Ion colour tables
Every colour the transition-metals paper can ask for, on one page — the table I wish I'd had taped above my desk. Flip on Test yourself and it becomes a quiz.
Reading mode — every colour on show.
Aqua ions in solution
| Ion | Metal | Colour |
|---|---|---|
| [Fe(H2O)6]2+ | Iron(II) | pale green |
| [Cu(H2O)6]2+ | Copper(II) | pale blue |
| [Co(H2O)6]2+ | Cobalt(II) | pink |
| [Fe(H2O)6]3+ | Iron(III) | yellow pale violet when pure — hydrolysis makes real solutions yellow |
| [Al(H2O)6]3+ | Aluminium | colourless |
| [Cr(H2O)6]3+ | Chromium(III) | violet usually looks green in the lab (ligand swaps with the anion) |
NaOH, ammonia and carbonate
A few drops of NH3 give the same hydroxide precipitate as a few drops of NaOH — the excess columns are where they differ.
| Ion | NaOH (few drops) | NaOH (excess) | NH3 (excess) | Na2CO3 |
|---|---|---|---|---|
| Fe2+ | green ppt Fe(OH)2 — darkens brown on standing | insoluble in excess | insoluble in excess | green ppt FeCO3 |
| Cu2+ | blue ppt Cu(OH)2 | insoluble in excess | dissolves — deep blue [Cu(NH3)4(H2O)2]^2+ | blue-green ppt CuCO3 |
| Fe3+ | brown ppt Fe(OH)3 | insoluble in excess | insoluble in excess | brown ppt Fe(OH)3 + CO2 fizz |
| Al3+ | white ppt Al(OH)3 | dissolves — colourless [Al(OH)4]^- | insoluble in excess | white ppt Al(OH)3 + CO2 fizz |
| Cr3+ | green ppt Cr(OH)3 | dissolves — green [Cr(OH)6]^3- | dissolves slowly — purple [Cr(NH3)6]^3+ | green ppt Cr(OH)3 + CO2 fizz |
The 3+ aqua ions are acidic enough to fizz with carbonate (CO2), so they give the hydroxide, never a carbonate. 2+ ions give the metal carbonate, no fizz.
Ligand substitution
[Cu(H2O)6]2+ + 4Cl− (conc. HCl)
→ [CuCl4]2− yellow-green
octahedral → tetrahedral (Cl− too big for six)
[Co(H2O)6]2+ + 4Cl− (conc. HCl)
→ [CoCl4]2− blue
pink → blue, octahedral → tetrahedral
[Cu(H2O)6]2+ + excess NH3
→ [Cu(NH3)4(H2O)2]2+ deep blue
partial substitution — only 4 of 6 ligands swap
Oxidation-state colour ladders
Vanadium
zinc + acid walks it all the way down
Chromium
dichromate ⇌ chromate flips with acid/alkali
Manganese
MnO4− is self-indicating in titrations
Drill these until they stick
The flashcard decks test every one of these colours (and the rest of the spec) with spaced repetition.
AQA A-level Chemistry flashcardsStruggling with transition metals? The first tutoring lesson is free.
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