The carbon atoms of glucose are numbered 1 to 6, starting from the carbon atom of the aldehyde group in the open-chain form. In the ring, carbon 1 is the carbon atom bonded to both the ring oxygen and an \mathrm{{-}OH} group.
When the \mathrm{{-}OH} group on carbon 1 of one α-glucose molecule and the \mathrm{{-}OH} group on carbon 4 of another lose a water molecule between them, the two rings are joined through an oxygen atom. This gives maltose, \mathrm{C_{12}H_{22}O_{11}}.
A bond that joins two sugars through an oxygen atom in this way is called a glycosidic bond, and the one in maltose is an α-1,4-glycosidic bond. Acids and enzymes hydrolyse disaccharides into two monosaccharide molecules.
Maltose is made of two glucose units. It forms when the enzyme amylase breaks down starch, and is found in malt and starch syrup.
Sucrose, ordinary table sugar, is made of a glucose unit and a fructose unit, and is obtained from sugar cane and sugar beet. Lactose, found in milk, is made of a galactose unit and a glucose unit.
Cellobiose, obtained by breaking down cellulose, is also made of two glucose units, but they are joined by a β-1,4-glycosidic bond rather than the α-1,4-glycosidic bond in maltose.
In maltose and lactose, one of the two rings still has an \mathrm{{-}OH} group on carbon 1. That ring can open into a chain with an aldehyde group, so maltose and lactose reduce Fehling's solution and give a silver mirror.
The part of a ring that can open in this way, where one carbon atom is bonded to both the ring oxygen and an \mathrm{{-}OH} group, is called a hemiacetal.
In sucrose, the hemiacetal carbon atoms of both units, carbon 1 of glucose and carbon 2 of fructose, are joined to each other through an oxygen atom. Neither ring can open, so sucrose does not reduce Fehling's solution and does not give a silver mirror.
When sucrose is heated with dilute acid, or treated with the enzyme invertase, it is hydrolysed into glucose and fructose: \mathrm{C_{12}H_{22}O_{11}} + \mathrm{H_2O} \longrightarrow \mathrm{C_6H_{12}O_6} + \mathrm{C_6H_{12}O_6}.
The equal mixture of glucose and fructose that forms is called invert sugar. Glucose and fructose are both reducing sugars, so invert sugar reduces Fehling's solution, although sucrose does not.
Honey is mostly glucose and fructose, because bees break down the sucrose in nectar with invertase.
Light whose vibrations all lie in one direction is called polarized light. When polarized light passes through a sucrose solution, the direction of vibration is turned to the right, that is clockwise as seen looking towards the light. This ability to turn polarized light is called optical activity.
After hydrolysis, the solution turns the light to the left instead, because fructose turns it to the left more strongly than glucose turns it to the right. The direction of rotation is reversed, or inverted, and this is where invert sugar gets its name.
The angle measured under standard conditions is called the specific rotation. It is +66.5° for sucrose, +52.7° for glucose and -92.4° for fructose, so a mixture of equal amounts of glucose and fructose gives about -20°.