C21 qualitative analysis

Qualitative AnalysisQualitative Analysis
Chapter 21
LEARNING OUTCOMES
 Identify gases – H2, O2, CO2, NH3, SO2, Cl2 and H2O
 Identify cations – Pb2+
, Al3+
, Ca2+
, Zn2+
, Fe3+
, Fe2+
,
NH4+, Cu2+
 Identify anions – CO3
2-
, SO4
2-
, SO3
2-
, NO3
-
, Br-
, I-
, Cl-

What is QA?
 QA stands for Qualitative Analysis.
It is a process that a chemist carries out
to identify an unknown substance.
 The analysis will identify the cation(s)
and anion(s) present in the unknown sample.
Chapter 21

The QA Process
1. Make preliminary observations about the
unknown substance.
2. Carry out tests in the order given.
3. Make observations and record them in a suitable
format.
4. Draw conclusions from the observations
5. Identify the unknown substance.
Five important steps
Chapter 21

The colour tells us about the possible cations
present in the substance.
What is the colour of the
substance?
- Is it white, colourless,
blue, yellow or some other
colour?
What is the nature of the
substance?
- Is it crystalline, metallic
or in powder form?
Preliminary Observations
Chapter 21

Colour Possible identity of substance
White (colourless) Sodium (Na+
), potassium(K+
), ammonium
(NH4
+
)
Black Copper(II) oxide (CuO),
Iron(II) oxide (FeO)
Blue/green Copper(II), Cu2+
or Iron(II), Fe2+
Yellowish/brown Iron(III), Fe3+
Greyish, metallic Metals: Zinc, Iron, Magnesium,
Aluminium
Chapter 21

Is it soluble in water?
 The solubility tells us about the
possible anions present in the
substance.
Insoluble
Soluble
Chapter 21

Salt Solubility in water
Carbonates Insoluble except those of sodium,
potassium and ammonium
Chlorides Soluble except those of lead(II) and
silver.
Nitrates All are soluble.
Sulphates Soluble except those of lead(II) and
Barium
Table of Solubility
Chapter 21

 Solid : 1 small spatula, just enough
to cover the bottom of a test tube.
 Liquid : About 2 cm-depth in a test tube.
 Reagent : Pour slowly and carefully
down the side of the test tube.
Or
use a dropper and drop it slowly into the test tube.
Chapter 21
Quantity of Substance

Using a Bunsen burner
Heating Liquids
 Use a gentle non-luminous flame (air-hole half open) for
heating a liquid in a test tube.
 Move the test tube up and down the flame and stop the
heating as soon as the liquid starts to boil.
Heating Solids
 When heating a solid, place it in a dry, clean test tube. Heat
gently at first, then strongly.
 For your own safety, wear protective goggles and point the
test tube away from yourself and others when heating.
Chapter 21

Recording Results
 Record all your observations
immediately or you may forget.
 Record the change in colour
before and after the test.
 Record the formation of a
precipitate - its colour and
whether it is soluble in
excess (extra amount) of
the reagent.
Before After
Ppt. is
soluble
White ppt.
formed
Chapter 21

Recording Results
formulae for names of chemicals
ppt. for precipitate
aq. for aqueous solution
 You may use phrases and short forms:
 Record the liberation of a gas by noting if any bubbles
(effervescence) are seen. State the test you carried out to
identify the gas, and the result you obtained.
Chapter 21

Quick Check 1
1. State the colour of the following
substances:
(a) Sodium carbonate: __________
(b) Potassium sulphate: _________
(c) Calcium chloride: ___________
(d) Iron(II) sulphate: ___________
(e) Iron(III) sulphate: ___________
(f) Copper(II) sulphate: _________
(g) Copper(II) carbonate: ________
(h) Copper(II) oxide: ___________
Solution
Chapter 21

Quick Check 1 (cont’d)
2. State whether the following substances are soluble
or insoluble in water:
(a) Sodium carbonate: ________________
(b) Magnesium carbonate: _____________
(c) Calcium chloride: _________________
(d) Zinc sulphate: ____________________
(e) Calcium carbonate: ________________
(f) Lead(II) chloride
(g) Lead(II) nitrate: ___________________
Solution
Chapter 21

1. State the colour of the following substances:
(a) Sodium carbonate: white
(b) Potassium sulphate: white
(c) Calcium chloride: white
(d) Iron(II) sulphate: greenish-yellow
(e) Iron(III) sulphate: reddish-brown
(f) Copper(II) sulphate: blue
(g) Copper(II) carbonate: green
(h) Copper(II) oxide: black
Return
Chapter 21
Solution to Quick Check 1

2. State whether the following substances are soluble or
insoluble in water:
(a) Sodium carbonate: soluble
(b) Magnesium carbonate: insoluble
(c) Calcium chloride: soluble
(d) Zinc sulphate: soluble
(e) Calcium carbonate: insoluble
(f) Lead(II) chloride: insoluble
(g) Lead(II) nitrate: soluble
Return
Chapter 21
(cont’d)

Detection of gases
 How do we tell if a gas is given off?
(a) Effervescence
- Bubbles of gas are seen in
the solution.
(b) Fumes
- White fumes are seen when a
substance is heated.
Chapter 21

Testing for gases
Look here
Not here
 Note the smell of the gas by
holding the mouth of the tube a
few cm from the nose and fanning
the gas towards you.
 Note the colour of the gas by
looking through the side of the
test tube above the substance
or solution.
Chapter 21

Testing for gases
Testing with litmus paper
Wet the litmus papers (one piece of
blue and one piece of red) with
distilled water, then hold it near the
mouth of the test tube.
Do not allow the litmus paper to
touch the sides of the test tube or
drop it into the reagent, as it will react
with the reagent and give a false
result.
Chapter 21

Table: Testing for gases
Gas Colour Odour Litmus paper
(moist)
Oxygen Colourless Odourless No effect
Hydrogen Colourless Odourless No effect
Carbon dioxide Colourless Odourless Blue to red
Ammonia Colourless Pungent (like urine) Red to blue
Chlorine Light greenish-
yellow
Pungent
(Like swimming pool water)
Blue to red then
white
Sulphur
dioxide
Colourless Pungent
(like burning match)
Blue to red
Chapter 21

Test for gases with lighted splint
POP
Place a lighted (burning)
splint near the mouth of the
test tube.
(A) If the lighted splint
extinguishes with a “pop”
sound, then the gas is
hydrogen.
Chapter 21

Test for gases with lighted splint
(B) If the lighted splint burns more
brightly, then the gas may be oxygen.
In this case, you can confirm its identity by
using a glowing splint.
(D) If the burning splint is extinguished,
then the gas may be carbon dioxide,
water vapour, or some other gases.
(C) Insert a glowing splint (no fire) into the
mouth of the test tube. If the glowing
splint is relighted (rekindled) and burns
brightly, then the gas is oxygen.
flame
extinguished
Chapter 21

Testing for carbon dioxide
Note
The carbon dioxide should not be passed for too long, as the
limewater will turn clear again (due to the formation of soluble
calcium hydrogen carbonate), giving a false impression of a
negative test.
• Pour limewater (calcium hydroxide solution) into a test tube until it
reaches a depth of 2 cm from the bottom of the test tube.
• Stopper the reacting test tube with a delivery tube and pass the
gas into the limewater.
• If a white ppt. is formed, the gas is carbon dioxide.
Chapter 21

Quick Check 2
Solution
Complete the table below.
Test Observation Inference
1. Heat a little of solid X
strongly in a test tube.
Gas evolved extinguishes a burning
splint. It turns limewater chalky.
2. Heat the solution P gently
and test for any gas evolved.
A pungent gas was given off. It turned
moist red litmus to blue.
3. Heat solid Q strongly in a
test tube. Test for any gas
evolved.
A colourless and odourless gas is given
off. The gas relights a glowing splint.
4. Add solid X to the solution
Z and warm.
Test for any gas evolved.
Bubbles of gas were given off. The
burning splint is extinguished with a
“pop” sound.
Chapter 21

1. Heat a little of solid X
strongly in a test tube.
Gas evolved extinguished a
burning splint. It turned limewater
chalky.
Gas produced is carbon
dioxide.
CO3
2-
may be present.
2. Heat the solution P
gently and test for any
gas evolved.
A pungent gas was given off. It
turned moist red litmus to blue.
Gas evolved is NH3.
NH4
+
may be present.
3. Heat solid Q strongly
in a test tube. Test for
any gas evolved.
A colourless and odourless gas
is given off. The gas relighted a
glowing splint.
Gas evolved is oxygen.
4. Add solid X to the
solution Z and warm.
Test for any gas
evolved.
Bubbles of gas were given off.
Gas extinguished a lighted splint
with a ‘pop’ sound.
Gas evolved is hydrogen.
Solid X may be a metal
and solution Z may be an
acid.
Return
Chapter 21

Identification of cations
Aluminium, Al 3+
Iron(II), Fe2+
Calcium, Ca2+
Iron(III), Fe3+
Zinc, Zn2+
Ammonium, NH4
+
Copper(II), Cu2+
Lead(II), Pb2+
Chapter 21
 The cations can be identified using sodium hydroxide solution and
aqueous ammonia (ammonium hydroxide).
The cations react with the hydroxide ions in the alkali to form
insoluble metal hydroxides. These hydroxides appear as precipitates.
The cations can be identified from:
(a) the colour of the precipitate,
(b) the solubility of the precipitate in excess of the reagent.

 The unknown substance must be in solution form.
If not, a solution must be made as follows:
- soluble salt : dissolve in distilled water.
- insoluble salt : dissolve in a little dilute nitric acid.
Test using sodium hydroxide solution:
(a) Add sodium hydroxide solution drop
by drop to the unknown solution.
(b) Observe the precipitate formed and
note its colour.
(c) Add more sodium hydroxide solution (excess).
Shake the test tube and examine whether the precipitate
dissolves in excess sodium hydroxide solution.
Ppt.
formed
Colourless/
soluble
Chapter 21

 Ammonia gas has a pungent
smell (like urine) and turns moist
red litmus paper blue.
 This confirms the presence of
ammonium ion (NH4
+
) in the
unknown.
d) If no precipitate is formed, warm the
mixture and test for ammonia gas.
Chapter 21

Test for cations (Table 1)
Cation Add a little NaOH (aq) Add excess NaOH (aq)
Al 3+
White ppt ppt. dissolves to form colourless solution
Ca 2+
White ppt ppt. remains insoluble
Zn 2+
Pb 2+
Cu 2+
Blue ppt ppt. remains insoluble
Fe 2+
Dirty-green ppt ppt. remains insoluble (turns brown on
standing)
Fe 3+
Reddish-brown ppt ppt. remains insoluble
NH4
+
No ppt. When warmed,
NH3 gas given off.
No change
Chapter 21

Identification of cations (using aq. NH3)
Test using aqueous ammonia:
(a) Add ammonia solution drop by drop
to the unknown solution.
(b) Observe the precipitate formed and
note its colour.
(c) Add more ammonia solution (excess).
Shake the test tube and examine
whether the precipitate dissolves in
excess ammonia solution.
Chapter 21
Ppt.
formed
Colourless/
soluble

Test for cations (Table 2)
Cation Add a little NH3 (aq) Add excess NH3(aq)
Al3+
White ppt. Ppt. remains insoluble
Ca2+
No ppt (or very slight ppt.) No change
Zn2+
White ppt. Ppt. dissolves to form colourless solution
Pb2+ White ppt. Ppt. remains insoluble
Cu2+
Blue ppt. Ppt. dissolves to form dark blue solution.
Fe2+
Dirty-green ppt. Ppt. remains insoluble (turns brown on
standing)
Fe3+
Reddish-brown ppt. Ppt. remains insoluble
NH4
+
No ppt. No change
Chapter 21

Quick Check 3
1. What are some of the preliminary observations that you can make on an
unknown sample, before carrying out any test?
2. When a solid is insoluble in water, what acid is used? Why?
3. Which cations will produce a white precipitate with sodium hydroxide solution
and aqueous ammonia?
4. Which cation will produce a white precipitate which is soluble in both aqueous
ammonia and sodium hydroxide?
5. Which cations will produce a white precipitate which is soluble in excess
sodium hydroxide, but not in excess aqueous ammonia?
6. Which cation will produce a white precipitate with sodium hydroxide solution
but not with aqueous ammonia?
7. Which cation will produce a blue precipitate with sodium hydroxide?
8. Which cation will produce a deep blue solution with aqueous ammonia?
Chapter 21
Solution

Return
1. Observe the colour and texture of the unknown
sample, and determine if it is soluble in water.
2. Dilute nitric acid is used to dissolve the solid
because nitrates of all metals are soluble in water.
3. Aluminium, lead and zinc ions
4. Zinc ions
5. Aluminium and lead ions
6. Calcium ion
7. Copper(II) ion
8. Copper(II) ion
Chapter 21

Identification of Anions
Carbonate, CO3
2-
Chloride, Cl-
Iodide, I-
Sulphate, SO4
2 -
Nitrate, NO3
-
 Each anion has a different specific test.
 For carbonate, you need not make a solution to carry out the test.
For other anions, you must make a solution of the unknown in distilled
water only.
*If you use dilute nitric acid to dissolve it, you are introducing the nitrate ions
into the unknown.
Chapter 21

Test for carbonate
 Add some dilute
hydrochloric acid (or
nitric acid) to the
unknown.
 Pass the gas into
limewater.
 Bubbles of gas are
given off.
 The gas turns
limewater chalky.
The gas is CO2 .
Therefore, CO3
2-
is present in the
unknown.
Chapter 21

Test for chloride
 Add a little silver
nitrate solution to
the unknown.
 Add dilute nitric
acid to the above,
then shake.
 A white ppt is
formed.
 White ppt is
insoluble in nitric acid.
White ppt is AgCl.
Therefore, Cl-
is
present.
Chapter 21

Test for iodide
 Add a little lead(II)
nitrate solution to the
unknown.
 Add dilute nitric
acid to the above,
then shake.
 A yellow ppt is
formed.
 Yellow ppt is insoluble
in nitric acid.
Yellow ppt is PbI2.
Therefore, I-
is
present in the
unknown.
Chapter 21

Test for sulphate
 To the unknown
solution, add a little
barium chloride (or
barium nitrate)
solution.
 Then add some
dilute hydrochloric acid
(or nitric acid) and
shake.
 A white ppt is
formed.
 White ppt is
insoluble in
hydrochloric acid
(or nitric acid).
White ppt is
BaSO4.
Therefore, SO4
2-
is present in the
unknown.
Chapter 21

Test for nitrate
 To the unknown, add a
little sodium hydroxide
solution.
Then add a few pieces of
aluminium foil (or a
spatula of aluminium
powder).
 Warm the mixture
gently.
 Test for ammonia gas
evolved.
 A pungent gas is
given out.
 The gas turns moist
red litmus paper blue.
NH3 gas is
produced.
NO3
-
is present in
the unknown.
Chapter 21

Summary of Tests for Anions (Table 3)
Anion Test Observation
Carbonate,
CO3
2-
Add dilute HCl. Test for gas
evolved.
Effervescence. Gas evolved
turned limewater chalky.
Chloride,
Cl-
Add AgNO3 solution,
followed by dilute HNO3
White ppt. insoluble in HNO3
Iodide,
I-
Add Pb(NO3)2 solution,
followed by dilute HNO3
Yellow ppt. insoluble in HNO3
Sulphate,
SO4
2-
Add BaCl2 solution,
followed by dilute HCl.
White ppt. insoluble in HCl
Nitrate,
NO3
-
Add NaOH solution, then Al foil.
Warm gently.
NH3 gas evolved turned red
litmus blue
Chapter 21

Quick Check 4
Solution
1. When we add aqueous silver nitrate to an unknown, what anion(s) are we
looking for?
2. When we add aqueous barium chloride or barium nitrate to an unknown,
what anion(s) are we looking for?
3. When we are told to add dilute hydrochloric acid to an unknown, what
anion are we testing for and what should we get ready?
4. When we are told to add sodium hydroxide and aluminium powder to an
unknown solution, what anion are we testing for and what should we get
ready?
5. When we add silver nitrate or barium chloride solution to an unknown,
what reagent should we also add? What is the reason for this?
Chapter 21

Return
1. Chloride, Cl-
or iodide, I-
2. Sulphate, SO4
2-
3. Carbonate, CO3
2-
. We should get a delivery tube and test tube
containing some limewater ready.
4. Nitrate, NO3
-
. We should get a piece of moist red litmus paper
ready.
5. We should add some dilute nitric acid. This is to exclude any
carbonate present from forming a precipitate.
Chapter 21

Quick Check 5
1. Dissolve a sample of S1 in distilled
water and divide it into 3 portions.
S1 dissolved to form a pale blue
solution.
2. To the first portion, add aqueous
sodium hydroxide slowly until in excess.
A blue ppt was formed.
Ppt was insoluble in excess NaOH.
3. To the second portion, add aqueous
ammonia slowly until in excess.
Ppt dissolved to form a deep blue
solution.
4. To the third portion, add barium chloride
solution, then dilute HCl.
A white ppt was formed.
Ppt was insoluble in HCl.
Conclusion: S1 is _________ .
Complete the table below and identify the substance S1.
Solution
Chapter 21

1. Dissolve a sample of S1 in distilled
water and divide it into 3 portions.
S1 dissolved to form a pale blue
solution.
Cu2+
may
be present
2. To the first portion, add aqueous
sodium hydroxide slowly until in excess.
Ppt was insoluble in excess NaOH.
Cu2+
is
present.
3. To the second portion, add aqueous
ammonia slowly until in excess.
Ppt dissolved to form a deep blue
solution.
Cu2+
is
confirmed.
4. To the third portion, add barium
chloride solution, then dilute HCl.
A white ppt was formed.
Ppt was insoluble in HCl.
SO4
2-
is
present.
Conclusion: S1 is copper(II) sulphate.
Complete the table below and identify the substance S1.
Solution
Chapter 21

C21 qualitative analysis

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C21 qualitative analysis