IGCSE CHEMISTRY
SECTION 2 LESSON 5
Content
The iGCSE
Chemistry
course
Section 1 Principles of Chemistry
Section 2 Chemistry of the Elements
Section 3 Organic Chemistry
Section 4 Physical Chemistry
Section 5 Chemistry in Society
Content
Section 2
Chemistry
of the
Elements
a)
b)
c)
d)
e)
f)
g)
The Periodic Table
Group 1 Elements
Group 7 Elements
Oxygen and Oxides
Hydrogen and Water
Reactivity Series
Tests for ions and gases
Lesson 5
g) Tests for
ions and
gases
g) Tests for ions and gases
2.37 describe tests for the cations:
i Li+, Na+, K+, Ca2+ using flame tests
ii NH4+, using sodium hydroxide solution and identifying
the ammonia evolved
iii Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution
2.38 describe tests for the anions:
i Cl-, Br- and I-, using dilute nitric acid and silver
nitrate solution
ii SO42-, using dilute hydrochloric acid and barium
chloride solution
iii CO32-, using dilute hydrochloric acid and identifying
the carbon dioxide evolved
2.39 describe tests for the gases:
i hydrogen
ii oxygen
iii carbon dioxide
iv ammonia
v chlorine.
What’s a cation (and now
we mention it, what’s an
anion?)
What’s a cation (and now
we mention it, what’s an
anion?)
What’s a cation (and now
we mention it, what’s an
anion?)
Cations are
positively
charged ions
What’s a cation (and now
we mention it, what’s an
anion?)
Cations are
positively
charged ions
Cations are
positively
charged ions
Sodium ions, Na+, and
Magnesium ions, Mg2+,
are examples of cations
Sodium ions, Na+, and
Magnesium ions, Mg2+,
are examples of cations
Group 1 metals form
monovalent cations
Eg. Lithium forms Li+
Sodium ions, Na+, and
Magnesium ions, Mg2+,
are examples of cations
Group 2 metals form
divalent cations
Eg. Calcium forms Ca2+
Sodium ions, Na+, and
Magnesium ions, Mg2+,
are examples of cations
Group 3 metals form
trivalent cations
Eg. Aluminium forms Al3+
What’s a cation (and now
we mention it, what’s an
anion?)
Anions are
negatively
charged ions
Anions are
negatively
charged ions
Chloride ions, Cl-, and
oxide ions, O2-, are
examples of anions
Chloride ions, Cl-, and
oxide ions, O2-, are
examples of anions
Group 6 elements form
divalent anions
Eg. sulphide forms S2-
Chloride ions, Cl-, and
oxide ions, O2-, are
examples of anions
Group 7 elements form
monovalent anions
Eg. fluoride forms F-
Common cations
Monovalent
Divalent
Trivalent
Lithium Li+
Potassium K+
Sodium Na+
Copper(I) Cu+
Silver Ag+
Hydrogen H+
Barium Ba2+
Calcium Ca2+
Magnesium Mg2+
Zinc Zn2+
Iron(II) Fe2+
Tin(II) Sn2+
Lead(II) Pb2+
Copper(II) Cu2+
Aluminium Al3+
Iron(III) Fe3+
Common anions
Monovalent
Divalent
Trivalent
Bromide BrChloride ClIodide IHydroxide OHNitrate NO3-
Oxide O2Carbonate CO32Sulphate SO42Sulphite SO32Sulphide S2-
Phosphate PO43-
Lesson 5
g) Tests for
ions and
gases
g) Tests for ions and gases
2.37 describe tests for the cations:
i Li+, Na+, K+, Ca2+ using flame tests
ii NH4+, using sodium hydroxide solution and identifying
the ammonia evolved
iii Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution
2.38 describe tests for the anions:
i Cl-, Br- and I-, using dilute nitric acid and silver
nitrate solution
ii SO42-, using dilute hydrochloric acid and barium
chloride solution
iii CO32-, using dilute hydrochloric acid and identifying
the carbon dioxide evolved
2.39 describe tests for the gases:
i hydrogen
ii oxygen
iii carbon dioxide
iv ammonia
v chlorine.
Li+, Na+, K+, Ca2+ using
flame tests
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
1. Clean the flame test metal loop wire by dipping it
into hydrochloric acid and then holding it in a hot
Bunsen flame.
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
1. Clean the flame test metal loop wire by dipping it
into hydrochloric acid and then holding it in a hot
Bunsen flame.
2. Repeat this until the wire doesn't produce any
colour in the flame.
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
1. Clean the flame test metal loop wire by dipping it
into hydrochloric acid and then holding it in a hot
Bunsen flame.
2. Repeat this until the wire doesn't produce any
colour in the flame.
3. When the wire is clean, moisten it again with
some of the acid and then dip it into a small amount
of the solid you are testing so that some sticks to
the wire.
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
4. Place the wire back in the flame again.
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
4. Place the wire back in the flame again.
5. If the flame colour is weak, it is often worthwhile
to dip the wire back in the acid again and put it back
into the flame as if you were cleaning it. You often
get a very short but intense flash of colour by doing
that.
Li+, Na+, K+, Ca2+ using
flame tests
Instructions for a flame test:
4. Place the wire back in the flame again.
5. If the flame colour is weak, it is often worthwhile
to dip the wire back in the acid again and put it back
into the flame as if you were cleaning it. You often
get a very short but intense flash of colour by doing
that.
RESULT?
Li+, Na+, K+, Ca2+ using
flame tests
Li+, Na+, K+, Ca2+ using
flame tests
Flame test results:
CATION
FLAME TEST COLOUR
Lithium
RED
Sodium
STRONG PERSISTENT
ORANGE
Potassium
Calcium
LILAC (PINK)
ORANGE-RED
NH4+, using sodium hydroxide
solution and identifying the
ammonia produced
NH4+, using sodium hydroxide
solution and identifying the
ammonia produced
NH4+ is the ammonium ion.
Although not a metal, it is
classified as a cation
because it forms a
positively charged ion.
NH4+, using sodium hydroxide
solution and identifying the
ammonia produced
Test for NH4+ :
All ammonium salts react with dilute alkalis, such as
sodium hydroxide, to give ammonia.
NH4Cl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) + NH3(g)
NH4+, using sodium hydroxide
solution and identifying the
ammonia produced
Test for NH4+ :
All ammonium salts react with dilute alkalis, such as
sodium hydroxide, to give ammonia.
NH4Cl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) + NH3(g)
Because it is an alkaline gas, ammonia will turn red
litmus blue
NH4+, using sodium hydroxide
solution and identifying the
ammonia produced
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When an iron(II) salt is added to
sodium hydroxide, a dirty green
precipitate of iron(II) hydroxide is
formed.
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When an iron(II) salt is added to
sodium hydroxide, a dirty green
precipitate of iron(II) hydroxide is
formed.
Eg.
Iron
+ Sodium  Iron
+ Sodium
sulphate hydroxide hydroxide sulphate
FeSO4
+ 2NaOH  Fe(OH)2 + Na2SO4
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When an iron(III) salt is added to
sodium hydroxide, a orange /
brown precipitate of iron(III)
hydroxide is formed.
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When an iron(III) salt is added to
sodium hydroxide, a orange /
brown precipitate of iron(III)
hydroxide is formed.
Eg
Iron + sodium 
chloride hydroxide
iron + sodium
hydroxide chloride
Fe(Cl)3 + 3NaOH  Fe(OH)3 + 3NaCl
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When a copper(II) salt is added to
sodium hydroxide, a pale blue
precipitate of copper(II) hydroxide
is formed.
Cu2+, Fe2+ and Fe3+ using sodium
hydroxide solution
When a copper(II) salt is added to
sodium hydroxide, a pale blue
precipitate of copper(II) hydroxide
is formed.
Eg.
Copper + Sodium  Copper + Sodium
sulphate hydroxide hydroxide sulphate
CuSO4 + NaOH  Cu(OH)2 + Na2SO4
Lesson 5
g) Tests for
ions and
gases
g) Tests for ions and gases
2.37 describe tests for the cations:
i Li+, Na+, K+, Ca2+ using flame tests
ii NH4+, using sodium hydroxide solution and identifying
the ammonia evolved
iii Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution
2.38 describe tests for the anions:
i Cl-, Br- and I-, using dilute nitric acid and silver
nitrate solution
ii SO42-, using dilute hydrochloric acid and barium
chloride solution
iii CO32-, using dilute hydrochloric acid and identifying
the carbon dioxide evolved
2.39 describe tests for the gases:
i hydrogen
ii oxygen
iii carbon dioxide
iv ammonia
v chlorine.
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
STANDARD PROCEDURE:
Add dilute nitric acid to a
solution of the halide, and
then add silver nitrate
solution.
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
RESULT:
With a chloride solution, a
white precipitate of silver
chloride is formed:
NaCl + AgNO3  AgCl + NaNO3
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
RESULT:
With a bromide solution, a
pale yellow precipitate of
silver bromide is formed:
NaBr + AgNO3  AgBr + NaNO3
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
RESULT:
With an iodide solution, a
yellow precipitate of silver
iodide is formed:
NaI + AgNO3  AgI + NaNO3
Cl-, Br- and I- using dilute nitric
acid and silver nitrate solution
Halide Results:
Chloride
Bromide
Iodide
SO42- , using dilute hydrochloric
acid and barium chloride solution
SO42- , using dilute hydrochloric
acid and barium chloride solution
STANDARD PROCEDURE:
Dilute hydrochloric acid is
added to a solution of the
sulphate and then barium
chloride solution is added.
SO42- , using dilute hydrochloric
acid and barium chloride solution
RESULT:
A white precipitate shows
the presence of a sulphate
BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2NaCl(aq)
SO42- , using dilute hydrochloric
acid and barium chloride solution
Barium sulphate precipitate
CO32- , using dilute hydrochloric
acid and identifying the carbon
dioxide evolved
CO32- , using dilute hydrochloric
acid and identifying the carbon
dioxide evolved
STANDARD PROCEDURE:
Dilute hydrochloric acid is
added to a solution of the
carbonate.
CO32- , using dilute hydrochloric
acid and identifying the carbon
dioxide evolved
RESULT:
A gas, carbon dioxide, is
produced which will turn
limewater cloudy (then
clear again)
CO32- , using dilute hydrochloric
acid and identifying the carbon
dioxide evolved
RESULT:
CaCO3(aq) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
Calcium + Hydrochloric  Calcium + water + carbon
Carbonate
Acid
Chloride
dioxide
CO32- , using dilute hydrochloric
acid and identifying the carbon
dioxide evolved
Limewater
Calcium
carbonate +
Hydrochloric
acid
Lesson 5
g) Tests for
ions and
gases
g) Tests for ions and gases
2.37 describe tests for the cations:
i Li+, Na+, K+, Ca2+ using flame tests
ii NH4+, using sodium hydroxide solution and identifying
the ammonia evolved
iii Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution
2.38 describe tests for the anions:
i Cl-, Br- and I-, using dilute nitric acid and silver
nitrate solution
ii SO42-, using dilute hydrochloric acid and barium
chloride solution
iii CO32-, using dilute hydrochloric acid and identifying
the carbon dioxide evolved
2.39 describe tests for the gases:
i hydrogen
ii oxygen
iii carbon dioxide
iv ammonia
v chlorine.
Tests for gases
You need
to know
these!
Tests for gases
1. Hydrogen
Hydrogen gas
Hydrochloric acid
Magnesium ribbon
Tests for gases
1. Hydrogen
Hydrogen gas
Hydrochloric acid
Magnesium ribbon
Tests for gases
1. Hydrogen
POP!
Hydrogen gas
Hydrochloric acid
Magnesium ribbon
Tests for gases
1. Hydrogen
POP!
To test for
hydrogen, use a
burning splint.
The gas will
explode with a
squeaky ‘pop’.
Hydrogen gas
Hydrochloric acid
Magnesium ribbon
Tests for gases
2. Oxygen
Oxygen gas
Hydrogen peroxide
Manganese (IV) oxide
Tests for gases
2. Oxygen
Glowing splint
Oxygen gas
Hydrogen peroxide
Manganese (IV) oxide
Tests for gases
2. Oxygen
Splint re-lights
Oxygen gas
Hydrogen peroxide
Manganese (IV) oxide
Tests for gases
2. Oxygen
To test for oxygen,
use a glowing splint.
The gas will cause the
splint to re-light.
Splint re-lights
Oxygen gas
Hydrogen peroxide
Manganese (IV) oxide
Tests for gases
3. Carbon dioxide
Delivery tube
Bubbles of carbon
dioxide
Hydrochloric acid
Calcium carbonate
Limewater
Tests for gases
3. Carbon dioxide
Delivery tube
Bubbles of carbon
dioxide
Hydrochloric acid
Calcium carbonate
Limewater
Tests for gases
3. Carbon dioxide
To test for carbon
dioxide, bubble the
gas through
limewater. After a
short while the
limewater will go
milky-white.
Delivery tube
Bubbles of carbon
dioxide
Hydrochloric acid
Calcium carbonate
Limewater
Tests for gases
3. Carbon dioxide
To test for carbon
dioxide, bubble the
gas through
limewater. After a
short while the
limewater will go
milky-white.
Delivery tube
If carbon dioxide
continues to be
bubbled through
limewater, the liquid
will eventually go
clear again.
Bubbles of carbon
dioxide
Hydrochloric acid
Calcium carbonate
Limewater
Tests for gases
4. Ammonia
Tests for gases
4. Ammonia
Tests for gases
4. Ammonia
Ammonia gas will turn moist litmus
paper from RED to BLUE
Tests for gases
5. Chlorine
Tests for gases
5. Chlorine
Tests for gases
5. Chlorine
Chlorine gas will turn moist litmus
paper from BLUE to RED, and will
then bleach it WHITE
Tests for gases
Summary
Test
Positive
result
Hydrogen
Burning splint
Burns with a
squeaky pop
Oxygen
Glowing splint
Relights a glowing
splint
Carbon dioxide
Bubble through
Limewater
Limewater turns
cloudy
Ammonia
Moist red litmus
paper
Turns from red to
blue
Chlorine
Moist blue litmus
paper
Bleaches the
paper
Gas
End of Section 2 Lesson 5
In this lesson we have covered:
Tests for Cations
Tests for Anions
Tests for the gases