8.3 Qualitative Analysis Flashcards
What is qualitative analysis?
Technique that relies on simple observations rather than measurements
eg - bubbles, precipitates, colour changes, or identification of gases
What gas do carbonates produce when they react with acids?
Carbon dioxide
Carbonate ion formula
CO3 2-
Test for carbonate ion
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
How to test for CO2
Bubble the gas through lime water
If it becomes cloudy, the gas is Carbon Dioxide
Reaction in limewater
CO2 + Ca(OH)2 —> CaCO3 + H2O
The precipitate Calcium Carbonate is formed, giving the milky colour
Which sulfate is insoluble in water?
BaSO4
Barium Sulfate
Sulfate ion formula
SO4 2-
Sulfate ion test
Aqueous Barium ions are added to a solution of an unknown compound, which forms a white precipitate Barium Sulfate
Ionic equation of Sulfate test
Ba2+ + SO42- —> BaSO4 (s)
How are Ba2+ ions added to the solution?
Aqueous Barium Chloride or Barium Nitrate
If a halide test is being carried out next, use Barium Nitrate, as Barium Chloride introduces Chloride ions
Which halides are insoluble in water?
Silver halides
Ionic equation for halide tests
Ag+ + X- —> AgX (s)
Halide test process
Add HNO3 and AgNO3 (aq) to an aqueous solution of a halide
Different colour precipitates will be formed
Only soluble Silver halide…
Silver fluoride
Different colour Silver halide precipitates
AgCl - white
AgBr - cream
AgI - yellow
How is the solubility of the silver halides tested? Why?
Using aqueous ammonia as the colours can sometimes be hard to tell apart
What happens when NH3 (aq) is added to AgCl?
It is soluble in dilute ammonia
What happens when NH3 (aq) is added to AgBr?
It is soluble in concentrated ammonia
What happens when NH3 (aq) is added to AgI?
It is insoluble in concentrated NH3
How are barium meals used in medicine?
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
Correct order for anion tests
1) Carbonate
2) Sulfate
3) Halides
Why is the carbonate test carried out first?
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
Why is the sulfate test second?
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
Why is the halide test carried out last?
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
What should you do if asked to analyse a mixture of chemicals?
Carry out the tests in the same sequence and on the same solution
Carbonate test in a mixture of chemicals
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
Why is nitric acid used for this carbonate test?
Sulfuric acid contains sulfate ions
Hydrochloric acid contains chloride ions
These will show up in sulfate and halide tests
Sulfate test in a mixture of chemicals
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
Halide test in a mixture of chemicals
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
How does the test for NH4 + ions work?
When heated together, ammonium ions and aqueous hydroxide ions react to form ammonia gas NH3
NH4+ ion test
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)
