3B - Tests for ions and gases

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Revision notes written by Widowmaker
Module 3B
Unit 3.1B: Laboratory chemistry
Specification
a) Recognise the results of reactions of compounds specifically mentioned in Units 1 and 2 of the specification and
the results of tests for simple ions: carbonate, hydrogencarbonate, sulphate(VI), sulphite, chloride, bromide,
iodide, nitrate, ammonium, lithium, sodium, potassium, magnesium, calcium, strontium and barium
Tests for ions
Ion
Carbonate
Formula
Test
CO32-
Hydrogencarbonate
HCO3-
Sulphate (VI)
SO42-
Sulphite
SO32-
Chloride
Cl-
Test 1
 Heat the solid in a test tube with a
Bunsen burner.
 It should decompose producing the
oxide and carbon dioxide. E.g.
CuCO3(s)  CuO(s) + CO2(g)
 Test for the gas using limewater Limewater should turn from colourless to
solution.
cloudy in the presence of carbon dioxide due
to precipitation of calcium carbonate.
Test 2
 Add dilute HCl to the solid.
Vigorous effervescence.
 Test for the gas evolved using Limewater should turn from colourless to
limewater solution.
cloudy in the presence of carbon dioxide due
to precipitation of calcium carbonate.
Test
 Add calcium chloride to a No precipitate forms since calcium
hydrogencarbonate solution.
hydrogencarbonate is soluble.
Test
 Add barium chloride solution White precipitate of barium sulphate forms.
acidified with dilute HCl to the
test solution.
Test
 Warm the sulphite with dilute
HCl.
 Test for gases using acidified The solution turns green.
potassium dichromate(VI) solution
(or paper)
Test 1
 Add concentrated sulphuric acid to White steamy acidic fumes are seen - HCl
the solid chloride.
fumes.
Test 2
 Add dilute nitric acid to a solution
of a chloride to acidify the
solution. This eliminates any
carbonates or sulphites.
Observations
Revision notes written by Widowmaker


Bromide
Br-
Add silver chloride to the solution.
Add dilute ammonia solution.
Module 3B
White precipitate of AgCl forms.
Solid dissolves.
Test 1
 Add concentrated sulphuric acid to Steamy brownish acidic fumes are seen.
the solid bromide.
Test 2
 Add dilute nitric acid to a solution
of a bromide to acidify the
solution. This eliminates any
carbonates or sulphites.


Iodide
I-
Add silver chloride to the solution.
Add
concentrated
ammonia
solution.
Test 1
 Add concentrated sulphuric acid to
the solid iodide.
Cream precipitate of AgBr forms.
Solid dissolves.
Purple acidic fumes are seen. The mixture
turns to a brown slurry.
Test 2
 Add dilute nitric acid to a solution
of a iodide to acidify the solution.
This eliminates any carbonates or
sulphites.


Nitrate
NO3-
Add silver chloride to the solution.
Add
concentrated
ammonia
solution.
Test 1
 Heat solid nitrate.
 If group 1 solid (not Li) then will
decompose to give the nitrite and
oxygen.
 All other solid nitrates decompose
to give the metal oxide, nitrogen
dioxide and oxygen.
Yellow precipitate of AgI forms.
Solid is insoluble.
Oxygen gas is evolved that will relight a
glowing splint.
Brown gas is seen (NO2). Oxygen gas is also
evolved and will relight a glowing splint.
Test 2
 Boil
nitrate
solution
with
aluminium/Devarda’s alloy, in
sodium hydroxide solution.
 Test vapour with red litmus paper.
Litmus paper turns blue in the presence of
ammonia.
Ammonium
NH4+
Test
 Warm ammonium compound with
Revision notes written by Widowmaker
Lithium
Li+
Sodium
Na+
Potassium
K+
Magnesium
Mg2+
Calcium
Ca2+
Strontium
Sr2+
Barium
Ba2+
Module 3B
NaOH.
 Test vapours immediately using NH3 turns the litmus paper blue.
damp red litmus paper.
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A carmine red flame is seen.
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A yellow flame is seen.
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A lilac flame is seen.
Test
 Add NaOH solution to the A white solid forms which is insoluble in
magnesium solid.
excess NaOH(aq). This is Mg(OH)2(s)
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A brick red flame is seen.
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A crimson red flame is seen.
Test
 Dip nichrome wire in HCl.
 Dip wire in solid.
 Heat wire in centre of flame.
 Observe colour of flame.
A apple green flame is seen.
Revision notes written by Widowmaker
Module 3B
b) Recognise the chemical tests for simple gases, to include hydrogen, oxygen, carbon dioxide, ammonia, chlorine,
nitrogen dioxide and sulphur dioxide
Tests for gases
Name
Hydrogen
Oxygen
Formula
H2
O2
Carbon dioxide
CO2
Ammonia
NH3
Chlorine
Cl2
Test
Observations
Ignite gas.
Squeaky pop is heard.
Place a glowing splint in a sample of
the gas.
Bubble
gas
through limewater
(saturated
solution
of
calcium
hydroxide)
Test for gas using damp red litmus
paper.
Test 1
Test for gas using damp litmus paper
(red or blue)
The glowing splint relights.
A solution turns from colourless to cloudy. A
white (milky) precipitate of calcium
carbonate forms which is sparingly soluble.
Litmus paper turns blue.
Chlorine bleaches the litmus paper very
quickly.
Test 2
Test for gas using moist starch-iodide The paper turns blue-black.
paper.
Test 3
Pass gas through a solution of a The solution turns from colourless to orange.
bromide.
Test 4
The solution turns from colourless to brown
Pass gas through a solution of an (possibly with a black precipitate, iodine).
iodide.
Nitrogen dioxide
Sulphur dioxide
NO2
SO2
Not many tests for this gas.
The gas is brown.
Test 1
Bubble gas through a solution of The solution turns from orange to green.
potassium dichromate (VI) dissolved in
sulphuric acid.
Test 2
Bubble gas through a solution of The solution turns from purple to colourless.
potassium manganate (VII) dissolved in
sulphuric acid.
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