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Solid, liquid, gas...solid, liquid, gas...solid, liquid, gas... You've probably heard these
three words together so many times that they sound like some sort of meditative chant. But
don't slip into a blissful trance just yet, because we're going to review just what
these three states of matter are, and then we're going to introduce you to some other states
of matter that you might not have run across before. As you may notice, we've divided the
section on each state into two parts, a macroscopic part that talks about what each phase
looks and feels like to your senses, and a microscopic part that talks about what the molecules
of a substance in each phase are doing.
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Macroscopic
Solids are things that hold their shape. Rocks are solids. Your desk is made of solids. |
The molecules of a solid don't move around very much. They tend to stay put relative to each other. If a solid has molecules arranged in an orderly fashion, we say it is crystalline. If the molecules of a solid are not arranged in any order, we call the solid amorphous. Many polymers are crystalline solids, while others are amorphous solids. |
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Macroscopic
Liquids are different from solids in that they don't hold their shape. That is, they flow and we can pour them. The water in a glass is shaped like the glass, until you pour it into a bucket. Then it will be shaped like the bucket. But liquids do keep the same volume. If you poured 1 liter of water from one bucket to another, it would still take up 1 liter of space, no matter what the shape of the buckets. |
The molecules of a liquid move around a lot. They bounce off each other and spin around, and slide around from one side of the container to the other. They're always moving relative to each other. This is why liquids don't hold their shape and why they can be poured. You might think of a solid as a marching band, keeping in formation when it moves, while a liquid is more like an unruly mob without any formation. But even though the molecules of a liquid move relative to each other, they are still bound to each other through intermolecular forces. This is why liquids hold their volume. |
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Macroscopic Gases don't keep their shape, and don't keep their volume either. If you had one liter of a gas, such as nitrogen, and you pumped it into a 2-liter jar, the gas would swell to fill up the whole 2 liters. |
Another important difference between gases and liquids is that molecules in the liquid state interact with each other through intermolecular forces. These forces hold liquid molecules together. When molecules are in the gas state, they don't interact much. This is why liquids keep their volume but gases do not. Gas molecules aren't held together strongly, so they can spread out, filling as much space as they can. |
State Changes
Solids can melt and become liquids, and liquids can boil to become gases. Likewise, gases
can condense to become liquids, and liquids can freeze to become solids. Sometimes solids
can even become gases without ever becoming liquids. This is called subliming. But what
makes solids melt, and what makes gases condense?
The simple answer is heat. But what is heat? Most of you probably have felt hot or cold. But
what makes something hot or cold? Heat is a form of energy. Heat is the energy of moving
molecules. Let's think about an ice cube. An ice cube is a solid, that is, its molecules aren't
moving relative to each other. They may be shaking and vibrating, but they stay put. If we heat
the ice cube, its molecules start moving around more. If we heat the ice cube enough, the
molecules will start moving around relative to each other, and when this happens, the solid ice
melts and becomes liquid water. If we keep heating the liquid water, eventually the water
molecules will be moving so fast that they won't want to stay with each other anymore. When this
happens, the liquid water becomes a gas÷water vapor.
All this can happen backwards, too. If the water vapor gets cold enough, it will condense back
into liquid water, and if we keep cooling the water, it will freeze to become ice again.
But Wait, There's More!
So you've always been told that the three states of matter are solid, liquid, and gas, right?
It turns out there are some other states of matter that you don't always hear much about
in a science class. We're going to look at some of them.
The Supercritical State
This summer, set an ice cube on a hot sidewalk and you'll see first hand that any material
can be a solid, liquid, or a gas under the right conditions. Ice is a solid, but when it melts
on the hot sidewalk it becomes a liquid. When that liquid water evaporates, it becomes a
gas÷water vapor. It's always water, H2O. That isn't changing. The
little H2O molecules are just passing from one state to
another. So water can be a solid liquid or a gas, depending on two things: temperature and
pressure. We can even make a fancy little diagram that shows what state water (or any other
substance) will be at any temperature and pressure, like you can see below.
But look at the diagram. Above a certain temperature, the critical temperature, and
above a certain pressure (the critical pressure), the boundary between gas and liquid
disappears. Above this point, called the critical point, the substance acts like a gas in
some ways and like a liquid in some ways. The substance is now is a fourth state called the
supercritical state.
The supercritical state is important to polymers because carbon dioxide in the supercritical
state is a useful solvent in which to carry out certain polymerization reactions. Want to know
more? Then read Polymers and Supercritical
CO2.
The Liquid Crystalline State
Liquids flow, solids don't. Molecules in the solid phase can be ordered, but molecules in the
liquid state are not ordered. Or are they? It is sometimes possible for a material to flow like
a liquid even if its molecules are arranged in an orderly fashion. Think of a bunch of logs
floating down a river. They're flowing alright, but they always remain ordered, pointed in the
same direction. This is how the molecules in liquid crystalline states behave. They are
liquids÷they flow and can be poured. But their molecules tend to line up just like the logs in a
river. Usually this happens when the molecules of a material are long, thin, and rigid, like
logs.
This comes back to polymers eventually. In fact this all came back to polymers many years ago
when Stephanie Kwolek unwittingly brewed up the very first known
liquid crystalline polymer solution.
Glassy and Rubbery States
This last section is about two states that are only found in polymeric materials. Actually,
scientists are still arguing about whether these count as states or not, but they are important,
so we're going to talk about them. Have you ever left a flexible plastic bucket outside in the
winter time? If you have, you would have seen it become stiff and brittle. When polymers
are in the solid state, and amorphous (remember that word?), they can be in either the glassy
state or the rubbery state. When that bucket was warm, it was soft and flexible. It was in
the rubbery state. But when it gets cold enough, that bucket will become brittle, that is, it
will pass into the glassy state.
The temperature at which any polymer passes from the rubbery
state to the glassy state or vice versa is called the glass transition temperature.
Each amorphous polymer has a different glass transition temperature. So some polymers are in
the glassy state at room temperature, and others are in the rubber state. Rubber and soft
plastics are in the rubbery state at room temperature, while hard plastics are in the glassy
state at room temperature.
It is important to note that a polymer is a solid when it is in the glassy state or
the rubbery state. Both states can be thought of as different kinds of solid states.
Polymers & Liquid Crystals — an excellent
tutorial and resource from Case Western Reserve University.
For more information, at other Web sites...
The Glass Transition — part
of The Macrogalleria from the University of Southern Mississippi.