chemical analysis exam questions

Question 1

These alums contain aluminium ions (Al3+).
Describe how sodium hydroxide solution can be used to show this.
……………………………………………………………………………………………………

Answer 1

add excess sodium hydroxide to the aluminium hydroxide precipitate (or white precipitate) if the precipitate redissolves, aluminium ions are present.

Question 2

Aluminium ions do not give a colour in flame tests. However, flame tests can be used to
distinguish between these three alums.
Explain how these three alums could be identified from the results of flame tests.
………………………………………………………………………………………………………..

Answer 2

add sample of metal ion on a wire mounted in the handle
put through a blue bunsen burner flame if flame turns yellow, sodium ions are present if flame turns lilac potassium ions are present

Question 3

Describe a change that would be seen in each test tube.
Give a reason for each change.
Test tube 1..
Test tube 2.
(drawing of test tube 1 with calcium carbonate and ethanoic acid)
(test tube 2 is connected to test tube 1 and has limewater inside it)

Answer 3

test tube 1- fizzing effervescence because of carbon dioxide gas
test tube 2- cloudy because of the precipitate of calcium carbonate forms.

Question 4

what would we see if potassium chloride solution was tested with silver nitrate in the presence of dilute nitric acid

Answer 4

a white precipitate would form (explanation= because choride ions form white precipitate of silver chloride)

Question 5

what would we see if we tested calcium nitrate solution with silver nitrate solution in the presence of dilute nitric acid

Answer 5

no change

Question 6

what would we see if tested potassium chloride solution with barium chloride in the presence of dilute hydrochloric acid

Answer 6

no change

Question 7

The flame colour for potassium ions is …………………… .
The flame colour for calcium ions is …………………… .

Answer 7

potassium- lilac

calcium ions- orange red

Question 8

Give one reason why a flame test would not show the presence of both potassium
ions and calcium ions in a mixture.

Answer 8

the sample would contain a mixture of different ions so the colours would be masked. lilac and orange red colours would mix

Question 9

solution A gives a yellow flame test result and carbon dioxide gas is produced when HCL is added to it
give the name of solution A

Answer 9

solution A is sodium carbonate
explanation= it gives a yellow flame test result
carbon dioxide produced when hcl added means it is a carbonate

Question 10

solution B gives a crimson flame test result and forms a white precipitate which is insoluble in excess sodium hydroxide. it also forms a white precipitate when silver nitrate solution is added to it.

Answer 10

calcium chloride
explanation- (crimson flame test result does make us think it is lithium but it forms a white insoluble precipitate when reacted with sodium hydroxide means it must contain calcium ions. it is calcium ‘CHLORIDE’ because forms a white precipitate when silver nitrate solution is added to it. this is the test for hallide ions)

Question 11

solution C forms a dark green precipitate when sodium hydroxide is added to it
what metal ions are in solution c

Answer 11

iron (ii) ions.
explanation- the question is asking for metal ions when ‘sodium hydroxide’ is added this means it must be a test on a metal hydroxide. from metal hydroxide tests we know that if a green precipitate is formed, then iron (ii) ions must be present.

Question 12

there are 4 medicine bottles
bottle number 1)washing soda contains sodium carbonate
bottle number 2) common salt contains sodium chloride
bottle number 3)sodium nitrate
bottle number 4) sodium sulfate
You are provided with the following reagents:
• aluminium powder
• barium chloride solution acidified with dilute hydrochloric acid
• dilute hydrochloric acid
• silver nitrate solution acidified with dilute nitric acid
• sodium hydroxide solution.
• limewater
• red litmus paper
(i) Describe tests that you could use to show that these chemicals are correctly named.
In each case give the reagent(s) you would use and state the result.
Test and result for carbonate ions:

Answer 12

washing soda- sodium carbonate flame test yellow carbonate test or carbonate add dilute acid to the sodium carbonate the dilute acid should react and fizzing effervescence should be produced. bubble the gas through limewater of it turns cloudy then carbon dioxide is present and the washing soda is sodium carbonate
bottle 2 common salt- to test for sodium flame test yellow to test if chloride ions are present add dilute nitric acid and dilute silver nitrate solution. if a white precipitate is formed chloride ions are present
bottle 3 sodium nitrate- to test for sodium yellow flame test to test for ammonia red litmus paper should turn blue. there should be a pungent smell
to test for the sulfate ions add dilute hcl and barium chloride solution. should forma white precipitate

Question 13

Instrumental methods of analysis linked to computers can be used to identify chemicals.
Give two advantages of using instrumental methods of analysis.
………………………………………………………………………………………
( reference to the specification= Students should be able to state advantages of instrumental
methods compared with the chemical tests in this specification.

Answer 13

instrumental analysis is quicker- we are able to analyse samples more rapidly using flame emission spectroscopy than using flame tests
accurate- more likely to identify metal ions correctly than using flame tests
sensitive= instrumental methods such as FES (flame emission spectroscopy) can work on even tiny samples of metal chlorides

Question 14

The student concluded that the egg shell contains carbonate ions.
Describe how the student could identify the gas produced in test 1
test 1- dilute hcl was added to egg shell
observation- a gas was produced egg shell dissolved forming colourless solutions

Answer 14

(the clue is carbonate ions) so we have to explain the test for carbon dioxide gas
bubble the gas through limewater
if limewater turns cloudy, carbon dioxide is the gas present

Question 15

The student concluded that the egg shell contains aluminium ions.
Is the student’s conclusion correct? Use the student’s results to justify your answer
test 3- sodium hydroxide solution was added to the colourless solution from test 1
observation- A white precipitate formed that did not
dissolve in excess sodium hydroxide
solution.

Answer 15

student is incorrect cannot have aluminium ions as the precipitate did not re dissolve in excess sodium hydroxide. aluminium ions are present if precipitate redissolves and calcium and magnesium also form white precipitates

Question 16

The student concluded that the egg shell contains chloride ions.
Is the student’s conclusion correct? Use the student’s results to justify your answer

Answer 16

yes/.NObecause we are told that when silver nitrate solution is added, a white precipitate is formed. chloride ions are present if white precipitate forms. but chloride ions are present in hcl

Question 17

Some scientists wanted to investigate the amount of lead found in egg shells.
They used a modern instrumental method which was more sensitive than older methods.
(i) Name one modern instrumental method used to identify elements.

Answer 17

flame emission spectroscopy

Question 18

What is the meaning of more sensitive?

Answer 18

can detect even tiny sample of metal compound

can detect low concentrations of metal compounds

Question 19

The student dissolved X in water.
The student added dilute nitric acid and silver nitrate solution to the solution of X.
A white precipitate was formed.
Salt X contains chloride ions.
Explain why a white precipitate was formed

Answer 19

because silver chloride is formed which is insoluble.

Question 20

Salt X contains magnesium ions.
Name two other metal ions that would give a white precipitate when a few drops of
sodium hydroxide solution are added.
hint- the question is talking about metal hydroxide tests
think-what other two metals react with sodium hydroxide forming white precipitates.

Answer 20

• aluminium

- calcium

Question 21

Describe the two further tests the student would have to do to show that salt X
contains magnesium ions, and not the two metal ions you identified in part (c) (i).
Give the expected results of each test.
(it previously said salt X contains magnesiium ions and sodium hydroxide is added to form a white precipitate)

Answer 21

calcium- we can find out if the precipitate contains calcium ions by conducting a flame test. if flame turns orange red calcium ions are present
magnesium in a flame test would be colourless would not show a colour
aluminium- if we add sodium in excess the precipitate would not redissolve it would remain. this means that it does not contain aluminium ions as precipitates containing aluminium ions redissolve when sodium hydroxide is added in excess.
so the salt does not contain calcium ions as it shows a colourless flame and it does not contain aluminium ions as it does not redissolve.