Video Transcript
Which of the following is the term
for the movement of particles from where they are at a greater concentration to
where they are at a lesser concentration? (A) Equilibrium, (B) convection, or
(C) diffusion.
To answer this question, we need to
recall the name of the process in which particles move from a region of greater
concentration to a region of lower concentration.
So, first, let’s recall what we
mean by concentration. Concentration is a measure of how
much of a substance is contained in a given volume. For example, let’s imagine we have
two containers, each with the same volume but containing different amounts of a
particular gas. Here, we can see that the container
on the left contains more particles per volume than the container on the right. So we can say that the gas in the
container on the left is at a greater concentration than the container on the
right.
Now let’s think about how particles
might move from where they’re at a greater concentration to where they’re at a
lesser concentration. Imagine we have one large container
that contains a number of gas particles. Suppose all of these gas particles
have just been introduced to the container on the left-hand side. And this diagram shows the
positions of the gas particles shortly after. Notice that the gas particles are
all bunched up at the left end of the container. We can say that the concentration
of gas particles in this area is high, because there are lots of gas particles
occupying a small part of the container’s volume.
However, as time passes, the gas
particles will not stay in the same positions. We can recall that particles in
fluids, such as liquids and gases, move with what’s called Brownian motion. This refers to the motion of a
particle as it frequently collides with other particles, causing it to repeatedly
change direction. Because the particles collide very
frequently, each particle ends up following a very complicated, unique, and
seemingly random path.
One consequence of Brownian motion
is that particles tend to move so that they’re evenly spread throughout the
container. This means that, if they can,
particles will naturally tend to move to be at a lesser concentration rather than at
a greater concentration.
To see why this is, let’s take a
look at this particle here. If, for some reason, this particle
were to move to the left, where there’s already a high concentration of particles,
it wouldn’t get very far before it collided with another particle, causing it to
change direction and move back toward the right. However, when it does move to the
right toward where there is a low concentration of gas particles, there aren’t any
particles in its way for it to collide with. So the particle will keep going
until it reaches the wall of the container.
The same is true for the other
particles. It’s easier for them to move to the
right than to the left. So, gradually, they’ll tend to
spread out to the right until all the particles are evenly distributed throughout
the container. When the particles are spread out
like this, they occupy much more space than when they were bunched together at the
left end of the container. This means that the particles are
now at a lesser concentration than they were initially.
Now, to answer the question, we
need to recall the correct name for this process: diffusion. Diffusion is when particles of a
substance move from where they’re at a greater concentration toward a region of
lower concentration. In the example we just discussed,
we saw the particles diffusing towards the right until they were evenly
distributed. So option (C) appears to be the
correct answer. But let’s take a look at the other
options just to make sure.
Option (A) is equilibrium. When a system is at equilibrium, it
means that it’s balanced out in such a way that there is no net change in the system
over time. So equilibrium cannot correctly
describe the process that causes the concentration of the substance to change. However, we could say that the gas
particles are in equilibrium after diffusion has taken place.
Next, option (B) is convection. Convection is a term used to
describe how a fluid behaves when different parts of the fluid have different
temperatures. Now that’s not what we’re
discussing here, so this isn’t the correct answer either.
So we can be extra sure that the
correct answer is option (C). The term for the movement of
particles from where they’re at a greater concentration to where they’re at a lesser
concentration is diffusion.
