Associated with the three different phases are these properties but also what we call
a phase diagram.
We’re going to say this shows the effect that temperature, temperature down here, and
pressure have on a pure substance in a closed system.
So there’s no air entering the system.
We’re going to say any substance, when we do a phase diagram we can look at its three
phases.
We can have its solid phase, its liquid phase.
and its gas phase.
And the phase diagram basically explains at such a temperature and such pressure, what
phase will that matter exist as.
Will it exist as a solid, a liquid or a gas at this particular temperature and this particular
pressure?
That’s all the phase diagram is really useful for.
And what we’re going to say is this phase diagram represents a compound and the only
compound it doesn’t represent is water.
Water is a very unique substance.
If you think about it, if you freeze water, it becomes ice, right?
Let’s say you take that ice cube and you dunk it into a cup of water.
What usually happens is the ice cube actually floats on top of the water.
This means that the ice, the solid, is actually less dense than the liquid.
The solid ice weighs less, that’s why it’s able to float on the surface of the liquid.
This is a weird phenomenon that happens only with water.
All other substances, when you freeze them to a solid and you drop them in their liquid
form, the solid naturally just sinks to the bottom, which is what we expect, but water
isn’t that way.
And, again, why does water behave this way?
Because of the H bonding that exists in water.
The H bonding that exists between the molecules give them a certain orientation and when you
freeze them, this orientation gets changed even further giving us a special shape.
This special shape allows the solid to float on its liquid form.
So the phase diagram for water is different in this way.
You see this line here?
This line points towards the right.
That signifies that this phase diagram is any compound except water.
Water’s line points to the left.
It’s the only one where that line is going to move towards the left.
We’re going to say here, as the temperature and pressure increase, we’re going to say
that the liquid portion becomes more dense.
That’s what moving that line over to the left really signifies.
And we’re going to say how do we read this phase diagram?
Well, we’re going to say a phase diagram again shows all three states.
We’re going to say here, if we’re going straight from solid to gas, that phase change
is called sublimation and that can happen.
Carbon dioxide can go straight from a solid to a gas skipping the liquid phase altogether.
If we’re going from a gas to a solid, we’re depositing a solid so that’s deposition.
If we’re going from a solid to a liquid, then we’re going to say that that is melting
or what we call fusion.
So remember fusion or melting mean the same thing.
If we’re going from a liquid to solid, that’s freezing.
If we’re going from liquid to gas, that’s vaporization.
If we’re going from gas to liquid, that’s condensation.
We should know all of these different types of phase changes.
Next, we’re going to say that let’s say that this is point A, point B, point C and
point D.
We’re going to say at point B, we have something called the triple point.
At the triple point, the substance exists in all three phases.
At that point, exists as a liquid, a solid and a gas, all at once, the triple point.
We’re going to say at point D, point D is called the critical point.
The critical point is the last point on our phase diagram where our substance can exist
as a liquid or a gas.
If we go beyond the critical point, the substance becomes something entirely different.
It becomes a substance that we can’t recognize.
It becomes a substance called liquid plasma.
The temperature is so high, the pressure is so high that the compound doesn’t look like
a solid or a gas anymore.
We can’t recognize it.
It looks like liquid plasma.
If you wanted to realize what does liquid plasma looks like just think of lava or maybe
the sun.
If you could like physically reach the sun and grab a chunk of it, that’s what liquid
plasma would look like or lava will probably be the closest thing on earth that we can
say liquid plasma looks like.
So again, the critical point is the last point at which we can still recognize the substance.
Once we go a little bit beyond the critical point, we can’t tell if it’s a liquid
or a gas anymore and it looks like something called liquid plasma.
And then we’re going to say that these other major portions of the phase diagram.
So one, remember, that water has a different phase diagram from all other substances, and
then to recognize the different points associated with the phase diagram.
We have three phases of matter and the phase diagram just explains how temperature and
pressure play a role on going from one phase to another phase.