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Phase diagrams

Understanding and interpreting phase diagrams. Created by Sal Khan.

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• The idea of going from gas to solid by adding extra pressure is somewhat intuitive for me, but what is hard to picture is going from solid to liquid by adding more pressure? Why does this happen or what is the reasoning behind it.
• Water is kind of weird. Namely, the liquid form is actually denser than the solid form*, which is why ice floats. So more pressure would tend to favor a liquid. If you check out the CO2 phase diagram at , you'll see a boundary shape more like what you were expecting between solid and liquid.

*There are actually phases of water ice that are denser than liquid water at very high pressures. See the following for more detail:
http://en.wikipedia.org/wiki/Ice#Phases
• Can anyone explain the negative slope of the solid/liquid boundary for water versus the positive slope of the solid/liquid boundary for carbon dioxide (and I think for most other substances as well). In other words, it is confusing to me that if you had solid H20 at 0 degrees C and you increased the pressure above 1 atm, the ice would actually melt into liquid water.
• Can a vapour be converted into a solid or liquid without decreasing the temperature? If so, how?
• Yes! This is another great question. In the video here, Sal uses a horizontal line through the phase diagram. But, it doesn't have to be horizontal. Imagine a vertical line through this diagram-- for water, choose 100 degrees C. As long as you are at 100 C, you can change the phase by changing the pressure on the system. This kind of process is called "adiabatic".
• Does water have a plasma state?
• Water can be turned into plasma, but when that happens, most of its molecular bonds are broken, so it really becomes a mix of hydrogen and oxygen in the plasma state.

But remember that the plasma state isn't given by phase diagrams since plasma isn't due to a particular relationship between temperature and pressure. Plasma is a special gas-like state in which many of the atoms and molecules in s a substance are ionized. This can be due to extremely high temperatures, but plasma in reality can occur in a wide range of temperatures. The definition of plasma is also rather blurry and the physics of plasma is rather different than the physics of the other states of matter. There is no line that is crossed over in which a substance becomes a plasma like there is with the other states of matter.
• what does Sal mean by "1 atmosphere?"
• The atmosphere (abbreviated as atm) is an old unit of pressure.

For simplicity in the past scientist used the pressure of our atmosphere as the standard, since our planet has one atmosphere.

Equivalences for this are 760 mmHg (millimeters mercury) from a barometer or the SI unit and number would be about 100 kPa (kilo pascals, and is actually more precisely 101.325 kPa).
• I think the reverse sublimation is known as deposition .Right??
• Yes, the reverse of sublimation is deposition.
• So If I am exhaling at a place where theres 1atm and that place is say -80C, my exhaling breath (carbon dioxide) will become dry ice and start dropping from my nose? lol....but is that true?
• In theory, yes -- although, your breath would need a few moments to cool down, so the dry ice would form a meter or two away from your face. But since you would freeze to death a moment or two later, I don't think it would matter too much.
• as we go down the sea, pressure increases so that means temperature increases too right? so will it be hotter as we go down?
• No. Because of two reasons:
1.) This works well with a gas, but not with a liquid like water, which is basically incompressible.
2.) If you do this little compression experiment with an air-pump, it sure will get hotter. But if you wait for a little bit, the heat will dissipate away. The oceans are sitting there for a couple of millions of years. Plenty of time to distribute any excess heat.