Which process occurs when one element is simultaneously oxidized and reduced in a reaction?

• A. Disproportionation
• B. Oxidation number
• C. Half cell
• D. Anode

Answer: (A)

Disproportionation is a redox process in which an element in one substance is simultaneously oxidized to a higher oxidation state and reduced to a lower oxidation state within the same reaction. In other words, the element acts as both oxidizing and reducing agent at once. A classic example is the decomposition of hydrogen peroxide: 2 H2O2 → 2 H2O + O2. Here, oxygen within the same molecule is driven to two different oxidation states: some ends up as water (-2) and some as oxygen gas (0). This internal redox change—the same element being both oxidized and reduced in one reaction—is exactly what’s described. The other terms describe related ideas but not this process: oxidation number is a bookkeeping concept, a half cell is a piece of an electrochemical setup, and an anode is an electrode where oxidation occurs, not the specific reaction type.