Chemistry: Flame Tests, Precipitation, and Ion Identification Techniques
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22 Terms
What is a flame test?
A qualitative test used to identify certain metal cations from their characteristic flame colour.
Why do ions produce colours in flame tests?
Heat excites electrons to higher energy levels; when they fall back, they emit characteristic wavelengths of light.
What is the basic flame-test method?
Clean wire with HCl → dip into sample → place in flame → observe colour.
What are the main limitations of flame tests?
Some ions give no useful colour; similar colours are hard to distinguish; contamination can interfere.
What is the flame colour of Ba²⁺?
Pale/apple green.
What is the flame colour of Ca²⁺?
Brick red
What is the flame colour of Cu²⁺?
Blue-green.
What is a precipitation test?
Add a reagent that forms an insoluble salt; identify the ion from the precipitate and its colour.
What is observed when Mg²⁺ reacts with OH⁻?
White Mg(OH)₂ precipitate.
What is observed when Pb²⁺ reacts with OH⁻?
White Pb(OH)₂ precipitate; dissolves in excess OH⁻.
What is observed when Cu²⁺ reacts with OH⁻?
Blue Cu(OH)₂ precipitate.
What is observed when Fe²⁺ reacts with OH⁻?
Green Fe(OH)₂ precipitate.
What is observed when Fe³⁺ reacts with OH⁻?
Brown Fe(OH)₃ precipitate.
How can Ag⁺ be identified?
Add Cl⁻ → white AgCl precipitate; dissolves in NH₃.
What is a strong distinguishing test for Pb²⁺?
Add I⁻ → yellow PbI₂ precipitate.
What is a strong distinguishing test for Ba²⁺?
Add SO₄²⁻ → white BaSO₄ precipitate.
What are the precipitate colours for Cl⁻, Br⁻, and I⁻ with AgNO₃?
Cl⁻ = white; Br⁻ = cream; I⁻ = yellow precipitate.
How are AgCl, AgBr, and AgI distinguished using NH₃?
AgCl dissolves readily; AgBr dissolves in concentrated NH₃; AgI does not dissolve.
How is CO₃²⁻ tested?
Add acid → CO₂ effervescence; CO₂ turns limewater milky.
How is SO₄²⁻ tested?
Acidify then add Ba²⁺ → white BaSO₄ precipitate.
How is OH⁻ tested?
Red litmus turns blue.
How is PO₄³⁻ tested?
Ammonium molybdate → yellow precipitate.