=Reducing Action.=--All of the hexose sugars are active reducing agents. This is because of the aldehyde group which they contain. Many of the common heavy metals, when in alkaline solutions, are strongly reduced when boiled with solutions of the hexose sugars. Alkaline copper solutions yield a precipitate of red cuprous oxide; ammoniacal silver solutions give silver mirrors; alkaline solutions of mercury salts are reduced to metallic mercury, etc. Any sugar which contains a potentially active aldehyde group will exhibit this reducing effect and is known as a "reducing sugar." In some of the di- and tri-saccharides, the linkage of the hexose components together is through the aldehyde group, in such a way that it loses its reducing effect; such sugars are known as "non-reducing." Advantage is taken of this property for both the detection and quantitative determination of the "reducing sugars." A standard alkaline copper solution of definite strength, known as "Fehling's solution," is added to the solution of the sugar to be tested and the mixture boiled, when the characteristic brick-red precipitate appears. If certain standard conditions of volume of solutions used, length of time of boiling, etc., are observed, the quantity of cuprous oxide precipitated bears a definite ratio to the amount of sugar which is present, so that if the precipitate be filtered off and weighed under proper conditions, the weight of sugar present in the original solution can be calculated. The proper conditions for carrying on such a determination and tables showing the amounts of the various "reducing sugars" which correspond to the weight of cuprous oxide found, are given in all standard text-books dealing with the analysis of organic compounds. =Fermentability.=--The common hexoses are all easily fermented by yeast, forming alcohol and carbon dioxide, according to the equation C_{6}H_{12}O_{6} = 2C_{2}H_{5}OH + 2CO_{2}. The importance and biochemical significance of this reaction will be considered in detail in connection with the discussions of the relation of molecular configuration to biochemical properties (see page 56) and the nature of enzyme action (see page 194). =Formation of Hydrazones and Osazones.=--Another property of the hexoses which is due to the presence of an aldehyde group in the molecule, is that of forming addition products with phenyl hydrazine, known as "hydrazones" and "osazones." For example, glucose reacts with phenyl hydrazine in acetic acid solution, in two stages. The first, which takes place even in a cold solution may be represented by the equation C_{6}H_{12}O_{6}+C_{6}H_{5}·NH·NH_{2} = C_{6}H_{12}O_{5}:N·NH·C_{6}H_{5} Glucose Phenyl-hydrazine Glucose-hydrazone + H_{2}O. The structural relationships involved may be represented as follows: