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Qualitative Analysis

Test for Gases


Gas  
Test and Test Results
Ammonia (NH3)    
turns damp red litmus paper blue
Carbon dioxide (CO2)
gives white ppt with limewater, ppt dissolves with excess CO2
Chlorine (Cl2)
bleaches damp litmus paper
Hydrogen (H2)
produces "pop" sound with lighted splint
Oxygen (O2)
relights a glowing splint
Sulfur dioxide (SO2)
turns aqueous acidified potassium dichromate (VI) from orange to green

Test for Anions

AnionTestTest result
Carbonate (CO32-)
Add dilute acidEffervescence, carbon dioxide produced
Chloride (Cl-)
(in solution)
Acidify with dilute nitric acid, then add aqueous silver nitrate
White ppt
Iodide (I-)
(in solution)
Acidify with dilute nitric acid, then add aqueous lead(II) nitrate
Yellow ppt
nitrate (NO3-)
(in solution)
Add aqueous sodium hydroxide, then aluminium foil, warm carefully
Ammonia produced
Sulfate (SO42-)
(in solution)
Acidify with dilute nitric acid, then add aqueous barium nitrate
White ppt

Colours of some metal hydroxides

Metal hydroxide    
Colour
calcium hydroxidewhite
copper(II) hydroxidelight blue
iron(II) hydroxidegreen
iron(III) hydroxidered-brown
lead(II) hydroxidewhite
zinc hydroxidewhite

Source: SEAB

Testing for cations

  • Cations can be identified by their reactions with aqueous sodium hydroxide and aqueous ammonia
  • A precipitate is an insoluble solid.
  • When testing for cations, these precipitates only form when a metal ion in solution joins with hydroxide ions in solution to form an insoluble metal hydroxide
    • eg Cu2+ + 2OH- --> Cu(OH)2    (blue copper(II) hydroxide precipitate)
 CationAdd dilute NaOH
(5 drops)
Add excess dilute NaOH Add dilute aqueous NH3 (5 drops)Add excess dilute aqueous NH3 
 Al3+white ppt of aluminium hydroxideppt dissolves
colourless solution 
white ppt of aluminium hydroxideppt insoluble 
 Ca2+white ppt of calcium hydroxideppt insoluble no reaction no reaction 
 Cu2+blue ppt of copper(II) hydroxideppt insoluble blue ppt of copper(II) hydroxide ppt dissolves
deep blue solution 
 Fe2+dirty green ppt of iron(II) hydroxideppt insoluble dirty green ppt of iron(II) hydroxide ppt insoluble 
 Fe3+red-brown ppt of iron(III) hydroxideppt insoluble red-brown ppt of iron(III) hydroxide ppt insoluble 
 Pb2+white ppt of lead(II) hydroxideppt dissolves
colourless solution 
white ppt of lead(II) hydroxide ppt insoluble 
 Zn2+white ppt of zinc hydroxideppt dissolves
colourless solution 
white ppt of zinc hydroxide ppt dissolves
colourless solution 
NH4+ ammoniumammonia gas is produced on warming with dilute NaOH. This gas has a pungent smell and turns moist red litmus paper blue- no reaction
[Lead(II) ions can be distinguished from aluminium ions by the insolubility of lead(II) chloride.]

MCQ Questions

1. Which statement about salts is not correct?
a, salts are made by neutralising alkalis with acids
b. salts contain anions and cations
c. salts are made by dissolving metal oxides in acids
d. salts always contain water of crystallisation

2. An example of a salt which can be prepared by precipitation is
a. lead(II) nitrate
b. sodium carbonate
c. silver chloride
d. magnesium sulphate

3. A way of distinguishing dilute hydrochloric acid from dilute sulphuric acid is to
a. add universal indicator
b. add aqueous barium nitrate
c. add a metal carbonate
d. add magnesium ribbon

4. Which of these statements about solubility is true?
a. all sulphates are soluble in water except calcium and lead sulphate
b. all nitrates are insoluble in water except sodium and potassium nitrate
c. most metal oxides are soluble in water except those of Group I and II
d. most metal carbonates are soluble in water

5. Barium sulphate is insoluble in water. It is used in a 'barium meal' to allow X-ray studies of the intestines. It can be prepared by a precipitation reaction between two aqueous solutions. Which two substances would be suitable for preparing barium sulphate for use in X-ray radiography?
a. barium carbonate and sulphuric acid
b. barium chloride and sodium sulphate
c. barium oxide and potassium sulphate
d. barium nitrate and calcium sulphate

6. Which of these salts is best prepared by reaction with an acid and a base?
a. barium sulphate
b. copper(II) carbonate
c. magnesium sulphate
d. silver chloride

7. A solution of substance X gave a white precipitate when aqueous NaOH was added. However, when lead(II) nitrate solution was added to an acidified solution of X, a yellow precipitate formed. What is the correct identity of X?
a. calcium chloride
b. magnesium sulphate
c. sodium bromide
d. zinc iodide

8. Iron(III) hydroxide is precipitated out of solution when aqueous sodium hydroxide solution is added to iron(II) chloride solution.
Fe3+ (aq) + 3OH- (aq) --> Fe(OH)3 (s)

What is the minimum volume of 2 mol/dm3 aqueous NaOH required to precipitate the maximum amount of iron(III) hydroxide from 20cm3 of 1 mol/dm3 iron(III) chloride solution?
a. 10 cm3
b. 20 cm3
c. 30 cm3
d. 60 cm3

9. After acidification with dilute nitric acid, a colourless solution X reacts with aqueous silver nitrate to give a yellow precipitate. What could X be?
a. calcium iodide
b. copper(II) chloride
c. iron(II) iodide
d. sodium chloride

10. An element reacts with steam but not with cold water. Its oxide can be reduced by heating it with carbon. When it is placed in a solution containing iron(II) ions, a grey deposit is formed. The element is most likely to be
a. lead
b. magnesium
c. zinc
d. copper

MCQ Answers

1. d
2. c
3. b
4. b
5. b
6. c
7. d
8. c
9. a
10. c

Structured Questions Worked Solutions

1. A student was given an aqueous solution analyse. It contains copper(II) chloride and aluminium nitrate.
a. Describe how he could detect the presence of chloride ions in the aove solution.
b. Name the precipitate(s) formed when excess aqueous ammonia is added to the above solution.

Solution

1a. 
  • Add dilute nitric acid followed by aqueous silver nitrate. 
  • A white precipitate will be seen in the presence of chloride ions
OR
  • Add dilute nitric acid followed by aqueous lead(II) nitrate.
  • A yellow precipitate will be seen in the presence of chloride ions

2. W is an alkali and X is a salt. When the two solutions were mixed together, a reddish-brown precipitate Y was obtained. When a salt Z was added to solution W and heated, a pungent gas which turned moist red litmus blue was evolved. Suggest what could W, X, Y, and Z be.

Solution

W: Sodium hydroxide
X: Iron(III) chloride/sulphate/etc
Y: Iron(III) hydroxide
Z: Ammonium chloride/nitrate/etc

3. Give the name and formula of the ions present in each of the solutions X, Y and Z below - 
a. Solution X gives a white precipitate when dilute hydrochloric acid and aqueous barium chloride are added to it.
b. An alkaline gas is given off when sodium hydroxide solution is added to the colourless solution Z and the mixture heated.

Solution

3a. sulphate ion (SO42-)
3b. Ammonium ion (NH4+)

4. A similar reagent is added to zinc carbonate and sample S to initiate both reactions A and B.

ai. Name the reagent(s) required for Reaction A.
aii. Write down the chemical equation (with state symbols) for Reaction A.
bi. Give a possible identity of Sample S.
bii. Based on your answer in bi, write a chemical equation with state symbols for Reaction B.

Solution

ai. Hydrochloric acid
aii. ZnCO3 (s) + 2HCl (aq) ---> ZnCl2 (aq) + CO2 (g) + H2O (l)
bi. Zinc
bii. Zn (s) + 2HCl (aq) ---> ZnCl2 (aq) + H2 (g)

5.
a. Identify A to F.
b. Write an ionic equation (with state symbols) for the formation of the yellow precipitate.
c. Describe a test for the pungent gas R.
d. Pungent gas R dissolves in water to form a solution. Describe the observations when a few drops of copper(II) sulphate solution is added to the solution.

Solution

a. 
A: lead(II) carbonate
B: dilute nitric acid
C: ammonia gas
D: carbon dioxide
E: lead(II) nitrate 
F: lead(II) hydroxide

b. Pb2+ (aq) + 2I- (aq) ---> PbI2 (s)

c. Place two pieces of damp red and blue litmus papers at the mouth of the test tube. If the gas is ammonia, the damp red litmus paper will turn blue. A pungent gas will also be detected.

d. Blue precipitate is formed which dissolves in excess aqueous ammonia to form a dark blue solution.

6. In the experiment shown below, the gas X produced by the action of dilute sulphuric acid on the zinc granules was passed over two heated metallic oxides. A colourless liquid W was collected and the excess gas X was burnt off at Y.
a. What is gas X? Write the ionic equation for the formation of the gas.
b. State what is observed of:
i. zinc oxide
ii. copper(II) oxide
Write equation(s) for any change observed.
c. Explain your observation made in bi and bii.
d. Give a chemical test to identify liquid W.
e. Suggest a suitable drying agent to be placed inside the drying bulb.
f. Why was the excess gas X burnt off at Y?
g. What precautions should be taken before the excess gas was lit?

Solution

a.hydrogen gas
Zn + 2H+ ---> Zn2+ + H2

bi. Zinc oxide turns from white to yellow.
bii. Copper(II) oxide turns from black to pink
CuO + H2 ---> Cu + H2O

c. Zinc is above hydrogen in the reactivity series so zinc oxide will not be reduced by hydrogen to zinc. Upon being heated, zinc oxide will change its colour from white to yellow.
Copper is below hydrogen in the reactivity series so copper(II) oxide will be reduced to form pink copper.

d. W is tested with anhydrous cobalt(II) chloride paper. If W is water, the cobalt(II) chloride paper will turn from blue to pink.

e. fused calcium chloride

f. Because hydrogen gas is flammable and a mixture of hydrogen and air is very explosive.

g. Ensure that there is no leakage in the apparatus.




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