By Robert Hazen, Ph.D., George Mason University
Solids, liquids, gases, and plasma are all states of matter. Let’s consider that matter changes state. Matter can change from solid to liquid, from liquid to gas, and back again. These transformations are very familiar. They include freezing, melting, boiling, and condensing. Moreover, they are most commonly observed in our everyday world as a result of changing temperatures.
Change of State by Change in Temperature
Water is a liquid at room temperature. But it transforms to ice, to a solid, in the freezer. And then it transforms into a gas when it is boiled on the stove. These are changes of state that you make use of all the time. Now, if you could heat up water high enough to several thousand degrees temperature, you’d also turn water into a plasma. So that’s the fourth state.
A candle provides an interesting, and a more subtle, demonstration of the same sort of phenomenon. The candle wax itself is typically a carbon-based molecule. It’s about 20 carbon atoms long. These molecules are large enough with those 20 carbons to form a solid under room conditions. But when you light the candle, you’re increasing the temperature. And when you increase the temperature, phase transformations take place.
The first thing that’s happening is the candle wax melting right at the tip or at the base of the candle. And that’s roughly at a temperature of 100 to 200 or 300 centigrade. But as that liquid is pulled up into the wick, the flame itself heats up the liquid, it gets hotter and hotter, and the liquid becomes a gas, and it’s the gas that you see burning at the hottest part of the flame.
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Changes of State by Varying Pressure
Changes of state don’t necessarily have to use temperature. Pressure is a fascinating variable. Pressure is defined as a force per unit area. Now, the uniform high pressure that you experience when you go diving, where water is pressing you from all sides, that’s called hydrostatic pressure. It’s a kind of uniform pressure from all sides.
The atmosphere exerts uniform pressure on us of 14.7 pounds per square inch. By the way, if you have pressures that are a lot less than atmospheric pressure, less than 14.7 pounds per square inch, we call that a vacuum. And vacuums can also induce phase transitions.
If you lower the pressure, water boils at a lower temperature. If you take water high enough to a high enough elevation, it’ll boil at room temperature. And if you raise the pressure on water, you can turn the water as a liquid into water as a solid.
How a Diamond Anvil Cell Can Induce Change of State
The way scientists change water as a liquid to water as a solid in the laboratory is really interesting. They use something called a diamond anvil cell. This is an incredibly elegant device that involves two diamonds that are facing each other. They have little, flat surfaces.
You take a sheet of metal and drill a hole in it; that becomes a chamber to hold your sample. So you have a bottom flat surface that’s diamond, a circular cylinder of metal, and a top surface that’s diamond. And you squeeze down on the sample. With devices like this, the diamond anvil cell, you can achieve pressures of hundreds of thousands, even millions, of atmospheres just by tightening screws.
So this diamond anvil cell is the real workhorse of modern high-pressure research. For example, looking in the diamond anvil cell, you watch water freeze, you can watch it turn from a liquid to a solid and then change from one solid form to another just by varying pressure.
Learn more about isotopes and radioactivity.
Variation in State of Matter on a Phase Diagram
An interesting discovery of modern high-pressure research is that every material becomes a solid at sufficiently high pressure. Water becomes solid at 10,000 atmospheres. But hydrogen gas, oxygen, and all other materials become solid if you take them to high enough pressure. Hydrogen is normally a gas, but if you take it to 50,000 atmospheres, it turns into a crystal. And scientists actually determined, using x-ray diffraction, the crystal structure of those hydrogen crystals.
Well, the variation of the states of matter with temperature and pressure can be beautifully illustrated on a phase diagram. This is a graph of immense visual power. The phase diagram of water is an excellent example. At any given combination of temperature and pressure, water is either a solid, or it’s a liquid, or it’s a gas. So each region of this phase diagram that has pressure on one axis and temperature on the other axis can be coded, either as a solid or a liquid or a gas.
Learn more about properties of materials.
What Certain Lines on Phase Diagram Illustrate
Now there are certain lines on that pressure-temperature graph along which two states coexist. For example, solid and liquid coexist along the melting line, that’s zero degrees centigrade for water at room pressure. And also, at one specific temperature and pressure called the triple point, you get solid and liquid and gas all coexisting stably simultaneously.
For water, it turns out that the triple point occurs at about zero degrees centigrade and six-thousandths of an atmosphere. And there’s another interesting point—when water in the gas phase and water in the liquid phase becomes indistinguishable at the critical point.
The critical point for water occurs at about 374 degrees centigrade and 220 atmospheres. And it’s intriguing that those conditions actually exist in places on the ocean floor. By the way, on a graph like this, the plasma would be much, much higher temperature; it would be 1,000 degrees.
Common Questions about States of Matter and Changes of State
Increasing and decreasing the temperature leads to matter’s change of state. When the temperature is too high, the water turns to gas, and when the temperature drops, the liquid turns into ice.
Pressure can also induce a change of state. When you reduce pressure, water reaches the boiling point at a lower temperature. When you increase pressure, water molecules change from liquid to solid.
This tool has two diamonds facing each other. There is a flat surface between the two diamonds on which the material is placed. With the pressure of diamonds, a lot of pressure is applied to the sample, and the change of state occurs.