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MCAT Sample Questions : Physical Sciences

At the critical point, the density of liquid CO₂ is equal to the density of gaseous CO₂.This occurs at specific conditions of temperature and pressure.At temperatures and pressures above the critical point values, CO₂ is deemed supercritical.For a supercritical fluid, the density and ability to dissolve other substances are similar to values expected for liquids.The following figures give phase data for CO₂.

Figure 1 Phase diagram for CO₂

Some sample questions on this passage are as follows:

6. In Figure 2, which of the points (A-D) is the critical point for CO2?
   1. A
   2. B
   3. C
   4. D

   Answer: D

   Explanation: The liquid and vapor phases coalesce at point D of Figure 2, where the densities of liquid and gaseous CO₂are equal. Thus, answer choice D is the best answer

7. In an extraction of an organic oil, which of the following is an advantage of using supercritical CO₂?
   1. It reacts with most organic compounds.
   2. It is easily handled at room temperature.
   3. It crystallizes easily.
   4. It is easily removed by evaporation

   Answer: D

   Explanation:The question does not compare CO₂ to a specific solvent, so we are looking for an inherent property of CO₂ that makes it a good solvent for an organic oil. Supercritical CO₂ is similar to a liquid and can be easily removed by evaporation because it changes into a gas when the pressure is lowered. Answers A, B, and C are not true of CO₂, and answers A and C are not desirable properties of an extraction solvent. Thus, answer choice D is the best answer

8. Water is a liquid at room temperature, yet CO₂ at room temperature is liquid only at high pressures.Which of the following best explains this?
   1. CO₂ is polar and has strong intermolecular forces.
   2. CO₂ is nonpolar and has strong intermolecular forces.
   3. CO₂ is polar and has weak intermolecular forces.
   4. CO₂ is nonpolar and has weak intermolecular forces

   Answer: D

   Explanation: Polar water molecules are held together by relatively strong hydrogen bonds; whereas, the linear, nonpolar molecules of CO₂ are held together at room temperature by weak London dispersion forces. Thus, answer choice D is the best answer.

9. According to Figure 1, what is the critical temperature and pressure of CO2?
   1. –56.1^oC and 6.0 atm
   2. –54.1^oC and 119.0 atm
   3. 31.1^oC and 75.3 atm
   4. 25.0^oC and 1.0 atm

   Answer: C

   Explanation:The critical point, shown as a dot (·) in Figure 1, is near 30^oC and 80 atm. Answer choice C is the best answer

10. Which of the following compounds is most soluble in supercritical CO₂?
    1. NaCl
    2. C₂H₅OC₂H₅
    3. NH₄NO₃
    4. KOH

    Answer: B

    Explanation: According to the principle of “like dissolves like,” the covalent compound CO₂ is a better solvent for a covalent compound than it is for an ionic compound. Diethyl ether, C₂H₅OC₂H₅, is a covalent compound and NaCl, NH₄NO₃, and KOH are ionic compounds. Thus, answer choice B is the best answer.