A white powder could be almost anything. A handful of quick tests, a few drops of reagent and a careful look at the colour of a precipitate or a flame, will tell you which positive ion and which negative ion it contains.
Anions: carbonate fizzes with dilute acid, giving CO2 that turns limewater milky; halides are acidified with dilute nitric acid and tested with aqueous silver nitrate (chloride white, bromide cream, iodide yellow precipitate); sulfate is acidified and gives a white precipitate with aqueous barium nitrate; sulfite turns acidified aqueous potassium manganate(VII) from purple to colourless; nitrate warmed with aqueous sodium hydroxide and aluminium foil gives ammonia. Cations are tested with aqueous sodium hydroxide and aqueous ammonia: the colour of the precipitate, and whether it dissolves in excess, identifies the ion. Flame tests identify Li+, Na+, K+, Ca2+, Ba2+ and Cu2+ by colour.
Add acidified silver nitrate to each halide solution in the simulator and compare the three precipitate colours.
| Anion | Test | Positive result |
|---|---|---|
| carbonate, CO32− | add dilute acid; bubble the gas through limewater | effervescence; carbon dioxide turns limewater milky |
| chloride, Cl− | acidify with dilute nitric acid, add aqueous silver nitrate | white precipitate |
| bromide, Br− | acidify with dilute nitric acid, add aqueous silver nitrate | cream precipitate |
| iodide, I− | acidify with dilute nitric acid, add aqueous silver nitrate | yellow precipitate |
| sulfate, SO42− | acidify with dilute nitric acid, add aqueous barium nitrate | white precipitate |
| sulfite, SO32− | add acidified aqueous potassium manganate(VII) | turns from purple to colourless |
| nitrate, NO3− | add aqueous sodium hydroxide, then aluminium foil; warm carefully | ammonia given off: turns damp red litmus paper blue |
Extended, ionic equations: Ag+(aq) + Cl−(aq) → AgCl(s); Ba2+(aq) + SO42−(aq) → BaSO4(s). The acid is added first to remove carbonate ions, which would also give a precipitate.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Aqueous silver nitrate is added to an acidified solution and a cream precipitate forms. Which ion is present?
Aqueous sodium hydroxide gives a red-brown precipitate with a solution. Which ion is present?
Which flame colour does potassium give?
Which reagent is used to test for sulfate ions after acidifying?
Add a few drops of reagent, note the precipitate, then add the reagent in excess and note whether it dissolves.
| Cation | Aqueous sodium hydroxide | Aqueous ammonia |
|---|---|---|
| aluminium, Al3+ | white ppt, soluble in excess, giving a colourless solution | white ppt, insoluble in excess |
| ammonium, NH4+ | ammonia produced on warming (turns damp red litmus blue) | – |
| calcium, Ca2+ | white ppt, insoluble in excess | no ppt, or very slight white ppt |
| chromium(III), Cr3+ | green ppt, soluble in excess | green ppt, insoluble in excess |
| copper(II), Cu2+ | light blue ppt, insoluble in excess | light blue ppt, soluble in excess, giving a dark blue solution |
| iron(II), Fe2+ | green ppt, insoluble in excess | green ppt, insoluble in excess |
| iron(III), Fe3+ | red-brown ppt, insoluble in excess | red-brown ppt, insoluble in excess |
| zinc, Zn2+ | white ppt, soluble in excess, giving a colourless solution | white ppt, soluble in excess, giving a colourless solution |
Sorting the white precipitates: Al3+ and Zn2+ both dissolve in excess NaOH; only Zn2+ also dissolves in excess ammonia. Ca2+ stays as a precipitate in excess NaOH and gives little or no precipitate with ammonia.
Dip a clean nichrome or platinum wire (cleaned in concentrated hydrochloric acid) into the solid and hold it in a roaring blue Bunsen flame.
| Cation | Flame colour |
|---|---|
| lithium, Li+ | red |
| sodium, Na+ | yellow |
| potassium, K+ | lilac |
| calcium, Ca2+ | orange-red |
| barium, Ba2+ | light green |
| copper(II), Cu2+ | blue-green |
Halides are acidified with nitric acid, never hydrochloric (it adds chloride ions) or sulfuric acid. For sulfate, use barium nitrate after acidifying. Fe2+ gives a green precipitate and Fe3+ a red-brown one; do not swap them. Copper(II) hydroxide is light blue; the dark blue solution appears only with excess ammonia. Sodium's strong yellow flame can hide other colours, so the wire must be clean.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
A solution gives a white precipitate with aqueous sodium hydroxide that dissolves in excess. With aqueous ammonia it gives a white precipitate that does not dissolve in excess. Which ion is present?
Why is dilute nitric acid, not dilute hydrochloric acid, used to acidify a solution before testing for halide ions?
Which test identifies nitrate ions?
A blue solution gives a light blue precipitate with a little aqueous ammonia. What is seen when excess aqueous ammonia is added?
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