Phase Diagrams

Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Phase Diagrams of Chemical Thermodynamics. Watch this video to get all the mcat study tips you need to do well on this section of the exam!
Phase diagrams illustrate the physical states of substances at various pressure and temperature combinations. By knowing the temperature and pressure of a given molecule, you can pinpoint on its corresponding phase diagram exactly what state it exists in. Recall that the atoms of solid state materials occupy rigid positions in close proximity, in which neighboring atoms interact quite strongly with one another. The atoms of liquids interact moderately with one another, but the interactions are not quite as rigid as with solids. The atoms of gases are highly dissimilar to those of solids, in that the interactions between gaseous molecules can be thought of as much more negligible. Energy applied to a solid state molecule will break the attractions between neighboring atoms, allowing them to enter a liquid or gaseous state.
Phase diagrams
A single phase diagram has three regions representing the three states that a molecule can exist in. The units of a phase diagram’s x-axis are of temperature, while the units of the phase diagrams y-axis are of pressure. The leftmost region of a phase diagram represents solids. The middle region represents liquids. The rightmost region depicts gases. Considering the combination of pressure and temperature on a system, this make sense. Solids, generally exist at lower temperatures and higher pressures. Gases generally exist at high temperatures and low pressures. Liquids exist somewhere in between solids and gases.
Boundary lines exist between each region. The line between the solid and liquid regions is the solid-liquid boundary. The same convention holds for liquid/gas and solid/gas. Within a region, only one state of matter exists, but at the boundaries the two phases can coexist. This means that at every point along a boundary line you can have a mixture of both solids and gases, existing in equilibrium with each other.
Important points on a phase diagram
Also important are several key points on a phase diagram. Right in the center, at the intersection of the three phase boundaries, is what we call the triple point. The triple point is important because it's the correct pressure and temperature at which a balanced equilibrium of solids, liquids, and gases can exist. For instance, the overall range of pressures and temperatures on earth is close to the triple point of water, which is why, on a winter day, it's possible for there to be ice, water, and water vapor, all present at the same time.
At the very end of the phase boundary between liquid and gas is the point called the critical point. It denotes an upper limit for where we're able to distinguish liquids from gases. Past the critical point, liquids and gases are indistinguishable, and the state of matter at this point can be termed a “supercritical fluid”.
Case Study: Water
Most phase diagrams are highly similar, but one phase diagram to familiarize yourself with prior to taking the MCAT is the phase diagram of water. The solid-liquid boundary of water has a negative slope, which is highly unique. Liquid water is actually denser than solid ice. The implication is that unlike most other molecules, increasing the pressure applied to a molecule of solid state H2O will favor the formation of liquid H2O instead of solid H2O. Memorize the key features of water’s phase diagram, but also reason about the underlying aspects of water that lead to its particular phase diagram.
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[@sarahchong9616]

THANK YOU KEN TAO

[@yashdaryani8954]

How will high pressure in phase diagram of water prefer liquid state over solid state?? If it's a negative slope, wouldn't that mean a smaller value of temperature as opposed to larger? And solids exist at lower temperatures than liquids

[@yashdaryani8954]

@MedSchoolCoachMCATPrep

[@glaucusyellow]

Hi, you have to look at the increase in pressure with respect to constant temperature. As you increase the pressure (move up the y axis) the equilibrium starts to shift towards liquid water instead of solid (ice) water.