Select the compound that is
primarily implicated in the formation of acid rain. A) H₂O, B) CCl₄, C) KNO₃, D) NO₂,
or E) CO₂.
H₂O is the symbol for water. CCl₄ is the symbol for carbon
tetrachloride. KNO₃ is the symbol for potassium
nitrate. NO₂ is nitrogen dioxide. And CO₂ is the symbol for carbon
dioxide. Acid rain might sound like a very
simple thing. Generally, we think of anything
with a pH lower than seven as being acidic. So, surely, acid rain has to be
rain with a pH less than seven. In actual fact, ordinary rain can
have a pH anywhere between five and seven. All this means is that there’s an
acceptable level of acidity for rain based on the natural level of pollutants in the
Any rain with a pH of less than
five is classed as acid rain. And acid rain typically has a pH
around four. So, the word acid in acid rain
doesn’t necessarily mean the same thing as we would normally use it for. In this context, it means slightly
more acidic than normal. So, what we’re looking for is some
form of pollutant that is released into the atmosphere that will lower the pH of
rain below the acceptable level.
Option A, H₂O, doesn’t look like a
very promising candidate. Rain is mostly water, anyway, with
a natural pH of seven. Adding more water to rain in the
atmosphere is only going to reduce its acidity, not increase it. Carbon tetrachloride is an example
of a nonpolar molecule, and as such, it’s not water soluble. Even if it were water soluble, it’s
not acidic. So, even if it did dissolve, it’s
not going to change the pH of the rain. But at least carbon tetrachloride
might evaporate into the atmosphere.
Potassium nitrate, on the other
hand, is an ionic solid. So, even if it dissolves in water,
when the water evaporates, the potassium nitrate is going to be left behind. So, it’s not going to be present in
acid rain. Nitrogen dioxide, on the other
hand, is a water-soluble gas. When nitrogen dioxide is released
into the atmosphere, it reacts with water. This reaction produces the strong
acid, nitric acid, among other things. High levels of nitrogen dioxide are
going to lower the pH of water significantly, producing acid rain.
Nitrogen dioxide is a common
pollutant generated as a byproduct of internal combustion engines. It’s one of the class of compounds
we call nitrogen oxides. These commonly dissolve in water,
forming weak or strong acids. Carbon dioxide is also a
water-soluble gas, forming the weak acid, carbonic acid. Carbon dioxide is not especially
soluble in water, so the levels of carbonic acid possible for rain in the atmosphere
won’t be particularly high.
Combine this with the fact that
carbonic acid is a weak acid, we know that we’re not going to drop the pH by a
particularly large amount by introducing carbon dioxide. So, carbon dioxide is not primarily
implicated in the formation of acid rain. It’s much more common to hear
carbon dioxide in connection with global warming because it’s a greenhouse gas.
Nitrogen dioxide, carbon
tetrachloride, and water are also greenhouse gases. But because of the levels of carbon
dioxide in the atmosphere, that’s what it’s most known for. And the other substances are
connected with other issues. However we look at it, the compound
that is primarily implicated in the formation of acid rain is NO₂.