Qualitative Analysis CAPE

What is qualitative analysis

A series of tests used to identify the presence of specific cations and anions

What information does qualitative analysis not give

The amount of each ion present

What are the preliminary tests

Appearance, solubility, heat, flame test

What does a flame test identify

Metal cations by flame colour

Flame colour of sodium

Golden yellow

Flame colour of potassium

Lilac

Flame colour of lithium

Red

Flame colour of calcium

Brick red / crimson

Flame colour of barium

Light green

Flame colour of copper

Blue-green

Large crystals indicate

Water of crystallization present

Powdery solid indicates

No water of crystallization

Green salt indicates

Fe²⁺ present

Blue salt indicates

Cu²⁺ present

Coloured salt indicates

Transition metal present

White salt indicates

No transition metal present

Black solid indicates

CuO present

Yellow solid indicates

PbI₂ present

ZnO when heated

Yellow when hot, white when cool

Clear colourless solution

Soluble salt, no transition metal

Clear coloured solution

Soluble salt with transition metal

Undissolved solid

Insoluble salt

White ppt dissolves on heating and reforms on cooling

PbCl₂ present

How are gases identified

Odour, litmus, splint tests, limewater

Test for hydrogen

Pop with lighted splint

Test for oxygen

Relights glowing splint

Test for carbon dioxide

Turns limewater milky

Test for ammonia

Turns damp red litmus blue

Test for sulfur dioxide

Turns dichromate orange to green

Test for chlorine

Bleaches damp litmus paper

Test for nitrogen dioxide

Brown gas turns litmus red

What reagent is used for cation tests

NaOH and NH₃

What happens when NaOH is added to most cations

Insoluble hydroxide precipitate forms

Al³⁺ with NaOH

White ppt, soluble in excess

Pb²⁺ with NaOH

White ppt, soluble in excess

Zn²⁺ with NaOH

White ppt, soluble in excess

Ca²⁺ with NaOH

White ppt, insoluble

Mg²⁺ with NaOH

White ppt, insoluble

Cu²⁺ with NaOH

Blue ppt, insoluble

Fe²⁺ with NaOH

Green ppt turns brown

Fe³⁺ with NaOH

Red-brown ppt

Cr³⁺ with NaOH

Grey-green ppt, soluble in excess

Al³⁺ with NH₃

White ppt, insoluble

Pb²⁺ with NH₃

White ppt, insoluble

Zn²⁺ with NH₃

White ppt, soluble in excess

Ca²⁺ with NH₃

No ppt

Mg²⁺ with NH₃

White ppt

Cu²⁺ with NH₃

Blue ppt → deep blue solution in excess

Fe²⁺ with NH₃

Green ppt

Fe³⁺ with NH₃

Red-brown ppt

Cr³⁺ with NH₃

Grey-green ppt, insoluble

Which hydroxides are amphoteric

Al(OH)₃, Zn(OH)₂, Pb(OH)₂

Meaning of amphoteric

Reacts with both acids and bases

Zn(OH)₂ + excess OH⁻

Forms [Zn(OH)₄]²⁻

Al(OH)₃ + OH⁻

Forms [Al(OH)₄]⁻

Pb(OH)₂ + OH⁻

Forms [Pb(OH)₄]²⁻

Zn(OH)₂ + NH₃

Forms [Zn(NH₃)₄]²⁺ complex

Cu²⁺ + NH₃

Forms deep blue [Cu(NH₃)₄]²⁺

Why precipitates dissolve in excess

Complex ion formation shifts equilibrium

Condition for precipitate formation

Ionic product exceeds Ksp

Why heating favors decomposition

Endothermic reaction shifts equilibrium

Confirmatory test for Pb²⁺

Add KI → yellow PbI₂ ppt

Equation for Pb²⁺ test

Pb²⁺ + 2I⁻ → PbI₂(s)

Na₂CO₃ with Mg²⁺

White ppt MgCO₃

Na₂CO₃ with Zn²⁺

White ppt

Na₂CO₃ with Fe²⁺

Green ppt FeCO₃

Na₂CO₃ with Fe³⁺

Red-brown ppt

Na₂CO₃ with Pb²⁺

White ppt PbCO₃

Na₂CO₃ with Cu²⁺

Blue-green ppt

Na₂CO₃ with Cr³⁺

Green ppt + CO₂

Na₂CO₃ with Al³⁺

White ppt + CO₂

Na₂CO₃ with NH₄⁺

NH₃ gas + CO₂ + H₂O

Equation for NH₄⁺ with carbonate

2NH₄⁺ + CO₃²⁻ → 2NH₃ + H₂O + CO₂

Test for carbonate ions

Add acid → effervescence, CO₂ turns limewater milky

Test for sulphite ions

Add acid → SO₂ gas turns dichromate green

Test for sulphate ions

Add Ba²⁺ → white ppt insoluble in acid

Test for nitrate ions

Brown ring test

Observation in brown ring test

Brown ring forms at interface

Other nitrate tests

Copper turnings test, Devarda’s alloy test

What happens in Devarda’s test

Nitrate reduced to ammonia

Chromate colour in alkaline solution

Yellow

Chromate in acidic solution

Orange (dichromate)

Equation for chromate equilibrium

2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O

What indicates ammonia gas in NaOH test

Turns red litmus blue

Which ions produce ammonia with NaOH

NH₄⁺ only

Why Fe²⁺ turns brown

Oxidation to Fe³⁺ in air

Key idea of qualitative analysis

Identify ions based on characteristic reactions and observations