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.
