What could indicate that a reaction is oxidation?

An oxidation reaction is characterized by the loss of electrons from a substance. When an atom or molecule loses electrons, its oxidation state increases, which is a hallmark of oxidation in chemical reactions. This loss can occur in various contexts, such as when metals react with oxygen to form oxides.

In addition, oxidation often involves the reaction of a substance with oxygen, but the fundamental concept is tied to electron transfer. Thus, identifying reactions where a substance undergoes an increase in oxidation state, typically through the loss of electrons, is key to recognizing oxidation.

Considering the other options, gaining protons is associated with reduction reactions (gaining electrons and typically protons). An increase in temperature can occur in exothermic or endothermic reactions but does not inherently indicate oxidation. Finally, a decrease in molecular weight does not provide a direct measure of whether a substance is undergoing oxidation, as it could result from various other reactions or changes. Therefore, the correct identification of oxidation via the loss of electrons is critical in differentiating these processes.