Video Transcript
In this video, we will learn the
meaning of vapor pressure, how it relates to boiling point, and the factors that
affect the vapor pressure of a liquid. Before we go any further, it’s
important to clear up something. The term vapor pressure, as in the
vapor pressure of a liquid like water, has a specific meaning in chemistry. It is not necessarily the pressure
of gas above a liquid or the pressure contribution of a particular gas above a
liquid. This can be annoying and confusing,
but listen on and we’ll see what vapor pressure usually means to a chemist.
Here’s some water sitting in a
beaker. If we heated to the boiling point
of water, which is 100 degrees Celsius, it will boil and bubble. If we heat it for long enough, all
the water will boil. Now, let’s go back and take a
closer look. Here we are at the water that’s
less than 100 degrees Celsius. Let’s say it’s at room temperature,
25 degrees Celsius. Zooming in, the surface of water is
a complex, rippling surface with water molecules moving all over the place. But we’re going to keep it simple
and draw a straight blue line. Above the water, we have air, which
is about 1000 times less dense than the water. In air, we have various gases —
nitrogen, oxygen, argon, carbon dioxide, and so on. But even at 25 degrees, we don’t
just have water in the water. We have water vapor in the air.
Liquids do evaporate even below
their boiling point. Puddles don’t boil after all, but
they still disappear eventually in the heat of the sun. Vapor pressure is a measure that
helps us understand what goes on between water at room temperature and water
boiling. But to understand this a little
better, we’re going to need a more controlled environment. If we have water open to the air,
our pressure meter will measure atmospheric pressure. As the water warms up and more and
more water evaporates, the pressure meter will always read one atmosphere. That’s because we’re in an open
system. The pressure of the whole
atmosphere isn’t going to be affected by us boiling a little water in the lab. Instead, we need a closed system so
we can study what’s going on.
Let’s put the beaker in an
airtight, pressure-resistant box and heat the water up like we did before. Now, when we warm the water, we see
the pressure rise. However, something odd happens. The water isn’t boiling, even
though it’s at 100 degrees Celsius. And it’s not boiling at 105 degrees
or 110 degrees. We keep heating, and at 120 degrees
Celsius, the water finally bubbles and boils. We know that water boils at 100
degrees Celsius when the external pressure is one atmosphere. But the pressure inside the box now
reads two atmospheres. By boiling the water, we vastly
increased the pressure inside the box.
We now have a controlled
environment where we can measure the pressure, but the behavior is very
different. So how do we make sure that the
water inside the box boils at 100 degrees Celsius? All that air at the start is what’s
providing the extra pressure. So let’s remove it at the start
with a vacuum pump. We now have our box with only water
in it. There’s liquid water in the beaker,
and there’s gaseous water filling the rest of the box.
Now, let’s do the heating
again. At 25 degrees, we see a pressure
that’s almost zero, 0.02 atmospheres. At 50 degrees Celsius, the system
stabilizes at about a tenth of an atmosphere. And the pressure inside the box is
about half an atmosphere around 80 degrees Celsius. And as we reach 100 degrees Celsius
and a pressure of one atmosphere, the water boils. If we continue to heat above 100
degrees, the water will eventually boil away. But it does depend on the size of
the container. All we were interested in finding
was the boiling point, 100 degrees Celsius.
Now we have a way of measuring the
pressure of the vapor of a substance at a specific temperature. We can look at vapor pressure. Vapor pressure is the pressure
exerted by gas in dynamic equilibrium with its liquid state in a closed system in
the absence of other substances like air. We can see the meaning of the term
dynamic equilibrium if we take a closer look. Water molecules will be evaporating
from the surface of the liquid into the gas phase; they’re evaporating. But at the same time, water
molecules in the gas phase will be hitting the surface of the liquid and
condensing. To be in dynamic equilibrium, the
rate of evaporation must equal the rate of condensation. This is another way of saying we
measure vapor pressure when the system has stabilized. And the last thing is that the
vapor pressure of a substance is a measured quantity at a specific temperature.
Now let’s have a look at the
relationship between the vapor pressure of a substance and its boiling point. When we refer to a substance’s
boiling point, we generally assume that the pressure is one atmosphere, since that’s
the condition we’re used to. So we can define boiling point
simply as the temperature at which the vapor pressure of a substance is one
atmosphere. But there’s a general definition of
a boiling point which is dependent on the external pressure, whether it’s one
atmosphere or something else. We can use the relationship between
boiling point and vapor pressure to predict the behavior of liquids at other
temperatures. We know the boiling point of water
is 100 degrees Celsius, so the vapor pressure of water at 100 degrees Celsius must
be one atmosphere.
At temperatures below 100 degrees
Celsius, the vapor pressure of water is less than one atmosphere. But what does the curve look like
for a substance with a lower boiling point like ethanol with a boiling point of 78.4
degrees Celsius? Wherever we look, the vapor
pressure for ethanol’s liquid versus water as a liquid is higher. Ethanol having a lower boiling
point is more volatile. When doing comparisons, we need to
make sure we’re doing things at the same temperature. This principle holds when we’re
dealing with two different liquids. When dealing with solids and
liquids, it’s a bit more complicated. So far, we’ve only considered the
vapor pressure of pure substances like water. What if we have a solution?
A solution still has a vapor
pressure. We can place a solution in our box,
remove the air, and measure the pressure of the gas after the system has
stabilized. If we add salt to water, will the
vapor pressure go up or down? When we add sodium chloride, we
form sodium and chloride ions, but water can still evaporate. However, the rate of condensation
will be greater than the rate of evaporation. So we see a reduced vapor pressure
and an increased boiling point relative to pure water. And the graph of the vapor pressure
against temperature will look something like this. We can think of the sodium ions and
chloride ions as reducing the surface area available for water to evaporate, slowing
the rate of evaporation.
This reduction in the vapor
pressure of a solution relative to the pure solvent is an example of what we call a
colligative property. The reduction of the vapor pressure
is mostly dependent on the number of dissolved particles and not on the identity of
the particles. What’s also interesting is that
adding salt water reduces its melting point as well as increasing its boiling
point.
Next, we’re going to move away from
the box and look at real ways of measuring vapor pressure. There are many ways of measuring
the vapor pressure of a substance, but one of the most simple is called a
manometer. A manometer is most useful when the
vapor pressure of a substance is relatively high. There are two main types of
manometer, but we’re only going to look at one type, the open-ended manometer. The key part of a manometer is a
U-shaped tube usually made of glass. Inside the glass, there’ll be some
liquid. It helps if the liquid is very
dense and has a very low vapor pressure itself. This is why liquid mercury is very
commonly used. One end of the manometer is left
open to the air, so the pressure on that end is always one atmosphere.
The other end can be connected to
our flask. In the flask, we put our pure
testing substance and make sure there’s no air. When the system stabilizes, the
vapor pressure of the substance in the flask will be competing against atmospheric
pressure, pushing the mercury up or down. If at the given temperature the
vapor pressure of our substance is greater than one atmosphere, the pressure on the
left-hand side will be greater than the pressure on the right. So the mercury will rise on the
right and fall on the left. But if the vapor pressure is less
than one atmosphere, then the mercury will be higher on the left than the right
because atmospheric pressure will be greater than the vapor pressure.
The difference in height of the two
columns, combined with the density of mercury, gives us a measure of the
pressure. But we won’t be looking into that
calculation in this video. What’s important to realize is that
the greater the vapor pressure, the lower the level on the left-hand side and the
greater the level on the right-hand side. Now, it’s time to do some practice
questions.
The image below shows three
manometers, each containing a sample of ethanoic acid. For which manometer is the
temperature of the ethanoic acid the highest?
We can use manometers to measure
the vapor pressure of pure substances. In all the manometers, we have
exactly the same amount of ethanoic acid. Above the liquid and in the tubing,
we’re going to have gaseous ethanoic acid. The other part of the manometer is
the U-shaped tube that contains a dense liquid. Here I’m assuming that it’s
mercury. If we heat the ethanoic acid in the
flask, we’re going to increase the amount of ethanoic vapor in the tubing. This is going to increase the
pressure on this side of the manometer. This vapor pressure is going to
compete with the external pressure applied to the other side of the U-shaped
tube. We can assume it’s one
atmosphere. The most important thing is that
it’s consistent between A, B, and C.
The question is asking, for which
manometer is the temperature of the ethanoic acid the highest? We can label the temperatures 𝑇 A,
𝑇 B, and 𝑇 C. The lower the level of the mercury
on the side of the flask, the higher the vapor pressure in the flask. We can put the manometers in order
of the height of the mercury on the side of the flask. The height is greatest for A and
lowest for C. This means the pressure inside C
must be the highest of the three, and the temperature of the ethanoic acid in C is
greater than the ethanoic acid in B or the ethanoic acid in A. Therefore, the manometer in which
the temperature of the ethanoic acid is the highest is C.
Let’s finish off with the key
points. The vapor pressure of a substance
is the pressure of the gas that is achieved when we establish dynamic equilibrium
with the liquid substance in a closed system in the absence of other substances at a
given temperature. The boiling point of a substance is
the temperature at which its vapor pressure is equal to the external pressure. More volatile substances, which are
substances with higher vapor pressures at a given temperature, will have lower
boiling points. And adding solutes to solvents like
water lowers the vapor pressure, raises the boiling point, and lowers the melting
point.
