Virtual CrezLab - Qualitative Analysis

Experiment A
	Test	Observations
	R is a colourless aqueous solution of a compound containing a cation and an anion which could possibly be iodide.
a	To a portion of solution R, add an equal volume of dilute nitric acid.	No visible reaction, a colourless solution remains.
b	To the mixture from (a), add a small volume of lead(II) nitrate.	Yellow ppt. formed.
c(i)	To a portion of solution R, add copper(II) sulphate solution.	A grey ppt. in a brown solution is obtained.White ppt. reappears.
c(ii)	To the mixture in c(i), add sodium thiosulphate solution until no further change is observed.	Brown solution decolourised. A white ppt. in a colourless solution is obtained.

For Questions 1 to 8, refer to Experiment A.
The purpose of adding the dilute nitric acid in step (a) is to

A.	acidify the mixture.
B.	determine if R reacts with the acid.

For Question 2, choose the reason/justification.

A.	All nitrate salts are soluble.
B.	Dilute nitric acid is a strong oxidising agent.
C.	The acid will react with any carbonate ion if they are present in R.
D.	Dilute nitric acid is a good solvent.

How is the yellow precipitate formed?

A.	Displacement reaction
B.	Precipitation reaction

For Question 3, choose the reason/justification.

A.	The lead(II) ion is more reactive than the cation from R.
B.	The iodide ions combine with the nitrate ions.
C.	The lead(II) ion is less reactive than the cation from R.
D.	The lead(II) ions combine with the iodide ions.

How is the brown solution formed?

A.	Redox reaction
B.	Decomposition reaction

For Question 5, choose the reason/justification.

A.	The iodide ions combine with copper(II) ions to form a new soluble compound.
B.	Copper(II) ion acts as a reducing agent to reduce iodide ions to iodine molecules.
C.	The copper(II) ions cause R to decompose to form iodine solution.
D.	Copper(II) ion acts as an oxidising agent to oxidise iodide ions to iodine molecules.

In step c(ii), what reaction has occurred to cause the brown solution to decolourise?

A.	Redox
B.	Precipitation

For Question 7, choose the reason/justification.

A.	Iodine combined with thiosulphate ions to form a new insoluble substance.
B.	Thiosulphate ions reduce iodine molecules back to colourless iodide ions.
C.	Iodine combined with thiosulphate ions to form a new soluble substance.
D.	Thiosulphate ions oxidise iodine molecules back to colourless iodide ions.

Experiment B
	Test	Observations
a	Add zinc powder to a test tube half filled with copper(II) sulphate solution. Stir for about 5 minutes. Allow the zinc to settle and then carefully pour off the solution.	Effervescence occurs. A reddish brown deposit formed on zinc powder. Blue solution decolourises. Colourless solution is obtained.
b(i)	To the solution add aqueous sodium hydroxide until a change is observed.	A white ppt. is formed.
b(ii)	Add excess aqueous sodium hydroxide to the result of test 2(a).	Ppt. dissolves to form a colourless solution.

For Questions 9 to 14, refer to Experiment B.
A student concludes that the zinc has displaced copper out of the copper(II) sulphate solution causing the blue solution to decolourise. Do you agree with the student’s conclusion?

A.	Yes
B.	No

10.

For Question 9, choose the reason/justification.

A.	Copper metal is more reactive than zinc metal
B.	Zinc ion displaces the cation from the blue solution.
C.	Copper metal is less reactive than zinc metal.
D.	Zinc ions lose electrons to cations in the solution.

11.

How is the reddish brown deposits formed?

A.	Redox reaction
B.	Precipitation reaction

12.

For Question 11, choose the reason/justification.

A.	Zinc metal acts as a reducing agent to reduce copper(II) ions to copper atoms.
B.	Copper(II) ion loses electrons and reduces zinc ion to zinc atom.
C.	Zinc ions combine with copper(II) ions to form a new insoluble substance.
D.	Copper(II) ion acts as an oxidizing agent to oxidize zinc ions to zinc atoms.

13.

The student also concludes that the white precipitate in test (b) is zinc hydroxide.
Why does the white precipitate disappear in excess aqueous sodium hydroxide?

A.	It dissolves in aqueous sodium hydroxide
B.	It reacts with aqueous sodium hydroxide

14.

For Question 13, choose the reason/justification.

A.	More solvent is added so there is more volume for the white precipitate to dissolve in it.
B.	There is a chemical reaction between the white precipitate and excess sodium hydroxide forming product(s) which is/are soluble.
C.	No further reaction is seen except for the disappearance of the light blue precipitate, and no new reagent is added.
D.	Hydroxide ion displaces the cation from the white precipitate.

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