Which statement about oxygen used in aircraft systems is true?

The statement that oxygen used in aircraft systems is 99.5 percent pure and practically water-free is accurate because aviation oxygen must meet extremely high purity standards to ensure safety and efficiency in the cabin atmosphere control system. Pure oxygen is essential for maintaining the appropriate physiological conditions for passengers and crew at high altitudes, where atmospheric pressure is significantly lower.

In aircraft, any impurities or moisture in the oxygen can pose problems such as corrosion in the systems or reduced efficiency in oxygen delivery. The high purity standard helps to prevent the risk of combustion-related issues that could arise from the presence of other gases or contaminants. Therefore, ensuring that the oxygen is both highly pure and free from water is critical for safety reasons, highlighting why this statement is true.