Video: Understanding the Method of Atmospheric Spread of Pollutants

Nitrogen dioxide is an atmospheric pollutant produced by some cars. Which of the following statements explains how the gas spreads out into the atmosphere? [A] Statement A [B] Statement B [C] Statement C [D] Statement D.

04:49

Video Transcript

Nitrogen dioxide is an atmospheric pollutant produced by some cars. Which of the following statements explains how the gas spreads out into the atmosphere? (A) The NO2 molecules move from an area of high concentration near the car to an area of low concentration in the atmosphere by evaporation. (B) The NO2 molecules move from an area of low concentration near the car to an area of high concentration in the atmosphere by diffusion. (C) The NO2 molecules move from an area of high concentration near the car to an area of low concentration in the atmosphere by diffusion. (D) The NO2 molecules move from an area of low concentration near the car to an area of high concentration in the atmosphere by evaporation.

In this question, we’re looking at nitrogen dioxide molecules, or NO2 molecules, as they spread out into the atmosphere. Part of solving this question is to determine whether the molecules move from an area of high concentration to an area of low concentration or the other way around, from an area of low concentration to an area of high concentration. Concentration essentially refers to how densely packed together the molecules are. An area of high concentration of NO2 molecules means that a large number of NO2 molecules are packed into a small space, whereas an area of low concentration of NO2 molecules means that the molecules are relatively spread out from one another.

The other part of this question is to determine whether this process is an example of diffusion or evaporation. Once we determine how the concentration changes and whether it’s an example of diffusion or evaporation, we can select the correct answer.

Let’s start with the diffusion-versus-evaporation question. First, the definition of evaporation is the process wherein a liquid turns into a gas. However, in this question, nitrogen dioxide is a gas that spreads out into the atmosphere, another gas. So it is not correct to say that this is the process where liquid becomes a gas, or evaporation. Therefore, we can eliminate choice (A) and choice (D). The process of a gas or liquid spreading out into another gas or liquid is known as diffusion, a gas spreading out into another gas. That’s exactly what is happening in this question. So we can say that this process is called diffusion, leaving answer (B) or answer (C) to be the correct one.

The next thing to figure out to differentiate between these two answers is whether this process involves moving from an area of low concentration to high concentration or from an area of high concentration to low concentration. It may help to think about a simpler example of diffusion, such as a drop of ink landing in a glass of water and spreading out to make the entire glass a certain color. Is the ink more concentrated or more densely packed at the beginning or at the end of this process?

The answer is that the ink begins with a high concentration and ends with a low concentration. At the beginning, the ink is very tightly packed together in a single drop, whereas by the end, that ink has spread out to fill the entire glass. If we keep this example in mind as we consider the specific situation of the problem, where a car is producing nitrogen dioxide gas, it follows that the area with the highest concentration of nitrogen dioxide gas would be near the car’s exhaust. As the nitrogen dioxide molecules move away from the car and mix with the atmosphere, they spread out, lowering the concentration of nitrogen dioxide gas. So we can say that this situation involves the NO2 molecules moving from an area of high concentration near the car to an area of low concentration in the atmosphere.

In fact, by definition, all instances of diffusion involve something starting at a high concentration and spreading out to a lower concentration. The answer that matches our two conclusions is (C). The NO2 molecules move from an area of high concentration near the car to an area of low concentration in the atmosphere by diffusion.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.