MCAT Physical : Phase Diagrams

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

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Example Question #1 : Phase Diagrams

A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove.

The temperature outside is –10 degrees Celsius. The scientist asks the students to consider the following when answering his questions:

Gibbs Free Energy Formula:

ΔG = ΔH – TΔS 

 

Liquid-Solid Water Phase Change Reaction:

H2O(l) ⇌ H2O(s) + X

The scientist prepares two scenarios. 

Scenario 1:

The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup.

Scenario 2: 

The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly.

After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment. 

Scenario 3:

Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes.

Scenario 4:

The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen.

Which of the following best describes the Liquid-Solid Water Phase Change Reaction above?

Possible Answers:

Both an aqueous redox and physical reaction

Physical reaction

Chemical reaction

Both a chemical and physical reaction

Aqueous redox reaction

Correct answer:

Physical reaction

Explanation:

This is an example of a phase change physical reaction at the liquid-solid interface. There is no change in the organization of chemical bonds; therefore, it is not an example of a chemical reaction. There is also no change in the bonding partners of the constituent oxygen and hydrogen atoms of water. As a result, there is no change in oxidation numbers.  Without a change in oxidation number values, it is not a redox reaction.

Example Question #1 : Phase Diagrams

Which best describes the significance of a "critical point" on a phase diagram?

Possible Answers:

The lowest temperature and pressure at which a substance may exist in a gaseous phase

The highest temperature and pressure at which a substance may exist in distinct solid and liquid phases

The temperature and pressure at which a substance may exist in equilbrium between the solid, liquid, and gas phases

The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases

The lowest temperature and pressure at which a substance may exist in a liquid phase

Correct answer:

The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases

Explanation:

The critical point is defined by the highest temperature and highest pressure at which a substance exists in distinct liquid and gas phases. Beyond this point, there is no distinction between the liquid and gaseous phases.

The temperature and pressure at which a substance may exist in equilibrium between the solid, liquid, and gas phases is known as the triple-point.

Example Question #1 : 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.

Phase_diagram_ps

What is the name of Point D?

Possible Answers:

Boiling point

Triple point

Freezing point

Critical point

Correct answer:

Triple point

Explanation:

Point D occurs at the intersection of the three segments, and is called the triple point. All three phases (solid, liquid, and gas) co-exist at the triple point.

Point B corresponds to the critical point. The freezing and boiling points are dependent on pressure, and are depicted as the lines between regions 1 and 2 (freezing) and between regions 2 and 3 (boiling).

Example Question #4 : 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.

Phase_diagram_ps

What processes are occurring at Point D?

Possible Answers:

Deposition (and sublimation)

Condensation (and vaporization)

All of the above

Freezing (and melting)

Correct answer:

All of the above

Explanation:

Point D is called the triple point. At the triple point, all three phases (solid, liquid, and gas) co-exist. The processes that are occurring are condensation and vaporization, sublimation and deposition, and freezing and melting.

Condensation and vaporization allow liquid and gas to co-exist. Sublimation and deposition allow solid and gas to co-exist. Freezing and melting allow solid and liquid to co-exist. In these terms, it makes sense that all six processes occur at the triple point.

Example Question #2 : 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.

Phase_diagram_ps

What is the name of Point B?

Possible Answers:

Freezing point

Triple point

Melting point

Critical point

Correct answer:

Critical point

Explanation:

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. 

 

Example Question #3 : 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.

Phase_diagram_ps

At temperatures and pressures above those at Point B, the material is called a __________.

Possible Answers:

Plasma

Liquid

Gas

Supercritical fluid

Correct answer:

Supercritical fluid

Explanation:

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. 

Example Question #7 : 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.

Phase_diagram_ps

What processes are occurring at Point A?

Possible Answers:

None of these

Condensation/vaporization

Deposition/sublimation

Freezing/melting

Correct answer:

Freezing/melting

Explanation:

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. 

Example Question #5 : 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.

Phase_diagram_ps

What processes are occurring at Point C?

Possible Answers:

None of these

Vaporization/condensation

Deposition/sublimation

Freezing/melting

Correct answer:

Vaporization/condensation

Explanation:

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. 

Example Question #7 : 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.

Phase_diagram_ps

What processes are occurring at Point E?

Possible Answers:

Vaporization/condensation

Freezing/melting

Deposition/sublimation

None of these

Correct answer:

Deposition/sublimation

Explanation:

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. 

Example Question #10 : 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.

Phase_diagram_ps

What molecule has a phase diagram similar to the one presented?

Possible Answers:

Water

Bromine

Mercury

Hydrogen

Correct answer:

Water

Explanation:

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.

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