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Chapter 9: Gases and Vapors

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Which of the following statements regarding the physical properties of nitrous oxide is FALSE?

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(A) Nitrous oxide is colorless.

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(B) Nitrous oxide is odorless.

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(C) Nitrous oxide cannot support combustion.

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(D) Nitrous oxide has a boiling point below room temperature.

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(E) Nitrous oxide is nonflammable.

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The answer is C. Nitrous oxide is a colorless and odorless gas. While it is nonflammable, it is an oxidizing agent and does support combustion. For this reason, its concentration should be kept to a minimum in cases where the risk of surgical or airway fire are elevated. Its boiling point is well below room temperature (-88°C), meaning that at standard temperature and pressure, nitrous oxide exists in a gaseous form, in contrast to the potent volatile anesthetic agents. However, because its critical temperature is above room temperature (36°C), nitrous oxide can be liquified when sufficient pressure exists inside a cylinder.

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Ref: Butterworth JF IV, Mackey DC, Wasnick JD. Morgan and Mikhail's Clinical Anesthesiology. 5th ed. New York, NY: McGraw Hill; 2013.

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Which of the following potent volatile agents is most likely to evaporate when left in an open container in the operating room?

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(A) desflurane

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(B) enflurane

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(C) halothane

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(D) isoflurane

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(E) sevoflurane

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The answer is A. The degree to which a liquid will evaporate depends on the vapor pressure, or the pressure exerted by a vapor when it is in equilibrium with the liquid phase. The boiling point of a liquid is the temperature at which vapor pressure equals atmospheric pressure. A high vapor pressure and/or a low boiling point will favor evaporation of the liquid phase into a vapor phase. As can be seen from Table 9-1, desflurane has the highest vapor pressure and lowest boiling point. In fact, at high altitudes, where boiling points are not as high, desflurane may boil at normal room temperature of 20°C.

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Table 9-1. Vapor pressures and boiling points for the potent volatile anesthetic agents.

Volatile agent

Vapor pressure at 20°C (mm Hg)

Boiling point (°C)

Desflurane

669

22.8

Enflurane

172

56.5

Halothane

243

50.2

Isoflurane

238

48.5

Sevoflurane

157

58.5

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Ref: Butterworth JF IV, Mackey DC, Wasnick JD. Morgan and Mikhail's Clinical Anesthesiology. 5th ed. New York, NY: McGraw Hill; 2013.

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