Qualitative Analysis

What is the test for ammonium ions (NH4+)

1. Add NaOH

2. Gently heat

3. Test any gas produced with damp red litmus

What observations can be made from a positive test for NH+ ions

Damp red litmus turns red to blue

What are the reactions that take place during the test for ammonium ions (give equations)

• NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)

• NH3(g) + H2O(l) → NH4+(aq) + OH-(aq)

Why does the red litmus have to be damp during the test for ammonium ions

To provide water for the ammonia to dissolve and reform the NH4+ ions

What is the test for sulphate ions (SO4²-)

1. Add dilute HCl

2. Add BaCl2(aq) or Ba(NO3)2(aq)

What can be observed in the positive test for SO4²- ions

A white ppt

Give the ionic equation for the positive test for SO4²- ions with the Ba(NO3)2

Ba²+(aq) + SO4²-(aq) → Ba(SO4)²-(s)

Why is dilute HCl added in the test for sulphate ions, and why not H2SO4

• HCl is added to remove any CO3²- forming a precipitate of BaCO3.

• H2SO4 contains SO4²- ions

What is the test for carbonate ions (CO3²-)

1. Add dilute HCl

2. Test gases formed by bubbling through limewater (Calcium hydroxide solution)

What is the positive result for presence of carbonate ions

Limewater goes from colourless to cloudy

Give the equations for the reactions involved in the test for CO3²- ions

• CO3²-(aq) + 2H+(aq) → CO2(g) + H2O(l)

• CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)

What is the test for HCO3- ions

1. Add dilute HCl

2. Test gases formed by bubbling through limewater (Calcium hydroxide solution)

What is the positive result for presence of HCO3- ions

Limewater goes from colourless to cloudy

Give the equations for the reactions involved in the test for HCO3- ions

• HCO3-(aq) + H+(aq) → CO2(g) + H2O(l)

• CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)

What are the test for halide ions

1. Add a few drops of dilute nitric acid

2. Add a few drops of silver nitrate solution (AgNO3)

3. Add dilute ammonia

4. If not dissolved add concentrated ammonia

Why is dilute nitric acid added in the test for halide ions and why is HCl not used.

• dilute nitric acid removes any carbonate ions which would form precipitate of silver carbonate

• HCl(aq) contains Cl- ions

What is a positive test for Cl- ions

• White precipitate

• Dissolves in presence of dilute ammonia

What is a positive test for Br- ions

• Cream ppt

• Insoluble with dilute ammonia

• Dissolves with concentrated ammonia

What is a positive test for I- ions

• Yellow ppt

• Insoluble with dilute ammonia

• Insoluble with concentrated ammonia

What are the tests for group 2 cations

• add NaOH (forms a hydroxide)

• Add H2SO4 or Na2SO4 (forms a sulphate)

• Test is dependant on the solubility of the metal hydroxide/sulphate

What can be observed during the positive tests for Mg2+ ion

• With NaOH, white ppt

• With H2SO4 or Na2SO4, colourless solution formed

What can be observed during the positive tests for Ca2+ ion

• With NaOH, faint white ppt

• With H2SO4 or Na2SO4, colourless solution

What can be observed during the positive tests for Sr2+ ion

• With NaOH, colourless solution

• With Na2SO4 or H2SO4, faint white ppt

What can be observed during the positive tests for Ba2+ ion

• With NaOH, colourless solution

• With H2SO4 or H2SO4, white ppt