Ion Identification
This is part of the HSC Chemistry course under the topic Analysis of Inorganic Compounds.
HSC Chemistry Syllabus
Conduct qualitative investigations – using flame tests, precipitation and complexation reactions as appropriate – to test for the presence in aqueous solution of the following
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cations: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+)
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anions: chloride (Cl–), bromide (Br–), iodide (I–), hydroxide (OH–), acetate (CH3COO–), carbonate (CO32–), sulfate (SO42–), phosphate (PO43–)
Identifying Cations Using Flame Test, Precipitation and Complexation
This video explores a range of qualitative tests that can be conducted in a school laboratory to identify metal cations including:
- Barium
- Calcium
- Magnesium
- Lead
- Silver
- Copper(II)
- Iron(II) and Iron(III)
Common Tests for Metal Cations
Precipitation Tests
- Distinguishes cations based on the aqueous solubility of ionic compound formed when an anion is added.
- In general, large divalent cations are more likely to produce precipitate e.g. Ba2+, Pb2+
- Most effective when used to distinguish cations that produce compounds with vastly different solubility products (Ksp) with a given anion.
- For example, adding SO42– to distinguish between Ba2+ and Cu2+ BaSO4(s) is a white precipitate whereas CuSO4(aq) is a light blue aqueous solution.
Flame Tests
- Flame test is a qualitative application of spectroscopy which involves the interaction between matter e.g. metals and electromagnetic radiation e.g. light.
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Spectroscopy: when atomic electrons absorb heat from the flame, they are excited to a higher orbit (excited state).
- Electrons only absorb discrete amounts of energy that equal to the difference in energy levels between orbits.
- Excited electrons can return to their normal orbits (ground state) and release energy in form of electromagnetic waves. In flame tests, the electromagnetic waves have wavelengths in the visible light spectrum.
- The colour of emitted light depends on its wavelength (l) and thus, the discrete amount of energy originally absorbed during electronic excitation.
- Electron orbits of metal ions have different energy levels due to their different ionic structure. This causes different amounts of energy to be absorbed when they are exposed to a Bunsen burner flame. Consequently, when excited electrons return to their normal orbits, each metal ion produces a different wavelength of visible light, and thus shows a different colour.
- Flame test is only useful for identifying certain metal ions, specifically metal ions that emit visible light.
Complexation Tests
- A metal complex refers to a centrally position metal ion surrounded by molecules, bound by coordinate bonds.
- For example, Co2+ ion forms a pink complex with six water molecules and a blue complex with four chloride ions.
- Fe3+ ion forms a yellow complex with six water molecules and a blood red complex with a thiocyanate ion (SCN–).
- Complexation is only useful for identifying transition metal ions as non-transition metals cannot form metal complexes.
- Transition metal complexes have the unique ability to absorb visible light waves.
- Perceived colour of a solution is the colour of visible light that it does not For example, a Co2+ ion solution is pink because it does not absorb pink visible light.
Barium (Ba2+)
Calcium (Ca2+)
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Pale green flame of Ba2+ |
Magnesium (Mg2+)
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Lead(II) (Pb2+)
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Identifying Transition MetalsSilver (Ag+)
$$AgCl(s) + 2NH_3(aq) \rightarrow [Ag(NH_3)_2]^+(aq) + Cl^-(aq)$$
$$Ag^+(aq) + OH^-(aq) \rightarrow AgOH(s)$$ $$2AgOH(s) \rightarrow Ag_2O(aq) + H_2O(l)$$
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Copper(II) (Cu2+)
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Iron(II) (Fe2+)
Purple permanganate decolorises when Fe2+ ions (pale green) are present.
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Iron(III) (Fe3+)
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Concentrated solution of iron(III) chloride. The yellow appearance is due to iron(III) hexahydrate complex |
Iron(III) thiocyanate complex |
Sequence of Tests for Identifying Cations
- Pb2+ and Ag+ are identified with Cl– Ag+ precipitates are usually white/cream in appearance. PbI2 is distinctively bright yellow. The precipitation tests for these two ions should be conducted first within a sequence of tests.
- Ca2+ and Ba2+ are identified with SO42– When both ions are present, a flame test should be conducted. Ca2+ and Ba2+ions produce red and pale-green coloured flames respectively.
- Cu2+, Fe2+ and Fe3+ are identified with OH– Cu(OH)2 is a blue, Fe(OH)2 is green and Fe(OH)3 is brown.
- Mg2+ ions are best identified last within a sequence of test. It should not produce a colour in the flame test and precipitate with Cl– ions in the first step. However, it does produce a white precipitate with SO42– and OH–
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Flame test |
Test using NaCl(aq) |
Test using Na2SO4(aq) |
Test using NaOH(aq) |
Lead Pb2+ |
X |
White precipitate |
White precipitate |
White precipitate |
Silver Ag+ |
X |
White precipitate |
White precipitate |
Brown precipitate (Ag2O) |
Barium Ba2+ |
|
X |
White precipitate |
X |
Calcium Ca2+ |
![]() |
X |
White precipitate |
X |
Magnesium Mg2+ |
X |
X |
White precipitate |
White precipitate |
Copper(II) Cu2+ |
![]() |
X |
X |
Blue precipitate |
Iron(II) Fe2+ |
X |
X |
X |
Green precipitate |
Iron(III) Fe3+ |
X |
X |
X |
Brown precipitate |
Identifying Anions Using Precipitation and pH Indicators
This video explores a range of qualitative tests that can be conducted in a school laboratory to identify anions including:
- Halides: chloride, bromide and iodide
- Acetate
- Phosphate
- Sulfate
- Hydroxide
- Carbonate
Common Tests for Identifying Anions
Chloride (Cl–)
Bromide (Br–)
Iodide (I–)
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Hydroxide (OH–)
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Fe(OH)2, Fe(OH)3, Cu(OH)2. Source: Collins UK |
Acetate (CH3COO–)
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Carbonate (CO32–)
$$HNO_3(aq) + CO_3^{2-}(aq) \rightarrow NO_3^-(aq) + H_2O(l) + CO_2(g)$$
$$Ca(OH)_2(aq) + CO_2(aq) \rightarrow CaCO_3(aq) + H_2O(l)$$
$$CO_3^{2-}(aq) + H_2O(l) \leftrightharpoons HCO_3^-(Aq) + OH^-(aq)$$
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Sulfate (SO42–)
Precipitation of barium sulfate BaSO4 (s)
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Phosphate (PO43–)
$$PO_4^{3-}(aq) + H^+(aq) \leftrightharpoons HPO_4^{2-}(aq)$$
$$PO_4^{3-}(aq) + H_2O(l) \leftrightharpoons HPO_4^{2-}(aq) + OH^-(aq)$$ Thus, salts containing phosphate are basic
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Ammonium phosphate molybdate |
Sequence of Tests for Identifying Anions
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Test using HNO3(aq) |
Test using Ba(NO3)2(aq) |
Test using AgNO3(aq) |
Chloride Cl– |
X |
X |
White precipitate |
Bromide Br– |
X |
X |
Cream precipitate |
Iodide I– |
X |
X |
Yellow precipitate |
Sulfate SO42– |
X |
White precipitate in acidic conditions |
White precipitate |
Phosphate PO43– |
X |
White precipitate in alkaline conditions. Dissolves when nitric acid is added. |
X |
Carbonate CO32– |
Bubbling. Formation of CO2 |
White precipitate |
White precipitate |
Hydroxide OH– |
Temperature increase due to neutralisation |
X |
Brown precipitate(Ag2O) |
Acetate CH3COO– |
Smell of vinegar. Formation of acetic acid |
X |
White precipitate |
Cross-solubility table between cations & anions outlined by the HSC syllabus
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Chloride Cl– |
Bromide Br– |
Iodide I– |
Sulfate SO42– |
Phosphate PO43– |
Carbonate CO32– |
Hydroxide OH– |
Acetate CH3COO– |
Lead Pb2+ |
White |
White |
Yellow |
White |
White |
White |
White |
Sparingly soluble (white) |
Silver Ag+ |
White |
Cream |
Yellow |
White |
Yellow |
Yellow |
Brown |
Sparingly soluble (white) |
Barium Ba2+ |
✓ |
✓ |
✓ |
White |
White |
white |
✓ |
✓ |
Calcium Ca2+ |
✓ |
✓ |
✓ |
White |
White |
White |
Sparingly soluble (cloudy white) |
✓ |
Magnesium Mg2+ |
✓ |
✓ |
✓ |
✓ |
White |
White |
White |
✓ |
Copper(II) Cu2+ |
✓ |
✓ |
White |
✓ |
Blue-green |
White |
Blue |
✓ |
Iron(II) Fe2+ |
✓ |
✓ |
✓ |
✓ |
Brown |
White |
Green |
✓ |
Iron(III) Fe3+ |
✓ |
✓ |
✓ |
✓ |
Brown |
Orange-red |
Brown |
✓ |
Note: all nitrates, sodium, potassium and ammonia containing ionic compounds are aqueous.