, Mn2+
(b) Transfer two spatula end-fulls of K to a test tube, shake it with about 10 cm3 of water and filter.   Keep both the filtrate and the residue.	Colorless filtrate   White residue	Zn2+, Pb2+, Mn2+ SO42-, Cl– CO32-
(c) Divide the filtrate into five portions (i) To the first portion add ammonia solution Drop wise until in excess	White ppt. insoluble in excess rapidly turns brown	Mn2+
(ii) To the second portion add sodium hydroxide drop wise until in excess.	White ppt. insoluble in excess rapidly turns brown	Mn2+
(iii)               Place the third part of the filtrate in a small porcelain dish; add 2-3 drops of concentrated nitric acid, followed by a little amount of solid sodium bismuthate.	A purple solution	Mn2+ confirmed present
(iv) To the fourth part of the filtrate add a few drops lead nitrate solution and warm.	White ppt. insoluble on heating	SO42-
(v) Use the fifth portion to carry out a test of your choice to confirm the anion present in K.	A white ppt. formed with barium chloride solution insoluble in nitric acid	SO42- confirmed present
(d) Wash the residue, dissolve it in dilute hydrochloric acid and divide the solution into three portions. (i) To the first portion add sodium hydroxide drop wise until in excess.	Dissolves with effervescence giving of a gas that turned lime water milky   White ppt. soluble in excess	CO2 given off, CO32- confirmed present   Pb2+, Zn2+
(ii) To the second portion add ammonia solution drop wise until in excess	White ppt. soluble in excess	Zn2+ confirmed present
(iii) To the third portion of the solution, add 2-3 drops of potassium iodide solution	No observable change	Zn2+ confirmed present
(iv) to the fourth portion of the solution, add 2-3 drops of potassium hexacyanoferrate (II) solution	White ppt	Zn2+ confirmed present

 

Identify the

(i)  cations in K: Mn²+ and Zn²⁺

 

(ii)  and anions : SO₄^2- and CO₃^2-

Thanks Dr. Bbosa Science