Carbohydrates are compounds of carbon, hydrogen and oxygen. The formulae of many of them can be written in the form \mathrm{C}_m\mathrm{(H_2O)}_n, which is where the name comes from, although their molecules do not contain water molecules.
Sugars that cannot be hydrolysed into smaller sugars, such as glucose and fructose, are called monosaccharides. Glucose, fructose and galactose all have the molecular formula \mathrm{C_6H_{12}O_6}.
Two monosaccharide molecules joined with the loss of a water molecule make a disaccharide, \mathrm{C_{12}H_{22}O_{11}}, and many joined in the same way make a polysaccharide, \mathrm{(C_6H_{10}O_5)}_n, such as starch or cellulose. Hydrolysis splits disaccharides and polysaccharides into monosaccharides.
In crystals, a glucose molecule has a ring made of five carbon atoms and one oxygen atom. Of the two carbon atoms bonded to the ring oxygen, one also carries an \mathrm{{-}OH} group.
In β-glucose this \mathrm{{-}OH} group is on the same side of the ring as the \mathrm{{-}CH_2OH} group, and in α-glucose it is on the opposite side.
In water, the ring repeatedly opens and closes again. When it closes, the \mathrm{{-}OH} group can end up on either side, so α-glucose, β-glucose and the open-chain form are in equilibrium.
At equilibrium, about 36 per cent of the glucose is α-glucose and about 64 per cent is β-glucose, and only a very small amount is in the open-chain form. Only the open-chain form has an aldehyde group, \mathrm{{-}CHO}, and this group gives glucose its reducing properties.
When glucose solution is heated with Fehling's solution, the deep blue solution gives a red precipitate of copper(I) oxide, \mathrm{Cu_2O}. The copper(II) ions are reduced, and the aldehyde group is oxidized.
When glucose solution is warmed with ammoniacal silver nitrate solution, silver is deposited on the inside of the test tube and forms a mirror. This is the silver mirror reaction.
Only a little of the glucose is in the open-chain form at any moment, but as it reacts, more rings open to take its place. Sugars that react in these tests are called reducing sugars.
Fructose, found in fruit and honey, has the same molecular formula as glucose, \mathrm{C_6H_{12}O_6}. It is the sweetest of the common sugars.
In their open-chain forms, glucose has an aldehyde group at the end of the chain, while fructose has a ketone group on the second carbon atom. In water, fructose also forms rings, which are in equilibrium with its open-chain form.
Ketones do not usually reduce Fehling's solution, but fructose does, and it also gives a silver mirror. This is because the \mathrm{{-}CO{-}CH_2OH} part of fructose is easily oxidized in basic solution.
Yeast contains a set of enzymes called zymase, which breaks glucose down into ethanol and carbon dioxide. This is called alcoholic fermentation, and the overall reaction is \mathrm{C_6H_{12}O_6} \longrightarrow 2\mathrm{C_2H_5OH} + 2\mathrm{CO_2}.
The gas given off turns limewater milky, which shows that it is carbon dioxide.
Alcoholic fermentation is used to make beer, wine and other alcoholic drinks, and the carbon dioxide it gives off makes bread dough rise.