Video Transcript
In this video, we’re talking about
states of matter. And while there do exist more
exotic states of matter, we’ll keep our discussion focused on the three most common,
the ones that show up the most, solids, liquids, and gases.
Now, when we use this phrase “state
of matter”, what we’re referring to is just how it is that the atoms or the
molecules in a material are arranged. Say that we took a pan and we put a
solid object in it, in this case a chunk of ice. In this perspective, we see the
individual water molecules arranged in a very orderly structured pattern. One characteristic of solid
materials is that their particles are fixed in place. They’re not mobile enough to be
able to move past one another within that material. It’s also true of solid materials
that they’re very hard to compress. If you’ve ever tried to compress,
say, a block of wood or a countertop, you know what that means. And additionally, material which is
in a solid state does not flow. And that goes back to the fact that
the particles aren’t able to move past one another easily.
So we had this solid material, this
chunk of ice, and it’s sitting there in our pan. Not much is going on because, after
all, the particles in the ice are fixed in place. But then, let’s say we start to add
some energy to the system. We put a flame under this pan and
we start to heat the ice. If we apply heat for a long enough
time so that enough energy is added into this chunk of ice, eventually, it will
change its state of matter. By dumping all this heat energy
into the ice, what we’ve done is we’ve loosened the bonds between the individual
water molecules. As each water molecule is heated
and gains energy, it starts to distance itself from its neighbours. The orderly structure of the
arrangement of the molecules breaks down. And they start to move in a much
more disordered way.
This is a description of a state of
matter in the liquid phase. In this phase, particles are able
to move past one another. Liquids, like solids, are still
hard to compress. Water, for example, is so hard to
compress that often as a shorthand description, we just say it’s incompressible. A point of difference though
between liquids and solids is that liquids are capable of flowing while solids or
not. We went from a solid, a chunk of
ice, to a liquid, water, by adding heat energy to our system. If we continue to add even more
heat, soon enough we’ll see yet another state of matter. Eventually, the molecules in this
water become heated enough and so energetic that they spread apart from one another
and completely break any bonds between them.
When this happens, we have a gas,
in this case water vapour. In this state of matter, the
particles are so far apart that there’s very little resistance to motion in any
particular direction. The particles are spread out far
enough that they’re easy to compress back together. Gases, in general, are very
compressible. And lastly, gases are very capable
of flowing. In fact, gases and liquids are both
called fluids because of this capacity.
Now, we might wonder what happens
if we continue even longer to heat this material. Will we find a new state of
matter? Well, if we were doing this with
ice, what we would find is that all of these molecules would eventually heat hot
enough to just escape from this pan and fly off into the atmosphere. Adding more energy just disperses
the gas more.
In this process, we’ve talked about
moving from a solid to a liquid to a gas. In other words, we’ve been moving
left to right on our screen. But, in general, it’s also possible
for states of matter to transition in the other direction. Instead of going from a solid to a
liquid, for example, we could go from a liquid to a solid. Or, we could start with a gas and
go from a gas to a liquid. It’s even possible to completely
skip the liquid phase. We could go from a solid straight
to a gas or a gas directly to a solid. Each one of these six transitions
between phases has its own name.
When a solid becomes a liquid, like
we had when our ice melted into water, then that’s referred to as melting. Whereas when we go the other way, a
liquid to a solid, that process is known as freezing. Then, when we go from a liquid
state of matter to a gas state of matter, that process is known as evaporation. And then, going the opposite way,
from a gas to a liquid, that’s called condensation. We might be familiar with this if
you have a cold drink on a hot humid day. In this case, the water vapour from
the humid air around the cold drink condenses on the outside of the glass. Like we said, it’s also possible to
make a jump across one entire state of matter, the liquid phase. If we were to go straight from a
solid phase to a gas phase, that’s known as sublimation. And going straight from a gas phase
to a solid phase, without going through the liquid phase at all, is called
deposition.
Some of these words are familiar
and some, may be, less familiar. But all of them are helpful in
describing phase transitions between states of matter. As we talk about different states
of matter and phase transitions, that means moving from one state of matter to
another, it’s important to connect energy with that process of transition. Recall that with our ice in the
pan, the only way it was able to change from ice to liquid water to gas was because
we were adding energy into it. We were heating the ice. We can say that for a material to
change from one state of matter to another, that’s called a phase change, the
material needs to gain or lose energy.
For example, we saw that for a
block of ice, a solid, to turn into a puddle of water, a liquid, we needed to add
energy into the ice, the solid. But then, let’s say we wanted to go
the opposite direction. Say we wanted to freeze some
water. In that case, it would be necessary
to take energy away from the water. It would have to lose energy to
turn into ice. And it’s the same story if we took
liquid water and turned it into water vapour or condensed water vapour into a
liquid. The water would need to gain energy
to become a gas, water vapour. And the water vapour would need to
be cooled down. It would need to lose energy to
condense into a liquid.
So whenever there is a change from
one state of matter to another, we know that there is an exchange of energy, either
a gain or a loss. In this process of heating ice to
water and then water to vapour, there’s something very interesting to be seen if we
look at the temperature of our material, whatever state of matter it’s in, plotted
against the total energy added into that material. In our case, that energy was added
by putting a flame underneath the pan that was holding our water sample. So we were adding energy into our
sample. And overtime, we know that its
temperature increased and also that its state of matter changed.
But take a look at this curve of
the temperature of our sample, in degrees Celsius, versus the energy added. Notice that there are two flat
portions to the curve. In these portions, we were adding
energy into our sample. But its temperature wasn’t going up
at all. It was staying constant. Why would that be? Well, notice what temperatures on
the vertical axis these values correspond to. The lower flat portion corresponds
to zero degrees Celsius. And the upper flat portion
corresponds to 100 degrees Celsius. These temperatures we know are the
boundary temperatures between phases of matter, zero between solid and liquid and
100 degrees Celsius between a liquid and a gas.
The two flat portions of this curve
confirm what we said here, that in order to change the state of matter of a
material, that material needs to gain or lose energy. In other words, in these parts of
this graph, the energy added went not to increasing the temperature of our material,
in this case water, but rather to changing its state. Looking at the top flat portion, we
could say that here we had liquid water at 100 degrees Celsius. And here, we had water vapour at
100 degrees Celsius. Or, on the bottom portion, here to
the left-hand side of this, we had zero-degree ice. And on the right-hand side, we had
zero-degree water. Now that we have a sense for what
the states of matter are and how they change from one to another, let’s get a bit of
practice through an example.
Some water vapour in a container is
cooled. It first condenses to become water
and then freezes to become ice. The temperature of the contents of
the container are recorded every minute, and the results are shown in the graph. What state was the water in between
zero and five minutes?
Before we answer this question,
let’s take a look at the graph. It shows us the temperature of the
water in degrees Celsius versus the time in minutes after this water vapour has
begun cooling. If we look on the graph at the
initial moment, time 𝑡 equals zero, we see that the temperature of this water at
that point was above 120 degrees Celsius. This confirms what we’re told in
the problem statement that this water at this point is vapour. It’s in the gas phase. And, in fact, this is part of the
focus of this first question, which asks us what state was the water in between zero
and five minutes.
Looking on our horizontal axis, we
see zero is right here and five minutes is right there. If we trace those two time values
up until they intersect our curve, then we see the portion of the curve we’re
interested in. It’s this portion right here. We can see that at a time value of
exactly five minutes, that’s when the temperature of this water sample becomes equal
to a 100 degrees Celsius. So up until five minutes, the
temperature of this sample was always above that. That tells us the water is too hot
to be in its liquid phase and certainly too hot to be in its solid phase. This means that over this time
interval, from zero to five minutes, the water is in the gas phase. Now let’s look at a few more
questions about this graph.
Our next question asks, what state
was the water vapour in between 40 and 45 minutes?
To figure this out, we locate those
two time values on our horizontal axis. We can see that for this portion of
the curve, between 40 and 45 minutes, the temperature of our water is always at or
below zero degrees Celsius. And in addition to that, if we look
a little bit to the left of 40 minutes, at this flat portion of the curve here, we
can tell that what’s going on there is the water is going from a liquid to a solid,
liquid to ice. This means that at 40 minutes
exactly, we’ve completed that phase transition and our water is now entirely
frozen. It’s solid ice. This tells us that for this time
interval between 40 and 45 minutes, our water is in the solid state of matter.
Next, we want to know what word
describes what is happening between five and 15 minutes.
Between five and 15 minutes, we see
that our curve is going through this flat portion. It’s not changing temperature, but
time is elapsing. Based on the description in the
problem statement, as well as the title of this graph, we know that even though the
temperature of our water isn’t changing over this time, there’s still an energy
exchange going on. That is, energy is being taken away
from the water. Now, if energy is leaving the water
but its temperature isn’t changing, that can only mean one thing, that the water at
that point is going through a phase transition, a change from one state of matter to
another. And indeed, that’s exactly what’s
going on between five and 15 minutes on this curve.
Now, we saw earlier that over this
portion of the curve, from zero to five minutes, our water was in the gas state. And if we then look at the portion
of the curve after 15 minutes, this portion right here, we can see that based on the
temperature range of that curve, the water must be in the liquid state. That’s because it’s between zero
degrees and 100 degrees Celsius. This indicates that this transition
here between five and 15 minutes is from the gas to the liquid phase. Whenever material makes the phase
change of going from a gas to a liquid, there is a particular name for that
process. The word is condensation. That describes the particular
change in phase that we’re seeing between five and 15 minutes from a gas to a
liquid.
And finally, our last question asks
this: What word describes what is happening between 35 and 40 minutes?
Looking at that portion of the
curve, we see once again it’s flat, indicating that this is a period where our water
is being cooled. But its temperature is not
changing. That’s the hallmark of a phase
transition, a change from one state of matter to another. Now in this case, unlike before,
this change of phase is happening at a temperature of zero degrees Celsius. For water, that’s the crossover
temperature between the liquid and the solid phases. Because our water is being cooled
over this time period, we know that it’s moving from a warmer to a cooler state over
this interval. In other words, it’s going from a
liquid to a solid. The word for describing that phase
change, that transition from a material being a liquid to being a solid, is one
we’re familiar with. It’s freezing. As the water goes from a liquid
phase to a solid phase over this time interval, it’s going through the process of
freezing.
Let’s summarize now what we’ve
learned about states of matter. In this lesson, we saw that a
particular state of matter — whether a solid, a liquid, or a gas — describes how
molecules are arranged in a material. We saw that solid materials have a
very orderly structure. Liquids are less ordered, and gases
are less ordered even more. We also learned a bit about the
properties of these three different states. We saw that solids and liquids are
both essentially incompressible and that while solids are incapable of flowing,
liquids and gases can flow.
Along with this, we’d learned the
names for transitions between different states of matter. We learned that going from a solid
to a liquid is melting whereas going the opposite way is freezing. Going from a liquid to a gas is
evaporating whereas going from a gas to a liquid is condensing. And we also saw it’s possible to
skip the liquid phase entirely. And going from a solid to a gas is
called sublimation whereas going from a gas to a solid is deposition. And finally, we saw that there is a
connection between a material changing its state of matter and the energy of that
material. In particular, where there’s a
change in state, a material either gains or loses energy.
