Point B occurs at the highest pressure and temperature point on the phase diagram. This point is called the critical point, meaning that at temperatures and pressures beyond this point separate gas and liquid phases do not exist. 

Point B is known as the critical point. While identifying this point on the phase diagram is important, it is also important to know that at temperatures and pressures above the critical point, the solution is called a supercritical fluid, meaning that separate gas and liquid phases do not exist. 

First, it is important to identify what phases are occurring on each side of the line on which Point A rests. In section 1, the pressure is high and the temperature low, meaning the solution is a solid. In section two, both the pressure and temperature are intermediate, meaning the solution is a liquid. In other words, the segment that Point A is on is the equilibrium line between solid and liquid, thus, melting and freezing are occurring at Point A. 

First, it is important to identify what phases are occurring on each side of the line on which Point C rests. In section two, both the pressure and temperature are intermediate, meaning the solution is a liquid. In section three, both the pressure and temperature are high, meaning the solution is a gas. In other words, the segment that Point C is on is the equilibrium line between liquid and gas, thus, vaporization and condensation are occurring at Point C. 

First, it is important to identify what phases are occurring on each side of the line on which Point E rests. In section 1, the pressure is high and the temperature low, meaning the solution is a solid. In section three, both the pressure and temperature are high, meaning the solution is a gas. In other words, the segment that Point E is on is the equilibrium line between solid and gas, thus, deposition and sublimation are occurring at Point E. 

While this may seem like an obscure question, the MCAT specifically requires you to know the shape of the water phase diagram. Unique to only a few molecules, the solid phase in our diagram is less dense than the liquid phase, meaning that the solid/liquid phase line has a negative slope (this can be seen as segment AD in the above image). Water is one of the select few compounds with this characteristic.

The phase diagram for carbon dioxide (CO₂) shows that it will sublimate from a solid to a gas as temperature is increased at one atmosphere of pressure. If a solid immediately goes to a gas, we can conclude that the pressure is too low to allow the substance to first go through the liquid phase. As a result, we can conclude that the point in which CO₂ is in all three phases (the triple point) will take place at a higher pressure than 1atm. Because water (H₂O) is able to go through all of its three phases at a pressure of 1atm, we know that the triple point pressure is less than 1atm.

Section 1 is at high pressure and low temperature, meaning the solution is a solid. In section 2, the solution is at intermediate pressure and temperature, meaning it is a liquid. Section 3 is at high temperature and low pressure, meaning it is a gas. 

Because dry ice is frozen carbon dioxide, it does not have the same liquid-solid equilibrium line as water and traditional ice. Its liquid-solid equilibrium line is positive, which means that increased pressure will only cause the dry ice to remain solid. If it were negative, then increased pressure would cause melting as seen with water. Skating on solids is like skating on dirt or rocks. The other choices concerning solid and gas does not apply since no gases are involved. 

In the new cell, energy can still be produced, but because zinc ions have a lower reduction potential than copper ions, the copper will be reduced and the direction of electron flow will be reversed, as compared to the cell with silver in which copper was oxidized.