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Question Video: Determining Which Natural Environment Will Increase the Rate of Rusting Chemistry • 10th Grade

In which of the following environments would rusting of a steel plate occur most rapidly? [A] Desert [B] Riverbed [C] Rain forest [D] Ocean floor [E] Frozen soil

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Video Transcript

In which of the following environments would rusting of a steel plate occur most rapidly? (A) Desert, (B) riverbed, (C) rain forest, (D) ocean floor, or (E) frozen soil.

To answer the question, we must understand what rust is and under what conditions it is formed. Chemically, rust is hydrated iron(III) oxide, with the chemical formula Fe2O3⋅𝑛H2O. The 𝑛 signifies that the number of water molecules in the compound can vary. Rusting is a chemical reaction and takes place in steps. Rusting starts when iron metal dissolves in water, which forms iron two plus ions and two electrons. Since iron is losing electrons, it is known as oxidation. This means that something has to be reduced. And in this case, the chemical being reduced is the oxygen.

So during the next step, oxygen gas reacts with hydrogen ions that are also dissolved in the water, as well as the electrons that are given off by the iron. This will form water. The hydrogen ions and dissolved oxygen in the water oxidize iron two plus ions into iron three plus ions in addition to forming water. These iron three plus ions react with water to generate iron hydroxide and hydrogen ions in the final phase. Rust is formed when iron hydroxide is slowly transformed into hydrated iron(III) oxide.

In summary, iron dissolves in water to form iron two plus ions that are then oxidized into iron three plus ions. Hydrogen ions are absorbed, and water is produced. The iron three plus ions then combine with water to make iron(III) hydroxide, which then forms hydrated iron(III) oxide.

As you can see, the formation of rust needs three components: iron, water, and oxygen. The oxygen is supplied either from the atmosphere or dissolved in water. So whenever iron is exposed to water and oxygen, rust will form even underwater.

This takes us back to our possible answers. We can already eliminate option (A) the desert, as there is no or very little water in the desert. The other options all have one thing in common, water, in varied forms and amounts, and oxygen, in varying concentrations.

The only option containing something else is the ocean floor. The ocean consists of saltwater. So the water in the ocean has dissolved oxygen as well as sodium chloride, or salt. If we consider the presence of NaCl and look once again at the chemical reaction, we can remember that iron undergoes an oxidation–reduction reaction. This involves the transfer of electrons. The sodium and chloride ions present in saltwater make saltwater a more effective electrolyte than freshwater, allowing electrons to be transferred more easily. And as a result, rust will form more quickly.

With this in mind, we can answer the question. The environment where rusting of a steel plate will occur most rapidly is (D) the ocean floor.

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