Anion tests

Question 1

What term is used to describe this type of analysis?

Answer 1

Qualitative analysis

Question 2

Define the term anion

Answer 2

Negatively charged ion

Question 3

Why is deionised water used in all of the tests?

Answer 3

Tap water contains too many dissolved ions which could interfere with the tests, however deionised water which contains no dissolved ions is perfect

Question 4

What is a precipitate?

Answer 4

Material that settles out of solution, insoluble in water

Question 5

In the test for the chloride ion, how can the presence of silver chloride be confirmed?

Answer 5

By adding dilute ammonia solution to the silver chloride, since silver chloride is soluble in dilute ammonia the cloudiness dissapears

Question 6

What is the difference between a sulfate ion and a sulfite ion and how can these two ions be distinguished from eachother in a lab

Answer 6

The sulfate ion has the formula SO4^2- and the formula of the sulfite ion is SO3^2-
Distinguished : adding barium chloride - white percipitate in each case
add dilute hydrochloric acid to the precipitate and the precipitate dissapears in the case of the sulfite ion but not in the case of the sulfate ion

Question 7

Why is it best to make up a fresh solution of limewater before using it to confirm the presence of CO2

Answer 7

Limewater reacts with carbon dioxide in the air which makes the solution milky

Question 8

In testing for the presence of the nitrate ion, why must extreme care be taken when using concentrated sulfuric acid

Answer 8

Conc sulfuric acid is very corrosive and causes burns to skin

Question 9

KCl, KNO3, Na2HPO4.12H2O,Na2SO3.7H2O, NaHCO3 and Na2SO4.10H2O.
Which of the substances listed above was identified by the addition of silver nitrate, AgNO3, solution
to a solution of each sample in turn? What observation indicated a positive test result?

Answer 9

KCl
white precipitate (ppt)

Question 10

KCl, KNO3, Na2HPO4.12H2O,Na2SO3.7H2O, NaHCO3 and Na2SO4.10H2O
One of the samples gave a brown ring when a little concentrated sulfuric acid was carefully poured
down the inside of a slanting test tube which contained a solution of the salt, together with another
reagent. What was the other reagent? Which salt was identifiable by the appearance of a brown
ring?

Answer 10

reagent - iron(II) sulfate

KNO3

Question 11

Describe how you would test the samples for the presence of the phosphate anion.

Answer 11

Add ammonium molybdate
few drops of concentrated nitric acid and warm gently
yellow precipitate (ppt) formed

Question 12

Balanced equation when hydrochloric acid is added to hydrogencarbonate

Answer 12

HCO3- + H+ -> CO2 + H20

Question 13

Balanced equation when hydrochloric acid is added to carbonate

Answer 13

CO3^2- + 2H+ -> CO2 + H20

Question 14

Balanced equation when magnesium sulfate is added to hydrogen carbonate

Answer 14

Mg2+ + 2HCO3- -> Mg(HCO3)2 (soluble)

Question 15

Balanced equation when mangensium sulfate is added to carbonate

Answer 15

Mg2+ + CO32- -> MgCO3 (arrow down)

Question 16

Balanced equation when barium chloride is added to sulfite

Answer 16

Ba2+ + So3^2- -> BaSO3 (arrow down)

Question 17

Balanced equation when barium chloride is added to sulfate

Answer 17

Ba2+ + SO4^2- -> BaSO4 (arrow down)

Question 18

Balanced equation when hydrochloric acid is added to sulfite

Answer 18

BaSO3 + 2HCl -> BaCl + SO2 + H20

Question 19

Balanced equation for when hydrogen carbonate is heated

Answer 19

Na2CO3 + MgSO4 -> Na2SO4 + MgCO3

Question 20

Describe the procedure for the test of carbonate and hydrogencarbonate ions

Answer 20

Add sodium carbonate and sodium hydrogen carbonate to two separate clean test tubes
Using a dropping pipette add HCl to both test tubes
Check for the release of CO2 using limewater = milky
Add sodium carbonate and hydrogen carbonate solutions to two other clean get tubes and using a dropping pipette add magnesium sulfate
Carefully heat the contents and note white precipitate of carbonate

Question 21

What is observed upon addition of HCl to
I) carbonate
ii) hydrogencarbonate

Answer 21

both -> effervescence and co2 gas given off

Question 22

What is the observation from the limewater test on the carbonate and hydrogencarbonate ion

Answer 22

both turn limewater milky

Question 23

What is the observation of the addition of magnesium sulfate to carbonate and hydrogen carbonate

Answer 23

carbonate -> white ppt

hydrogencarbonate -> no white ppt

Question 24

Observation upon hearing of carbonate and hydrogen carbonate

Answer 24

Carbonate - white ppt remains

Hydrogencarbonate - white ppt formed

Question 25

What is the observation of the addition of 1) BaCl2 and 2) HCl to sulfate and sulfate

Answer 25

1) both form white ppt
2) sulfate - ppt does not dissolve
sulfite - does dissolve

Question 26

Give a safety precaution

Answer 26

Wear safety glasses

Question 27

How do you identify unknown anions

Answer 27

Add unknown solutions to clean test tubes
Using dropping pipette add dilute hcl to test tube and record observations
If gas evolved establish using limewater weather it is CO2 or not. If it is, proceed with hydrogen carbonate and carbonate identification test (magnesium sulfate)
If no gas evolved, take further samples in clean test tubes and test in turn with barium chloride followed by dilute hcl, silver nitrate , cold saturated iron II sulfate followed by concentrated sulfuric acid and ammonium molybdate reagent until positive results obstained