Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 1 of 7

Unit 9.2:

Qualitative Analysis. Lesson 1 of 3

Qualitative Chemical Analysis.

In chemistry, qualitative analysis is the determination of the chemical composition of a sample. Qualitative analysis can tell you whether an atom, ion, functional group, or compound is present or absent in a sample, but it doesn't provide information about its quantity. Quantification of a sample, in contrast, is called quantitative analysis. Qualitative analysis includes a wide range of specific chemical tests to identify substances. However, before carrying out specific tests when trying to identify an unknown substance, it is good practice to first carry out some preliminary investigations. These can include:

taking note of appearance; check solubility of substance in water; investigate behaviour of substance when heated or burned.

In compliance with our current Chemistry syllabus, we shall focus on appearance of substances and then deal with specific chemical identification tests for a number of gases and ionic compounds. Solubility and effect of heat on substances are dealt with in other chapters and it is recommended that these are revised.

Appearance of substances.

• The first test that a good chemist should do, is to take note of the appearance of a substance under

test since this can give a hint of what the compound can be. For example: A crystalline solid is probably an ionic solid. A brittle

solid or a powder is a non-metal or covalent compound. There are exceptions however. For example, sodium hydrogen carbonate is ionic but can be described as a powder rather than crystalline. Solid iodine and sugar are crystalline, but both are covalent not ionic.

Liquids and gases are predominantly covalent in nature.

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 2 of 7

A shiny solid is a metal or graphite (carbon). Coloured ionic compounds typically show the presence of

a transition metals. Copper(II) (Cu2+) compounds: typically blue or green, except for copper(II) oxide which is black and copper(I) oxide which is of a reddish colour. Iron(II) (Fe2+) compounds: green; Iron(III) (Fe3+) compounds: rusty brown.

All compounds of group 1 & 2 metals are white.

Tests for Gases.

There are a variety of gases that can be tested. Properties of the gas give a hint of its identity. One can then proceed to a final, confirmatory test.

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 3 of 7

Gas Properties Confirmatory Test Equation

Oxygen O2

Colourless, odourless,

tasteless, neutral.

Relights a glowing splint.

n/a

Hydrogen H2

Colourless, odourless,

tasteless, neutral. Burns with a ‘pop’. 2H2(g) + O2(g) → 2H2O(l)

Carbon dioxide

CO2

Colourless, odourless,

tasteless, weakly acidic.

Turns lime water milky. CO2(g) + Ca(OH)2(aq) → CaCO3(s) +

H2O(l)

Sulfur dioxide

SO2

Colourless, pungent/choking

smell, acidic.

Turns potassium dichromate solution

from orange to green. (Also decolourises

purple potassium(VII) manganite solution.)

n/a

Hydrogen chloride

HCl

Colourless/fuming, pungent, acidic.

Forms white fumes with ammonia.

HCl(g) + NH3(g) → NH4Cl(s)

Ammonia NH3

Colourless, pungent (unclean toilets),

alkaline.

Forms white fumes with hydrogen chloride.

Turns wet red litmus blue.

HCl(g) + NH3(g) → NH4Cl(s)

NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)

Chlorine Cl2

Yellow-green, pungent, acidic.

Turns litmus paper red and then bleaches it

white.

Cl2(g) + H2O(l) → HCl(aq) + HOCl(aq)

Nitrogen dioxide

NO2

Brown, sharp/ biting odour, acidic.

Its brown coloration. n/a

Steam H2O(g)

Colourless, odourless,

tasteless, neutral.

Turns white anhydrous

copper(II) sulfate to blue.

(Also turns blue anhydrous cobalt(II)

chloride to pink.)

CuSO4(s) + 5H2O(g) → CuSO4.5H2O(s)

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 4 of 7

Exercise 1:

The letters A to F denote six gases. A: nitrogen dioxide B: oxygen C: hydrogen D: ammonia E: chlorine F: sulfur dioxide

a) Use the letters A to F to answer the following questions. Each letter may be used once, more than once or not at all. Select, from A to F: A coloured pungent gas which turns moist blue litmus red and contributes to acid rain. ______ A colourless pungent gas which forms dense white fumes when placed near concentrated

hydrochloric acid. ______ A colourless odourless gas which relights a glowing splint. ______ A coloured gas which first turns moist blue litmus red than bleaches it. ______ A colourless pungent gas which turns moist red litmus blue. ______ A colourless odourless gas that burns with a popping sound. ______ A colourless pungent gas which is produced when dilute aqueous NaOH is added to an aqueous

ammonium salt and the mixture heated. ______ A colourless pungent gas which turns moist blue litmus red and contributes to acid rain. ______

(8 marks)

Test for ammonia

Test for sulfur dioxide Hydrogen chloride & ammonia

Nitrogen dioxide

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 5 of 7

b) A colourless, pungent gas from the above list can change the colour of solutions of potassium dichromate and potassium permanganate when bubbled through them. Give the name of this gas and state the colour change observed when this gas is bubbled through potassium dichromate solution.

_____________________________________________________________________________ _____________________________________________________________________________

(2 marks) (Total: 10 marks)

Exercise 2:

The letters A to F denote six solid substances.

A: copper(II) sulfate B: iron(II) chloride C: iron(III) hydroxide

D: copper(II) oxide E: iodine F: graphite

a) Use the letters A to F to answer the following questions. Each letter may be used once, more than once or not at all. Select, from A to F

A black, dull, ionic substance. ______ A black, shiny, brittle substance. ______ A crystalline, covalent substance. ______ A substance whose hydrated form is crystalline and blue. ______ A substance that will give a green solution when dissolved in water. ______ An insoluble solid which is rusty brown in colour. ______ A substance that conducts electricity in its solid state. ______ A solid whose anhydrous form is used to test for the presence of water. ______

(8 marks) b) When a solution of iron(II) chloride is added to a solution of sodium hydroxide, a solid precipitate is produced.

Write a balanced chemical equation for the reaction, including state symbols.

_______________________________________________________________________ (3)

What colour would you expect this precipitate to be? ____________________________ (1)

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 6 of 7

c) Copper(II) oxide can be produced from the thermal decomposition of copper(II) carbonate. A gas is also produced.

What colour would you expect copper(II) carbonate to be? ________________________ (1) Give the name of the gas produced. _________________________________________ (1) Describe how you would test for this gas and state what you would observe. Include a balanced

chemical equation that explains these observations. _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ (6)

(Total: 20 marks)

By the end of this lesson you should be able to:

Understand the difference between Qualitative and Quantitative analysis in Chemistry. Describe the appearance of various substances. Use preliminary investigations on substances involving their appearance, solubility and behaviour

when heated to make predictions on their possible identity. Describe identification tests for the following gases: oxygen, hydrogen, carbon dioxide, ammonia,

chlorine, hydrogen chloride and sulfur dioxide.

Mr. Robert Zammit M.Sc.

Unit 9.2 – Qualitative analysis - Lesson 1 of 3 Page 7 of 7

Model answers to Exercise 1:

a) A; D; B; E; D; C; D; F (1 mark each)

b) Sulfur dioxide. (1 mark) Changes potassium dichromate solution from orange to green. (1 mark)

Model answers to Exercise 2:

a) D; F; E; A; B; C; F; A (1 mark each)

b) FeCl2(aq) + 2NaOH(aq) → Fe(OH)2(s) + 2NaCl(aq) (1 mark formulae; 1 mark balancing; 1 mark state symbols) Fe(OH)2 is muddy green in colour. (1 mark)

c) Copper(II) carbonate is blue-green in colour. (1 mark) Gas produced: carbon dioxide. (1 mark)

Bubble carbon dioxide through lime water solution. (2 marks) Limewater turns milky white as a white precipitate forms. (1 marks) CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)

(1 mark formulae; 1 mark balancing; 1 mark state symbols)