A chemical change in which a substance combines with oxygen, as when iron oxidizes, forming rust.

A chemical change in which a substance combines with oxygen, as when iron oxidizes, forming rust.

• A. Oxidation
• B. Hydration
• C. Reduction
• D. Nucleation

Answer: (A)

Oxidation is the process described here. It’s a chemical change in which a substance combines with oxygen, or more generally, loses electrons in a redox reaction. When iron rusts, it reacts with oxygen (and water) in the environment to form iron oxide, the reddish-brown rust you see. That incorporation of oxygen into the material is the telltale sign of oxidation. Hydration isn’t about reacting with oxygen; it’s about water associating with a substance to form a hydrated compound, so it doesn’t capture the oxygen-adding, redox nature of rusting. Reduction is the opposite process—gain of electrons or loss of oxygen—so it wouldn’t describe iron combining with oxygen to form rust. Nucleation refers to the initial formation of a new phase, such as crystals, and isn’t about a chemical change involving oxygen either.

Oxidation is the process described here. It’s a chemical change in which a substance combines with oxygen, or more generally, loses electrons in a redox reaction. When iron rusts, it reacts with oxygen (and water) in the environment to form iron oxide, the reddish-brown rust you see. That incorporation of oxygen into the material is the telltale sign of oxidation.

Hydration isn’t about reacting with oxygen; it’s about water associating with a substance to form a hydrated compound, so it doesn’t capture the oxygen-adding, redox nature of rusting. Reduction is the opposite process—gain of electrons or loss of oxygen—so it wouldn’t describe iron combining with oxygen to form rust. Nucleation refers to the initial formation of a new phase, such as crystals, and isn’t about a chemical change involving oxygen either.