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MCAT Sample Questions : Physical Sciences

Passage V

A chemist performed the following experiments to investigate the melting and freezing behaviors of acetamide.

Experiment 1: Melting

A large beaker of water was heated to a slow boil. A thermometer was placed in a test tube and 10 g of acetamide crystals was added. The test tube was then lowered into the boiling water (100o C). The temperature was immediately read, and was reread every 15 sec. The acetamide was stirred before each reading. When the temperature reached 80o C, the acetamide started melting. After a period of time, when all the acetamide had melted, the temperature began to increase again. Results are shown in Figure 1.

Experiment 2: Freezing

Trial 1

The test tube from Experiment 1 was removed from the hot water and left to cool in air at 20o C. The temperature readings and stirring were continued every 30 sec. The temperature dropped to 80o C, where it remained constant. The acetamide slowly began freezing and was completely solid after 23 min. After this, the temperature again decreased. The time for freezing was considered to be excessive, so another trial was completed.

Trial 2

The same test tube was placed in boiling water until the acetamide was completely melted. For this trial, however, the test tube was then placed in a beaker of water at 20o C. The results are shown in Figure 1.

Figure 1 Melting and freezing behavior of acetamide

Some sample questions on this passage are as follows:

  1. In Experiment 2, which of the following is the most important difference in the procedures used for Trials 1 and 2?

    1. The amounts of acetamide used in each test tube
    2. The surroundings that were used to cool the acetamide
    3. The temperatures at which the trials were started
    4. The lengths of time allowed for the acetamide to melt

    Answer: B

    Explanation: The only experimental difference in Trial 1 vs. Trial 2 is that, in Trial 2, the test tube is placed in water (20oC) to cool rather than in air (also 20oC). In other words, only the surroundings were different. Thus, answer choice B is the best answer.

  2. During Experiment 1, which of the following would most likely have occurred if the water had only reached 90o C before the test tube was placed into it?

    1. More water would have been needed to melt the acetamide.
    2. Less water would have been needed to melt the acetamide.
    3. The acetamide would not have melted.
    4. The acetamide would have taken longer to completely melt.

    Answer: D

    Explanation: The melting point of acetamide is 80oC; therefore, acetamide will melt when it is in a test tube that is placed in a water bath at 90oC. The temperature of the water in the bath, not the amount of water in the bath, determines whether or not the acetamide will melt. The period of time for acetamide to melt, starting at 90oC, is more than the corresponding period, starting at 100oC (i.e., the temperature of boiling water). Thus, answer choice D is the best answer.

  3. How are the designs of the two experiments important for producing useful results?

    1. Both processes, melting and freezing, take place under controlled conditions.
    2. Both processes, melting and freezing, take place without being controlled or monitored.
    3. The amounts of acetamide are shown to control the temperatures of melting and freezing.
    4. The amounts of acetamide are shown to control the times needed for melting and freezing.

    Answer: A

    Explanation: Without controlling the temperature (i.e., raising the temperature of the water bath above 80oC), the experimenter could not have observed melting or freezing. Without monitoring the time, the experimenter could not have determined the period of time for the samples to melt or freeze. The temperature of melting (freezing) of a pure substance such as acetamide is independent of the amount melted, and Experiment 2 shows that the surroundings control the period of time for freezing to occur. Thus, answer choice A is the best answer.

  4. During Trial 1 of Experiment 2, if the temperature readings were taken at 1-min intervals instead of 30-sec intervals, the acetamide would most likely have become completely frozen at:

    1. 11 min, 30 sec.
    2. 23 min.
    3. 46 min.
    4. a lower temperature.

    Answer: B

    Explanation: The time period of melting is independent of the time intervals used by the experimenter to record temperatures. The sample would freeze completely after 23 min regardless of the time interval used by the experimenter to record temperatures. Thus, answer choice B is the best answer.

  5. In Experiment 2, why was it necessary to place the test tube in hot water for Trial 2, in view of the fact that this was NOT done in Trial 1?

    1. The water was boiling for Trial 1, but it needed to be cold for Trial 2.
    2. The acetamide was cooled by air in Trial 1, but by water in Trial 2.
    3. The temperature lowered more quickly for Trial 2 than it did for Trial 1.
    4. The acetamide was liquid before Trial 1, but it was solid before Trial 2.

    Answer: D

    Explanation: After Experiment 1, the sample was removed from a hot water bath as a liquid. Subsequently, the sample froze during Trial 1. Therefore, the sample had to be reheated in a water bath above its melting point to start Trial 2 as a liquid. Thus, answer D is the best answer

  6. If the data for Trial 1 were plotted in Figure 1, compared to the data for Trial 2, they would:

    1. slope less steeply downward, and not all of the data could be shown.
    2. slope more steeply downward, and all of the data could be shown.
    3. slope upward, and all of the data could be shown.
    4. slope upward, and not all of the data could be shown.

    Answer: A

    Explanation: If the data for Trial 1 were plotted, the temperature would drop to 80oC and remain at this melting temperature for 23 min (or 23 min x 60 sec/min = 1380 sec). The line at 80oC would not slope downward at all in the figure, and it would extend well past 270 sec, the maximum time shown in the figure. Thus, answer choice A is the best answer.

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