Qualitative analysis

Question 1

Describe the test for anions

Answer 1

We would first test for carbonate ions:

Add a dilute nitric acid (HNO3)to a sample. In the presence of carbonate (CO2) gas bubbles will form (effervescence). Test for CO2 bubbling through limewater in the presence of CO2 limewater turns cloudy as COCO3 (s)-{calcium carbonate} and a white precipitate forms.

CaCO3 (aq) + HNO3 (aq) → Ca(NO3)2 (aq) + H2CO3 (aq)

After, we would test for sulfate ions:

We add barium nitrate {Ba(NO3)2} to a sample. If SO4^2- (sulfate ions} are present a white precipitate of BaSO4 (s) {barium sulfate}will form

Ba^2+ (aq) + SO4^2- (aq) —-> BaSO4 (s)

Finally, we would test for halide ions (X-):

We would add AgNO3 (aq)

AgCl will form a white precipice and is soluble to dilute NH3

AgBr will form a cream precipitate and is soluble to concentrated NH3

AgI will form a yellow precipitate and is insoluble in concentrated NH3

(If we are unable to differentiate by colour we can through its solubility to NH3)

Ag^+ (aq) + X^- (aq) —> AgX (s)

Question 2

Why do we test for anions in a certain order

Answer 2

In order to prevent a false positive result

e.g: If we skip the carbonate test and use the sulfate test, any carbonate present will react with barium nitrate to form barium carbonate, a white precipitate.This looks similar to barium sulfate, giving a false positive for sulfate ions.

If you test for halide ions before removing carbonate ions, silver carbonate (a pale yellow precipitate) may form, which can be mistaken for silver iodide — a false positive for iodide ions

Question 3

Why can we not use certain chemicals for the test for anions:

Answer 3

We cannot use HCl or H2SO4 for the carbonate test as it would leave chloride or sulfate ions which will interfere with later tests:

E.g
If HCl is introduced the chloride ion will react with the silver ion to form AgCl(s) and form a white precipitate. This causes false positive for Cl^-

E.g
If H2SO4 is introduced the sulfate ions will react with the barium ions to form BaSO4(s) and form a white precipitate. This causes a false positive for SO4^2-

If these chemicals were used then it would create false positives as the ion could react to give an incorrect result

Question 4

What must we add to the halide test to prevent a false negative

Answer 4

We add dilute nitric acid (HNO₃) before silver nitrate in the halide test to prevent false positives as it removes carbonate ions

Solutions can absorb carbon dioxide which can result in the formation of Carbonate ions resulting in a false positive for chloride ions when silver nitrate is added as a white precipitate forms. Adding nitric acid will prevent this

CO2(g) + H2O(l) → CO3^2-(aq) + 2H+(aq)

We cant use other substances such as HCl as it contains Cl^- ions which can react with Ag^+ to form a white precipitate. Thus a false positive for chloride

2Ag+(aq) + CO32-(aq) → AgCO3(s)A

Question 5

Describe a test for cations

Answer 5

Test for ammonium ions (NH4^+):

Add NaOH (aq) to a test tube with the sample and gently heat

NH4^+ (aq) + OH^- (aq) —> NH3 (g) + H2O (l)

Test the gas formed using damp red litmus paper by putting it over the mouth of the test tube

If the sample contains ammonium ions (NH4^+) then NH3(g) is formed and the litmus paper will turn blue- ammonia is a base