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Saved on this deviceLEARN · OBSERVE · INTERPRET
Your qualitative analysis toolkit
Never write only the reagent. A complete test answer states the reagent, procedure and positive observation.
01
Anion tests
Use selective reactions to identify negative ions.
Watch with purpose
Record the reagent and positive observation for carbonate, halide and sulfate ions. Notice why acidification matters.
| Ion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | Add dilute acid; test the gas with limewater | Effervescence; gas turns limewater milky |
| Chloride, Cl⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate | White precipitate |
| Bromide, Br⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate | Cream precipitate |
| Iodide, I⁻ | Acidify with dilute nitric acid, then add aqueous silver nitrate | Yellow precipitate |
| Nitrate, NO₃⁻ | Add aqueous sodium hydroxide and aluminium foil; warm | Ammonia produced; damp red litmus turns blue |
| Sulfate, SO₄²⁻ | Acidify with dilute nitric acid, then add aqueous barium nitrate | White precipitate |
| Sulfite, SO₃²⁻ | Add acidified aqueous potassium manganate(VII) | Purple solution decolourises |
Quick check: How do the silver halide colours change?
They deepen down the group: chloride is white, bromide is cream and iodide is yellow.
02
Cation tests
Compare precipitates with aqueous sodium hydroxide and ammonia.
Observation language
State the precipitate colour and whether it dissolves in excess reagent. “No reaction” and “insoluble in excess” are not interchangeable.
| Ion | Aqueous NaOH | Aqueous NH₃ |
|---|---|---|
| Al³⁺ | White ppt; soluble in excess | White ppt; soluble in excess |
| NH₄⁺ | On warming, ammonia evolves | — |
| Ca²⁺ | White ppt; insoluble in excess | No ppt or slight white ppt |
| Cr³⁺ | Green ppt; soluble in excess | Green ppt; insoluble in excess |
| Cu²⁺ | Light-blue ppt; insoluble | Light-blue ppt; dissolves in excess to deep-blue solution |
| Fe²⁺ | Green ppt; turns brown on standing | Green ppt; insoluble |
| Fe³⁺ | Red-brown ppt; insoluble | Red-brown ppt; insoluble |
| Zn²⁺ | White ppt; soluble in excess | White ppt; soluble in excess |
Memory anchor
Cu²⁺ is light blue; Fe²⁺ is green; Fe³⁺ is red-brown. Aluminium and zinc both form white precipitates soluble in excess NaOH.
03
Gas tests
Confirm a gas rather than relying on colour or smell.
Safety first
Never smell an unknown gas directly. Use the prescribed test paper, splint or reagent and observe from a safe distance.
| Gas | Test | Positive result |
|---|---|---|
| Ammonia, NH₃ | Damp red litmus paper | Turns blue |
| Carbon dioxide, CO₂ | Bubble through limewater | Limewater turns milky |
| Chlorine, Cl₂ | Damp litmus paper | Bleaches it |
| Hydrogen, H₂ | Lighted splint | Squeaky pop |
| Oxygen, O₂ | Glowing splint | Relights |
| Sulfur dioxide, SO₂ | Acidified potassium manganate(VII) | Purple solution decolourises |
04
Flame tests
Identify selected metal ions by characteristic emission colours.
Reliable technique
Clean the wire loop with hydrochloric acid between samples. Sodium contamination can mask other colours.
Lithium, Li⁺Crimson red
Sodium, Na⁺Yellow
Potassium, K⁺Lilac
Calcium, Ca²⁺Orange-red
Barium, Ba²⁺Light green
Copper(II), Cu²⁺Blue-green