Put the following tubes in the
order in which rusting will occur, starting with the slowest.
Each tube contains a nail with
different conditions. Rusting is a type of corrosion
affecting iron. Rusting occurs when iron is exposed
to oxygen and water, which forms hydrated iron(III) oxide. This means that any measure we take
to protect iron from oxygen or water will slow down the process of rusting. The process of rusting is also
accelerated when iron is exposed to salts or a low pH.
Tube (E) is the only one where the
nail is exposed to salt water. Since rusting occurs more quickly
when iron is exposed to salts, rusting will occur the fastest in tube (E). In test tubes (A), (B), and (D),
measures have been taken to protect the nail from oxygen or water. But in test tube (C), the nail is
exposed to tap water and oxygen from the air, so rusting will occur more quickly in
test tube (C) than in test tubes (A), (B), or (D).
If we look at test tube (D), the
nail has been protected from water by a stopper and the addition of anhydrous
calcium chloride. Anhydrous calcium chloride is a
desiccant, which means that it can absorb any water that enters the tube. This will protect the nail from
water for a long time, meaning that rusting will occur the slowest in this tube.
Now we just need to determine
whether rusting will occur more quickly in tube (A) or in tube (B). In tube (A), there’s no water, but
the nail is exposed to the air. Air has some amount of water vapor
in it, meaning that water will eventually make its way into the tube.
In tube (B), measures have been
taken to protect the tube from oxygen in the air. The tube is stoppered, and there’s
a layer of oil. However, this tube is full of tap
water. There’s always some amount of
oxygen gas that’s dissolved in tap water. This means there’s already both
oxygen and water in the tube. So rusting will occur more slowly
in tube (A) than in tube (B) because it will take some time for water vapor from the
air to enter the tube.
So putting the tubes in the order
that rusting will occur starting with the slowest is (D), then (A), then (B), then
(C), then (E).