fāˈlĭngz, deep-blue, alkaline solution used to test for the presence of aldehydes (e.g., formaldehyde, HCHO) or other compounds that contain the aldehyde functional group, -CHO. The substance to be tested is heated with Fehling's solution; formation of a brick-red precipitate indicates the presence of the aldehyde group. Simple sugars (e.g., glucose) give a positive test, so the solution has been used to test for the presence of glucose in urine, a symptom of diabetes; Benedict's solution, which gives the same test, is now more widely used. Fehling's solution is prepared just before use by mixing equal volumes of two previously prepared solutions, one containing about 70 grams cupric sulfate pentahydrate per liter of solution and the other containing about 350 grams Rochelle salt (potassium sodium tartrate tetrahydrate) and 100 grams sodium hydroxide per liter of solution. The cupric ion (complexed with tartrate ion) is reduced to cuprous ion by the aldehyde (which is oxidized) and precipitates as cuprous oxide (Cu₂O); for this reason, sugars that react with Fehling's solution are called reducing sugars.

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