The oxidation product of an aldehyde is either a carboxylic acid or the salt of a carboxylic acid, depending on the conditions under which the reaction occurs. A carboxylic acid forms in the presence of acidic conditions, whereas alkaline conditions result in the formation of a salt.
One example of an oxidative reaction involving an aldehyde is the chemical reaction that occurs when dilute sulphuric acid and a few drops of an aldehyde are used to acidify a solution of potassium dichromate(VI). The potassium dichromate(VI) ionizes, and the resultant dichromate (VI) ions are initially orange but turn into green chromium(III) ions when the aldehyde reduces them. In the process, the aldehyde becomes oxidized, forming a carboxylic acid. If the same procedure is carried out using a ketone, the solution remains orange because ketones resist oxidation under most conditions.
Another well-known oxidative reaction of an aldehyde involves Tollens' reagent and is nicknamed the "silver mirror test." To carry out this reaction, a chemist creates Tollens' reagent by adding a drop of sodium hydroxide to silver(I) nitrate, resulting in the formation of a silver(I) oxide precipitate. Next, the chemist redissolves this precipitate by adding ammonia. Tollen's reagent is now formed, and the chemist observes that a silver mirror appears to form on the test tube upon adding the aldehyde and heating the solution. In this case, the oxidative product is a salt.