3.3 - Qualitative analysis

Question 1

What is the test for group 7 elements called ?

Answer 1

Test for halides

Question 2

What is used to test for the presence of halogens ?

Answer 2

Aqueous silver nitrate solution [ AgNOv3 (aq) ]

Question 3

Describe how to test for halides ?

Answer 3

• Add aqueous silver nitrate solution [ AgNOv3 ( aq ) ] to an aqueous solution of a halide
• Note the colour of the precipitate that forms
• Add aqueous ammonia to test the solubility of the precipitate

Question 4

What are the colours of the precipitate when aqueous silver nitrate is added to aqueous solution of a halide ?

Answer 4

Chlorine = White precipitate
Bromine = Cream precipitate
Iodine = Yellow precipitate

Question 5

Why is aqueous ammonia used to test the solubility of the precipitate ?

Answer 5

• This is a confirmative test used to tell the different precipitate colours apart
• White, cream and yellow can be easily misinterpreted

Question 6

What is the solubility of the different precipitates when aqueous ammonia is added ?

Answer 6

Question 7

Write the overall equation and ionic which occurs when testing for the presence of halides ?

Answer 7

Overall : AgNov3(aq) + NaX(aq) –> AgX(s) + NaNOv3(aq)

Ionic : Ag+(aq) + X-(aq) –> AgX(s)

Question 8

Describe how to test for the presence of carbonate ions ?

Answer 8

• In a test tube, add dilute nitric acid to the solid or solution to be tested.
• If gas is produced, bubble the gas through lime water [ aqueous solution of calcium hydroxide Ca(OH) ]
• Note whether the lime water turns milky white/ cloudy

Question 9

Why does lime water go milky/ cloudy when testing for the preside of carbonate ions ?

Answer 9

Carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the lime water cloudy (milky)

Question 10

What is the positive result when testing for the preside of carbonate ions ?

Answer 10

Lime water goes cloudy/ milky white

Question 11

Write the overall and ionic equation for the reaction that occurs when testing for the preside of carbonate ions ?

Answer 11

COv2(g) + Ca(OH)v2 (aq) -> CaCOv3(s) + Hv2O(l)

Question 12

Describe the test for the preside of sulphate ions ?

Answer 12

• Add aqueous barium chloride/ nitrate solution [ BaClv2 (aq) / Ba(NOv3)(aq) ] to the solution being tested
• Note whether a white precipitate forms

Question 13

What is the positive result when testing for the presence of sulphate ions ?

Answer 13

A white precipitate forms

Question 14

Write the overall and ionic equation which shows what occurs when testing for the pests of sulphate ions ?

Answer 14

Overall : Nav2(SOv4)(aq) + BaClv2(aq) –> BaSOv4(s) + NaCl(aq)

Ionic : SOv4-2(aq) + Ba2+(aq) –> BaSOv4(s)

Question 15

When would you add barium nitrate instead of barium chloride when testing for sulphate ions ?

Answer 15

• If you intend to carry out a halide test afterwards
• If Barium chloride is used, then you are introducing chloride ions into the solution which will result in a white precipitate forming

Question 16

What is the sequence when testing a solution?

Answer 16

• Testing for Carbonate ions
• Testing for Sulphate ions
• testing for Halide ions

Question 17

What acronym can be used to remember the sequence you should test a solution in ?

Answer 17

CaSH = ( carbonate ions, sulphate ions and halide ions )

Question 18

Describe the test for the preside of ammonium ions ?

Answer 18

• Heat the solution with aqueous sodium hydroxide ( NaOH )
• Test the gas produced with moist pH indicator paper, Ammonia is alkaline and its presence will turn the paper blue.

Question 19

Why is it unlikely you will see ammonia gas when testing for ammonium ions ?

Answer 19

You are unlikely to see gas bubbles as ammonia is very soluble in water

Question 20

Write an ionic equation for the reaction that occurs when testing for the preside of ammonium ions ?

Answer 20

NHv4+(aq) + OH-(aq) —> NHv3 (g) + Hv2O(l)

Question 21

Explain why does the carbonate test have to be done first ?

Answer 21

• The carbonate test has to be done first to eliminate the carbonate ions
• Barium carbonate and Silver carbonate are both insoluble and produce a white precipitate for the sulphate and halide test
• This results in the production of a false positive

Question 22

Explain why the sulphate test has to be done before the halide test ? ( BaClv2 )

Answer 22

• If Barium chloride is used to test for the premise of sulphate ions, the sample/ solution will be contaminated with chlorine ions
• This will result in a white precipitate forming when silver nitrate is added
• This results in a false positive