CHEMISTRY : Week 1

 Chemistry Week 1

QUALITATIVE ANALYSIS OF SALT

What is qualitative analysis of salt?

It is a chemical technique to identify the ions present in a salt.

There are 3 steps of qualitative analysis :

         i.            PHYSICAL PROPERTIES

       ii.            ACTION OF HEAT

      iii.            TEST USING AQUEOUS SOLUTION + CONFIRMATORY TEST

PHYSICAL PROPERTIES (based on colour, solubility in water and physical state)

Solid	Aqueous	Possible salts, cations or metal oxide
White	Colourless	K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Pb2+, NH4+
Green	Insoluble	CuCO3
Light green	Light green	Fe2+
Blue	Blue	Cu2+
Brown	Brown	Fe3+
Black	Insoluble	CuO
Yellow (hot), white (cold)	Insoluble	ZnO
Brown (hot), yellow (cold)	Insoluble	PbO

ACTION OF HEAT (heat strongly)

Only for NO₃^- and CO₃^2-  because Cl^- and SO₄^2- cannot be decomposed by heat.

Able to detect the present of Cu^2+, , Zn²⁺, and Pb²⁺ ions because they have have special colours :

·         Cu²⁺ = Black

·         Zn²⁺ = Yellow when hot, white when cold

·         Pb²⁺  = Brown when hot, yellow when cold

solution  = metal oxide + gas

1.       Observe gas released to detect anion

Gas released	Properties	Gas	Anions present
Brown	Turns blue litmus paper to red	NO2 gas	NO32- ion
Colourless	Gas relights glowing splinter	O2 gas	O2- ion
Colourless	Turns lime water chalky	CO2 gas	CO22- ion
Colourless + pungent smell	Turns red litmus paper to blue	NH3  gas	NH4+ ion

2.       Observe the colour of residue to detect cation

Colour		Cations present
Black	CuO	Cu2+
Yellow (hot), white (cold)	ZnO	Zn2+
Brown (hot), yellow (cold)	PbO	Pb2+
White		Fe2+, Fe3+, Al3+, Ca2+, Mg2+, NH4+

CONFIRMATORY TEST FOR CATIONS

Using :

         i.            NaOH solution

       ii.            NH₃ solution

      iii.            Confirmatory test for specific ions

NH4+	Nesslers’ Reagent	Brown precipitate
Pb+	KI solution	Yellow precipitate

	Fe2+	Fe3+
Potassium hexacyanoferrate (ii)		Dark blue precipitate
Potassium hexacyanoferrate (iii)	Dark blue precipitate
Potassium thiocyanate		Blood red colouration

	Fe2+ uses Potassium hexacyanoferrate (iii) Fe3+ uses Potassium hexacyanoferrate (ii)

 

*For NaOH and NH₃ confirmatory test, refer to Chemistry Nilam page 171 and 172 (the mind map)

CONFIRMATORY TEST FOR ANION

Using ;

         i.            Any dilute acid (H₂SO₄ / HNO₃ / HCl / CH₃COOH)

       ii.            Ag⁺ (any soluble solution contains Ag⁺ ion, eg ; AgNO₃)

      iii.            Ba²⁺ (any soluble solution contains Ba²⁺  ion, eg ; BaSO₄)

     iv.            Dilute H₂SO₄ + FeSO₄ + concentrated H₂SO₄

		Observation
CO32- = any dilute acid	CO32- + 2H+ = CO2 + H2O	Lime water turns chalky
Cl- = AgNO3	Ag+ + Cl- = AgCl	White precipitate
SO42- = BaSO4	Ba2+ + SO42-  = BaSO4	White precipitate
NO3- = Dilute H2SO4 + FeSO4 + concentrated H2SO4		Brown ring formed

*If the solution contains CO₃^2- ion, when dilute acid is added, it will turns lime water chalky.