| Test | Positive when | Sign |
|---|---|---|
| Tollens' ([Ag(NH₃)₂]⁺) | the compound is an aldehyde (H on the C=O carbon) | silver mirror |
| Fehling's (Cu²⁺, blue) | the compound is an aliphatic aldehyde | brick-red Cu₂O |
| Iodoform (I₂ / NaOH) | the compound has a CH₃CO– group (methyl ketones, ethanal) | yellow CHI₃ |
Results follow these general rules under ordinary test conditions. Some special compounds, such as α-hydroxy ketones, can also reduce Tollens' and Fehling's reagents.