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.
