Section 2g tests for ions and gases
1. Cations
2.38 describe simple 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
Flame tests
cation
Li+
Na+
colour of flame
red
orange
cation
K+
Ca2+
colour of flame
lilac
brick red
A platinum or nichrome wire is cleaned in acid and dipped into the flame until it is colourless. It is then
cleaned in the acid again and then dipped into the sample that needs to be tested. The sample is then held in
a flame with the air inlet hole open.
1. Why is the wire cleaned thoroughly?
2. Suggest a reason why platinum is the best material for the wire.
3. Why is the air inlet hole wide open
Using sodium hydroxide solution
cation
NH4+
Cu2+
Fe2+
Fe3+
test
Add aqueous sodium hydroxide and warm, test
the gas given off with damp red litmus paper.
Add aqueous sodium hydroxide
test result
Red litmus turns blue indicating
presence of ammonia
Light blue precipitate, soluble in excess
Green precipitate, soluble in excess
Red-brown precipitate, soluble in excess
Questions
1. For the Cu2+, Fe2+and Fe3+ tests, find the name of the precipitate by writing a balanced chemical equation,
including state symbols, for each test. Assume each salt is a sulphate. The starting chemicals for the first
equation have been given.
(a) CuSO4 (aq) + NaOH (aq) 
+
precipitate =
(b)
precipitate =
(c)
precipitate =
2. For each of the above full symbol equations also write an ionic equation – again with state
symbols.
(a)
(b)
(c)
Section 2g tests for ions and gases
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2. Anions
2.39 describe simple 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
anion
ClBrISO42CO32-
test
acidify with dilute nitric acid, then add aqueous silver nitrate
acidify with dilute nitric acid, then add aqueous silver nitrate
acidify with dilute nitric acid, then add aqueous silver nitrate
acidify with hydrochloric acid, then add aqueous barium
chloride
add dilute acid, test gas produced with lime water
test result
white precipitate
pale cream precipitate
yellow precipitate
white precipitate
effervescence, carbon dioxide
produced
In the tests for the halides and sulphate ion and an acid is also added this is because the acid will react with
other substances other than the halide ions or the sulphate ion that could also make precipitates.
Questions
1. For the Cl-, Br -and I- tests, find the name of the precipitate by writing a chemical equation for each test,
including state symbols. Assume each salt is a sodium salt. The starting chemical for the first equation
has been given.
(a) NaCl (aq) + AgNO3 (aq) 
+
precipitate =
(b)
precipitate =
(c)
precipitate =
2. Write full ionic equations for each of the reactions above.
(a)
(b)
(c)
3. Name the insoluble solid formed during the sulphate ion test.
3. Gases
2.40 describe simple tests for the gases hydrogen, oxygen, carbon dioxide, ammonia, chlorine.
hydrogen (H2)
gas
lighted splint
test result
squaky ‘pop’
oxygen (O2)
glowing splint
relights
carbon dioxide (CO2)
bubble through limewater
milky, white precipitate
ammonia (NH3)
damp red litmus paper
turns blue
chlorine (Cl2)
damp red/blue litmus paper
bleaches
Section 2g tests for ions and gases
test
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