What is meant by the term “reducing sugar” and under what condition does this happen?

The term "reducing sugar" refers to a type of sugar that has the ability to reduce other substances. This reduction process involves the transfer of electrons from the reducing sugar to another substance, resulting in the reduction of that substance.

Reducing sugars are typically monosaccharides or disaccharides that contain a free aldehyde or ketone group. Examples of reducing sugars include glucose, fructose, and lactose.

The condition under which a sugar becomes a reducing sugar is when it undergoes a chemical reaction known as a redox reaction. In this reaction, the sugar molecule donates electrons to another molecule, which is then reduced. This can occur in the presence of a suitable oxidizing agent, such as an enzyme or a metal ion.

It is important to note that not all sugars are reducing sugars. Some sugars, such as sucrose, do not have a free aldehyde or ketone group and therefore cannot undergo the redox reaction to act as reducing sugars.

In summary, reducing sugars are sugars that have the ability to donate electrons and undergo a redox reaction. This occurs when the sugar molecule contains a free aldehyde or ketone group and is in the presence of a suitable oxidizing agent.