8.3 Qualitative Analysis

Question 1

Define qualitative analysis

Answer 1

Relies on simple observations rather than measurements.

Observations can include bubbles, precipitates or colour changes

Question 2

How do you test for carbonates? State the method

Answer 2

Carbonates react with acid to form CO2

1. In a test tube, add dilute nitric acid to the solid / solution
2. Bubbles mean it could be a carbonate
3. To prove it is CO2, bubble it through lime water ( calcium hydroxide - Ca(OH)2
4. If it is carbon dioxide, it will turn cloudy ( as it forms a fine white calcium carbonate precipitate )

Question 3

List the equations that go on in the carbonate test if you were to test sodium carbonate

Answer 3

Sodium carbonate and dilute nitric acid :

Na2CO3 (aq) + 2HNO3 (aq) -> 2NaNO3 (aq) + CO2 (g) + H2O

Carbon dioxide and lime water :

CO2 (g) + Ca(OH)2 -> CaCO3 (s) + H2O (l)

Question 4

How do you test for sulfate ions?

Answer 4

Most sulfates = soluble
Barium sulfate = insoluble

Add dilute HCl, followed by aqueous barium chloride (BaCl2)

White precipitate means its barium sulfate, so sulfate is present

Question 5

Write the ionic equation involved in the sulfate test

Answer 5

Ba2+ (aq) + SO42- (aq) -> BaSO4 (s)

Question 6

Write the equation for testing sulfates in a sample of sodium sulfate

Answer 6

Na2SO4 (aq) + BaCl2(aq) -> BaSO4(s) + 2NaCl (aq)

Question 7

How do you test for halides? Include the general equation

Answer 7

Most halides = soluble
Silver halides = insoluble

Aqueous silver ions react with aqueous halide ions to form precipitates of silver halides

Ag+ (aq) + X- (aq) -> AgX(s)

Question 8

State the method used to test for halide ions

Answer 8

1. Add aqueous silver nitrate (AgNO3) to an aqueous solution of a halide
2. The silver halide precipitate are different colours :
   Silver chloride = white, Silver Bromide = cream, Silver Iodine = yellow
3. Add aqueous ammonia to test the solubility of the precipitate. This stage is used as it hard to tell the colours apart.

Chloride = soluble in dilute NH3 
Bromide = soluble in concentrated NH3 
Iodide = insoluble in concentrated NH3

Question 9

What is the order you should do the tests in?

Answer 9

Carbonate
Sulfate
Halide

Question 10

Why do the tests have to be done in a certain order?

Answer 10

The carbonate test can produce bubbles, but the other two tests can’t. Therefore you can do it first without an incorrect conclusion.

To test for sulfates, you look for a white precipitate of BaSO4. But BaCO3 will also produce a white precipitate. So you need to do the carbonate test first.

In the halide test, you are looking for a precipitate, but silver carbonate (Ag2CO3) and silver sulfate (Ag2SO4) are both insoluble and will produce precipitates. Therefore, halide test is done last after ruling out carbonates and sulfates as possible.

Question 11

How would you test a solution for a mixture of ions rather than one specific one?

Answer 11

1. Carbonate test
   If bubbles, keep adding dilute nitric acid until it stops. This means carbonate ions have been removed.
2. Sulfate test
   Once done, filter solution to remove barium sulfate
3. Halide test
   Carry out as normal

Question 12

The other three tests are for anions. How do you test for cations? Give the equation and steps to the method

Answer 12

When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas, NH3.

NH4+ (aq) + OH-(aq) -> NH3(g) + H2O(l)

1. Add aqueous sodium hydroxide to solution of ammonium ion
2. Ammonia gas produced but no bubbles as it soluble
3. Mixture warmed and ammonia gas released
4. Test with moist pH indicator paper. Ammonia is alkaline and its presence will turn the paper blue