Qualitative analysis

Question 1

What do qualitative tests rely upon?

Answer 1

Qualitative tests rely upon simple observations rather than measurements and can be carried out on a test tube scale very quickly.

Question 2

When testing unknown in organic compounds, what do we carry out the test for first?

Answer 2

Anions

Question 3

In what order are the tests for anions done?

Answer 3

Carbonate test, sulphate test and then halide tests

Question 4

Why do we do the carbonate test first?

Answer 4

So it can be eliminated so carbonate compounds won’t give false positives, such as white precipitate

Question 5

what do carbohydrates react with acid to form?

Answer 5

as salt, a gas and a harmless liquid

Question 6

What do we use to identify carbonates?

Answer 6

this reaction of dilute nitric acid with an unknown solid or solution to identify carbonates.

Question 7

if we see bubbles, what could our compound be? (carbonate test)

Answer 7

a carbonate

Question 8

how can we prove the gas formed as carbon dioxide?

Answer 8

-Bubbled through limewater, it turns cloudy

Question 9

why is the carbonate test always undertaken first with an unknown compound?

Answer 9

as there is no risk of an incorrect conclusion being made

Question 10

What is sulphate test based on?

Answer 10

Precipitation formation

Question 11

What time is sulphate soluble in?

Answer 11

water

Question 12

What can be used to identify sulphate in unknown solutions?

Answer 12

Barium sulphate as it is very insoluble

Question 13

What is barium sulphate form when it forms?

Answer 13

white precipitate

Question 14

What do we normally use as a source of ba2+ ions in the sulfate test?

Answer 14

barium chloride or barium nitrate

Question 15

What is barium carbonate?

Answer 15

An insoluble white precipitate in water

Question 16

what can carbonate salts also be formed by?

Answer 16

Transitional metal ions

Question 17

If you want to do a test for sulphate and a test for halides one why do you need to use barium nitrate?

Answer 17

-barium chloride would also supply a source of chloride ions which will react with the silver nitrate forming a white precipitate and giving a false positive for chloride ions

Question 18

what is the test for halide ions based on?

Answer 18

Precipitate formation and solubility

Question 19

How can the halide ion be determined in aqueous solution?

Answer 19

solution silver nitrate followed by the addition of dilute ammonia

Question 20

why is the halide test left until last in the sequence?

Answer 20

as both silver carbonate and silver sulphate are insoluble white precipitate and give false positives for chloride ions

Question 21

what is the test for cations? (ammonium ions)

Answer 21

When ammonium (NH4+) containing salts are heated with aqueous hydroxide (OH-) the reaction produces ammonia gas (NH3).

Question 22

what is the ammonia formed in the test of the ammonium ions?

Answer 22

alkali and a gas

Question 23

how can we test ammonia in the test for ammonium ions (NH4+)?

Answer 23

1.Aqueous sodium hydroxide is added to the solution we want to test for ammonium ions.
2. To allow the ammonia to be released as a gas we have to heat the mixture (in a water bath or with a Bunsen burner).
3. The mixture does not present gas with bubbles as ammonia is much more soluble in water than carbon dioxide.
4. If ammonium ions are present the gas that is released is alkali and we test for this using moist pH indicator (red) paper (turns blue).