Now, what's the big deal about the aldehyde group? Well, from organic chemistry we know that when an aldehyde group is in the presence of some type of oxidizing agent, for example a copper ion with a charge of positive two, that will basically undergo an oxidation reduction reaction. So this will basically act as the reducing agent and the C two, the oxidizing agent, while that will act as the oxidizing agent and what we'll have is, will form a carboxylic acid. So the thing about this unmodified z glucose molecule is that it contains an aldehyde group when the glucose exists in its open chain form and the aldehyde group is reactive when in the presence of oxidizing agents. And when that happens, when we'll have some type of oxidizing agent in the presence of the unmodified glucose in its open chain form, will transform that glucose into a carboxylic acid. And such sugar molecules, unmodified sugar molecules that can basically react with oxidizing agents to form the carboxylic acids, these are known as reducing sugars because they act as reducing agents.