Final answer:
Hydrogen gas (H2) is the most common reducing agent that is colorless and odorless, used in various chemical reactions to donate electrons and reduce other substances.
Step-by-step explanation:
The most common reducing agent that is colorless and odorless is hydrogen gas (H2). In many chemical reactions, substances such as carbon (in the form of coke or coal), hydrogen gas, and various antioxidants (e.g., ascorbic acid and vitamin E) are used as reducing agents. These agents can donate electrons to other substances and therefore reduce them. Hydrogen gas, in particular, is known for its reducing properties and is involved in various reactions where it reacts with water vapor to form H2(g).
When considering other reducing agents like carbon or antioxidants, hydrogen stands out because it is a pure element, has no color, and does not have any odor, making it quite distinctive. For instance, in many industrial processes, hydrogen gas is used to reduce metal ores to the corresponding metals by removing oxygen, as well as in hydrogenation reactions in organic chemistry.