Fehling's solution is an aqueous solution that is made from sodium hydroxide, copper sulfate and potassium sodium tartrate. It is used to test substances for the presence of aldehydes and sugars.
The substance that needs to be tested for the presence of aldehydes is placed within Fehling's solution and then heated. If it contains aldehydes, a red precipitate nearly the color of brick will be produced.
The same results will occur if glucose is present in the substance and exposed to Fehling's solution. For this reason, it was used as a test for diabetes. Urine from a patient would be heated in Fehling's solution. If it produced the brick-red precipitate, the patient was likely to have diabetes or another blood sugar imbalance.