Video: Identifying the Compound Primarily Responsible for Acid Rain

Select the compound that is primarily implicated in the formation of acid rain. [A] H₂O [B] CCl₄ [C] KNO₃ [D] NO₂ [E] CO₂

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Video Transcript

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 atmosphere.

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.

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