MCAT Physical : Phase Diagrams
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Example Question #11 : Phase Diagrams
The phase diagram for water (H2O) shows that it can go through all three phases at a pressure of 1atm. The phase diagram for carbon dioxide (CO2) shows that it can only be a solid or a gas at 1atm.
Based on this information, which statement is true?
CO2 will not sublimate at a pressure of 1atm
CO2 has a higher triple point pressure than H2O
Raising the pressure when CO2 is a solid will result in CO2 liquid
CO2 does not have a liquid phase
CO2 has a higher triple point pressure than H2O
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.
Example Question #12 : Phase Diagrams
Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution.
What are the phases in sections 1, 2, and 3, respectively?
Solid, gas, liquid
Liquid, solid, gas
Gas, solid, liquid
Solid, liquid, gas
Solid, liquid, gas
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.
Example Question #33 : Phase Changes
A hockey player cannot skate on dry ice due to its __________.
positive slope of its solid-liquid equilibrium line
positive slope of its solid-gas equilibrium line
negative slope of its solid-gas equilibrium line
negative slope of its solid-liquid equilibrium line
positive slope of its solid-liquid equilibrium line
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.
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