After Seignette's salt has been added, the copper of the copper sulphate remains unchanged in the form of a doubly oxidised Cu++ ion. The action of the sugar reduces the copper by one level to Cu+, and the sugar molecule is oxidised accordingly. Yet as a single-charged Cu+ ion, the copper does not remain bonded in the compound, but is precipitated as copper oxide Cu+2O--. The copper oxide also causes the solution to take on a red-to-brown colour.
Fehling's solution reacts to all aldehydes (molecules with a -CH=O group at one end) and to molecules that easily rearrange into aldehydes. The only sugars this includes are so-called "single sugars" or monosaccharides, such as glucose or fructose. The solution only reacts to saccharose (household sugar) when the saccharose molecules are split, with each saccharose molecule forming one glucose and one fructose molecule. In a cola drink, for example, the phosphoric acid it contains causes the saccharose to split, making Fehling's test work.