Video: Identifying the Substance That Contributes to Acid Rain in a Set of Formulas

Which of the following contributes to acid rain? [A] CO₂ [B] O₃ [C] O₂ [D] Cl₂ [E] NO₂

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Video Transcript

Which of the following contributes to acid rain? A) CO₂, B) O₃, C) O₂, D) Cl₂, or E) NO₂.

Rain is considered to be acid rain if it has a pH less than five. Ordinary rain has a pH anywhere between five and seven. So, ordinary rain can be slightly acidic without being classified as acid rain. So, acid rain generally contains strongly acidic pollutants, which lower the pH below the typical level. These pollutants damage plants and cause chemical erosion. Our job is to select the substance that contributes meaningfully to acid rain.

The candidates are CO₂, carbon dioxide; O₃, ozone; O₂, which we generally call oxygen, molecular oxygen, or dioxygen; Cl₂, which is chlorine; or NO₂, nitrogen dioxide. To contribute to acid rain, the substance must be present in the atmosphere. It must be produced in meaningful quantities that would affect the pH of rain. And crucially, when it dissolves in water, it must produce acidic solutions. So, let’s put all the gases through these three tests.

Let’s start with carbon dioxide. Carbon dioxide is most definitely present in the atmosphere in meaningful quantities. The levels of carbon dioxide in the atmosphere present make a substantial contribution to global warming. Ozone is an allotrope of oxygen that can be found in high concentrations in the upper atmosphere. Although by high concentrations, we do mean relatively speaking. Ozone gas helps to shield us from the UV radiation from space. Oxygen obviously makes up a massive proportion of the atmosphere. It’s what supports combustion and allows us to continue to live. Oxygen makes up about 21 percent of the Earth’s atmosphere.

Chlorine, on the other hand, is not present by default in the atmosphere. Even the dissolved substances that form when it reacts with water are not present in meaningful concentrations. So, chlorine does not really contribute to the phenomenon of acid rain. And lastly, nitrogen dioxide is produced by internal combustion engines and similar processes and so is present in a meaningful concentration in the atmosphere. So, we can move on to the next criterion, whether or not the substance is produced in meaningful quantities.

Carbon dioxide is a product of respiration in pretty much all forms of life. Even plants produce it. Ozone is produced in lightning strikes and in the upper atmosphere where gases are bombarded with energy from space. Oxygen gas is produced in colossal quantities by plants. However, oxygen is also consumed by animals and plants, so the levels of oxygen stay pretty constant in the atmosphere. A gas whose concentration doesn’t really change won’t cause an intermittent phenomenon like acid rain. And skipping out on chlorine, we can look at nitrogen dioxide, which is produced in meaningful quantities by internal combustion engines.

The last question is perhaps the most significant. If the gas doesn’t dissolve to form an acidic solution in water, it can’t possibly contribute to acid rain. Carbon dioxide does dissolve in water, but only a little and only produces the weak acid carbonic acid. The weak carbonic acid solution is not sufficient enough to cause acid rain. However, large amounts of carbon dioxide are dissolving into the world’s oceans, changing the pH, which is having a dramatic effect on sea life all over the world. However, we’re only looking at acid rain here, so carbon dioxide is not our answer.

Ozone doesn’t dissolve in water to produce an acidic solution, so it’s not a candidate anymore. Oxygen doesn’t dissolve in water to produce an acidic solution, but chlorine gas does, forming hydrochloric acid. However, as we said before, chlorine isn’t present in the atmosphere in meaningful quantities. Our final candidate, nitrogen dioxide, is present in the atmosphere in meaningful quantities and produces strongly acidic solutions containing a mixture of nitric and nitrous acids. Nitrogen dioxide is a common biproduct of combustion, particularly in internal combustion engines, and of the five chemicals given it is the one that contributes to acid rain.

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