As you ascend, the composition of the air remains essentially the same; what changes with altitude?

As you ascend, the composition of the air remains essentially the same; what changes with altitude?

• A. Only the partial pressures of the gases change
• B. The air becomes pure oxygen
• C. The air becomes drier and cleaner
• D. The nitrogen content increases significantly

Answer: (A)

As you rise, the total air pressure falls, while the mix of gases stays essentially the same. Partial pressure is the portion of the total pressure that each gas contributes, calculated as its fraction in the air times the total pressure. Since the relative amounts of nitrogen, oxygen, and other gases remain about constant, their partial pressures all decrease in tandem with the dropping total pressure. That’s why the oxygen available to you per breath is reduced at higher altitudes, even though the air isn’t becoming pure oxygen. The other ideas don’t fit because altitude doesn’t inherently make the air pure oxygen, nor does it cause the nitrogen fraction to rise significantly. Humidity and dryness can vary, but dryness isn’t the fundamental change described by altitude; the key point is the change in partial pressures due to decreasing total pressure.

As you rise, the total air pressure falls, while the mix of gases stays essentially the same. Partial pressure is the portion of the total pressure that each gas contributes, calculated as its fraction in the air times the total pressure. Since the relative amounts of nitrogen, oxygen, and other gases remain about constant, their partial pressures all decrease in tandem with the dropping total pressure. That’s why the oxygen available to you per breath is reduced at higher altitudes, even though the air isn’t becoming pure oxygen.

The other ideas don’t fit because altitude doesn’t inherently make the air pure oxygen, nor does it cause the nitrogen fraction to rise significantly. Humidity and dryness can vary, but dryness isn’t the fundamental change described by altitude; the key point is the change in partial pressures due to decreasing total pressure.