8.3 Qualitative Analysis

Question 1

What is qualitative analysis?

Answer 1

Technique that relies on simple observations rather than measurements
eg - bubbles, precipitates, colour changes, or identification of gases

Question 2

What gas do carbonates produce when they react with acids?

Answer 2

Carbon dioxide

Question 3

Carbonate ion formula

Answer 3

CO3 2-

Question 4

Test for carbonate ion

Answer 4

Add nitric acid to the solid or solution to be tested
If there are bubbles, the unknown compound could be a carbonate
Test the gas that is produced

Question 5

How to test for CO2

Answer 5

Bubble the gas through lime water

If it becomes cloudy, the gas is Carbon Dioxide

Question 6

Reaction in limewater

Answer 6

CO2 + Ca(OH)2 —> CaCO3 + H2O

The precipitate Calcium Carbonate is formed, giving the milky colour

Question 7

Which sulfate is insoluble in water?

Answer 7

BaSO4

Barium Sulfate

Question 8

Sulfate ion formula

Answer 8

SO4 2-

Question 9

Sulfate ion test

Answer 9

Aqueous Barium ions are added to a solution of an unknown compound, which forms a white precipitate Barium Sulfate

Question 10

Ionic equation of Sulfate test

Answer 10

Ba2+ + SO42- —> BaSO4 (s)

Question 11

How are Ba2+ ions added to the solution?

Answer 11

Aqueous Barium Chloride or Barium Nitrate

If a halide test is being carried out next, use Barium Nitrate, as Barium Chloride introduces Chloride ions

Question 12

Which halides are insoluble in water?

Answer 12

Silver halides

Question 13

Ionic equation for halide tests

Answer 13

Ag+ + X- —> AgX (s)

Question 14

Halide test process

Answer 14

Add HNO3 and AgNO3 (aq) to an aqueous solution of a halide

Different colour precipitates will be formed

Question 15

Only soluble Silver halide…

Answer 15

Silver fluoride

Question 16

Different colour Silver halide precipitates

Answer 16

AgCl - white
AgBr - cream
AgI - yellow

Question 17

How is the solubility of the silver halides tested? Why?

Answer 17

Using aqueous ammonia as the colours can sometimes be hard to tell apart

Question 18

What happens when NH3 (aq) is added to AgCl?

Answer 18

It is soluble in dilute ammonia

Question 19

What happens when NH3 (aq) is added to AgBr?

Answer 19

It is soluble in concentrated ammonia

Question 20

What happens when NH3 (aq) is added to AgI?

Answer 20

It is insoluble in concentrated NH3

Question 21

How are barium meals used in medicine?

Answer 21

Used to enable doctors to see the outline of the gullet, stomach and upper small intestine to identify abnormalities such as ulcers or tumours
The white precipitate Barium Sulfate coats the inner lining of the gut which is visible on X ray

Question 22

Correct order for anion tests

Answer 22

1) Carbonate
2) Sulfate
3) Halides

Question 23

Why is the carbonate test carried out first?

Answer 23

When dilute acid is added, effervescence is produced from CO2 gas
Neither sulfate or halide ions produce bubbles with dilute acid
Test can be carried out without possibility of an incorrect conclusion

Question 24

Why is the sulfate test second?

Answer 24

After adding Ba2+ ions, a white precipitate of BaSO4 is produced
BaCO3 is also a white precipitate, so if this test is carried out on a carbonate, a white precipitate will still be formed
Therefore, carbonate test should be carried out first to make sure no carbonate is present

Question 25

Why is the halide test carried out last?

Answer 25

In this test, AgNO3 is added, looking for a precipitate
However, Ag2CO3 and Ag2SO4 are both insoluble in water and will form precipitates
Therefore, this test is carried out last, after carbonate and sulfate ions are ruled out

Question 26

What should you do if asked to analyse a mixture of chemicals?

Answer 26

Carry out the tests in the same sequence and on the same solution

Question 27

Carbonate test in a mixture of chemicals

Answer 27

If bubbles are seen, continue adding nitric acid until all bubbling stops
All carbonate ions will have been removed by then - no more to react in the other tests

Question 28

Why is nitric acid used for this carbonate test?

Answer 28

Sulfuric acid contains sulfate ions
Hydrochloric acid contains chloride ions
These will show up in sulfate and halide tests

Question 29

Sulfate test in a mixture of chemicals

Answer 29

To the solution left from the carbonate test, add an excess of Barium Nitrate (aq)
Any sulfate ions will precipitate out as Barium sulfate
Filter out the Barium sulfate

Question 30

Halide test in a mixture of chemicals

Answer 30

Add AgNO3 (aq)
Any carbonate and sulfate ions should have been removed, so precipitates formed must involve halide ions
Add NH3 to confirm which halide is present

Question 31

How does the test for NH4 + ions work?

Answer 31

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

Question 32

NH4+ ion test

Answer 32

NaOH (aq) is added to a solution of an ammonium ion
Ammonia gas is produced - unlikely to see gas bubbles as ammonia is soluble in water
Mixture is warmed, and ammonia gas is released
Test for gas with moist pH indicator paper (alkaline and will turn paper blue)