By Robert Hazen, Ph.D., George Mason University
Liquids, gases, and plasma are different states of matter. Liquids are collections of atoms and/or molecules that can change shape while retaining their volume. However, gas is nothing more than a collection of atoms/molecules that expand to fill whatever volume is available. And plasma is a strange state of matter, indeed. It’s produced when gas is subjected to extremely high temperatures.
Liquids and Gases
It’s such a familiar phenomenon that it’s hard to think that it’s anything extraordinary. But the fact is that a liquid, it’s holding its volume, but it’s changing its shape. Well, at the molecular scale, liquids behave sort of like a bag of flour. Some forces hold those liquid molecules together, but they are very weak forces.
The other state of matter is gas. Here, there is no fixed volume or fixed shape. If you could magnify a gas a billion times, what you’d see was the individual particles flying around all over; sort of like the ping-pong balls in the state lottery game. Gas pressure itself is the result of the collisions of these particles and the wall of the container.
The liquid molecules are free to move past each other because of the weak attraction between them. One’s the weak van der Waals attraction, and the other is hydrogen bonding. These cause the molecules, the atoms of liquid, to be attracted to each other, so they don’t fly off into space, but not so strong that they form a rigid crystal or a solid.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Properties of Liquids and Gases
This feature distinguishes liquids from gases. There is no attraction between the gas molecules, so they can move freely in space. Small gas molecules are O2, N2 , H2 , and CO2 that exist in nature.
By the way, the forces between liquid molecules lead to a phenomenon called surface tension. The reason that water forms little beads and droplets is that all the molecules tend to clump together. It’s not solid, but you do see a shape forming when a drop of water forms, so you can see that tendency to form hydrogen bonds.
Learn more about why atoms bond to one another.
Liquid Crystal and How It’s Formed
Now, an intriguing variation on the theme of liquid is something called a liquid crystal. This is a liquid that’s formed of polar molecules. These are molecules that have a positively charged end and a negatively charged end. And in the absence of an electric field, polar molecules of this sort just form a normal liquid.
But what happens when you apply an electric field to a liquid crystal. Because there is a positive and a negative end, all those molecules line up suddenly, just like they were in a magnetic field, just like a bunch of compass needles. And so you have an alignment of molecules, and as a result, the properties of the liquid change quite dramatically.
Remember, properties are the result of the molecules and atoms and how they’re bonded together. Indeed, what’s primarily dramatic is the change in the optical properties. So you can use liquid crystal displays, where the properties of the liquid change if you apply a field, you get a dark material, and if you take off the field, you get a light material. And so you can have liquid crystal displays in that way.
Learn more about the distinction between a phase transformation and a chemical reaction.
Plasma
Plasmas are the most abundant state of matter in the universe, and the reason is that plasmas form stars. Virtually all the visible matter we see when we look into the night sky are stars, and those stars are made entirely of plasma.
So that’s 99.99 percent of the visible universe. Plasma, however, is used in a few of today’s technologies. For example, in fluorescent light bulbs.
How Is Plasma Produced?
A plasma is produced when gas is subjected to extremely high temperatures. You have gas molecules colliding with each other all the time. But as you raise the temperature, the speed, the velocity of those molecules gets higher and higher and higher, and the collisions are with ever more force. And indeed, when you get to a high enough temperature, the gas molecules, when they collide with each other, start stripping off electrons.
An atom or a molecule with electrons removed is suddenly a charged particle. And you then have a sea of electrons that are also charged particles: positively charged atoms and molecules; negatively charged electrons forming a sea. Plasma has many properties, like metallic bonding. Only in this case, it’s sort of a gaseous state of a metal.
In the atoms and molecules being ionized, the electrons carry the electric charge. And so, for example, you can pass a current, an electric current can pass right through a plasma. Furthermore, plasma can be confined by a strong electromagnetic field. Because electrically charged particles are influenced by magnetic fields, you can imagine confining a mass of plasma in what’s called a magnetic bottle.
Common Questions about Liquids, Gases and Plasma
Liquids, gases, and plasma are three different states of matter with different shapes and properties. Liquid materials can take the shape of the container it’s placed in, but their volume remains unchanged. The volume of gas, on the other hand, varies according to the container it’s placed in. Plasma is a state of matter induced by high-temperature gas.
Liquid molecules tend to bond together, and in fact, weak bonding like hydrogen or van der Waals attraction occurs between them. Gas molecules tend to spin freely in space, and this is the difference between liquid and gas molecules. Plasma, on the other hand, can be a gaseous metal because it’s formed when gas is subjected to high temperatures.
Liquids, gases, and plasma are found naturally around us. Water is a clear example of liquid. Small air molecules such as oxygen, hydrogen, nitrogen, and carbon dioxide are examples of gaseous materials. The stars seen in the sky are all made of plasma. Plasma is also found in fluorescent light bulbs.
