A water molecule is lost when two glucose units come together to form a molecule of lactose. Which type of reaction is involved in the formation of the lactose molecule?

When two monosaccharaides e. g. glucose join together to form a disaccharide, the resulting bond is known as glycosidic bond and is formed through a condensation reaction where a water molecule is lost in the process.

The condensation reaction occurs when an alcohol group (OH) from a molecule reacts with the anomeric carbon of a sugar. The anomeric carbon is the central carbon of a hemiacetal. The OH group bonded to that carbon is replaced by the O of the alcohol, and the H of the alcohol is removed. Together they make water which is then released and lost.