Phase Change

States of matter	We say there are three states of matter: gas, liquid, and solid. These are also called "phases". Nitrogen and oxygen are gases, water and alcohol are liquids, iron and silver are solids. We also know that a given substance can exist in different phases at different temperatures. Water is a solid (ice) below 0^o C, a gas (water vapor) above 100^o C. Iron becomes a liquid when we heat it to a temperature of 1535^o C; this is its melting point. If we further heat the liquid to 3000^o C it boils; iron is a gas above this temperature.
Matter exists in an environment	To think about this more precisely, the substance we are interested in is located in an environment. If the environment is at temperature, T, the substance may not immediately be at that temperature, but eventually it will come to that temperature. For an example, say an ice cube is taken from the freezer where the temperature is -10^o C, and placed in a room where the air is at 20^o C. It will remain ice for a while, but after some time warms up to 0^o C, and melts. After more time the water warms up to 20^o C. Inside the freezer the environment is at -10^o C. If we put the water back in the freezeer, it must return to the solid state, ice.
Phase determined by temperature and pressure	The reason for emphasizing the environment is that there are actually two parameters that characterize an environment: one is temperature, the other is pressure. Most commonly the environment in which we observe materials is the earth's atmosphere near the earth's surface. In this environment the pressure doesn't vary much. Measured by the height of a mercury column in a barometer, the pressure is usually around 76 cm. It may vary down to 75 cm, up to 76.5 cm or so, but -- except for extreme weather conditions -- not much more than that. Therefore we are less aware of the fact that the phase of a substance depends on both the temperature and the pressure of the environment.
Melting and boiling points vary with pressure	At normal atmospheric pressures, and at a temperature of 99^o C, water is liquid. But at a pressure of 71 cm, and at 99^o C, water is a gas. Another way to say this is that the boiling point (the transition between liquid and gas) depends on the pressure. Under normal conditions the boiling point of water is 100^o C, but at low pressure the boiling point is lower, in fact, less than 99^o C. A rule for most substances is that the boiling point gets lower as the pressure gets lower, and that the melting point gets lower as the pressure gets lower. The melting point of water is an exception to this rule. It gets lower as the pressure gets higher. Some examples:
Lowered boiling point of H₂O	Atmospheric pressure is considerably lower at high altitudes, e.g. on a mountain. If food is cooked by putting it in boiling water, the temperature of that boiling water will be less than 100^o C at high altitude. So you will see instructions, say, on freeze-dried foods, to cook them for a longer time if you're at high altitude. A more extreme illustration of the same phenomenon is a demonstration often done in high school or college science courses: Place a beaker of water in a large jar attached to a vacuum pump. The pump removes air from the jar, and the air pressure inside the jar becomes quite low. Very quickly the water begins to boil -- even though the temperature is not high. The water is at room temperature, say 20^o C, and the boiling point has been reduced to a value below 20^o C.
Pressurized water in a reactor	The opposite is what happens in a water-cooled nuclear reactor. Water flowing through the reactor is kept at high pressure, so that its boiling point is higher than normal, perhaps 400^o C or higher. Thus the water itself may be raised to a temperature of 300^o C by the heat in the uranium fuel rods; but it stays in the liquid phase.
An experiment with melting ice	At the liquid/solid transition of H₂O, here is a simple experiment one can do at home. Place an ice cube on a small platform (an inverted mug) and stretch a wire across it. Attach weights to the ends of the wire, letting the weights hang down on the sides of the platform. The wire creates high pressure on the ice cube -- not the whole ice cube, but the part just underneath the wire. This lowers the melting point of that ice to a value below 0^o C, and so the ice melts more readily (more readily than the rest of the ice cube, for which the melting point is still 0^o C). You'll see the wire sinking down into a groove in the ice. If you wait a while (20 or 30 minutes) you may see the ice re-freeze above the wire, so that the wire is embedded in solid ice, like King Arthur's sword in the stone. The wire keeps sinking down.
Molten iron in the earth's interior	Deep in the earth's interior temperatures get very high, say around 2000^o C. The earth's core is largely made of iron, which has a melting point of 1535^o C. But this is the melting point at atmospheric pressure. Deep in the interior matter is under high pressure because of the weight of the earth above it. This makes the melting point of iron higher than 1535^o C -- say, 1800^o C. Iron is still in an environment of temperature 2000^o C, a temperature higher than its melting point. So the iron is in a liquid state. This is the condition in what's called the "outer core" (from a depth of 2900 km below the surface down to a depth of about 4000 km). [For comparison, the distance down to the center of the earth is about 6400 km; this is the earth's radius.]
The solid core	If you go even deeper, closer to the center of the earth, the pressure is even higher. This makes the melting point of iron go even higher, say to 2200^o C. In this region the environmental temperature (assuming it's still 2000^o C) is below the melting point. Here the iron must be solid. This region is called the "inner core". Earth has a solid inner core, and a liquid outer core. Of course no one's ever been there. But scientists can learn a great deal about the earth's interior by studying seismic waves. In this way the molten outer core was discovered in 1906 by Richard Oldham. The inner core was discovered in 1930 by Inge Lehmann. .