Several qualitative tests are used to detect the presence of reducing sugars. Two of them use solutions of copper(II) ions: Benedicts reagent (Cu in aqueous sodium citrate) and Fehlings solution (Cu in aqueous sodium tartrate). The reducing sugar reduces the copper(II) ions in these test solutions to copper(I), which then forms a brick red copper(I) oxide precipitate. Reducing sugars can also be detected with the addition of Tollens reagent, which consist of silver ions (Ag) in aqueous ammonia. When Tollens reagent is added to an aldehyde, it precipitates silver metal, often forming a silver mirror on clean glassware.
3,5-dinitrosalicylic acid is another test reagent, one that allows quantitative detection. It reacts with a reducing sugar to form 3-amino-5-nitrosalicylic acid, which can be measured by spectrophotometry to determine the amount of reducing sugar that was present.
Some sugars, such as sucrose, do not react with any of the reducing-sugar test solutions. However, a non-reducing sugar can be hydrolyzed using dilute hydrochloric acid. After hydrolysis and neutralization of the acid, the product may be a reducing sugar that gives normal reactions with the test solutions.
All carbohydrates are converted to aldehydes and respond positively in Molisch's test. But the test has a faster rate when it comes to monosaccharides.