Final answer:
Helium replaces nitrogen in heliox therapy to prevent narcotic effects and improve breathing efficiency. It's particularly useful for divers to avoid nitrogen narcosis and in medical settings to assist in respiratory conditions. Its non-reactive, low density, and high effusion rate make it ideal for various applications.
Step-by-step explanation:
In heliox therapy, helium is used to replace nitrogen in a therapeutic gas mixture. Helium is chosen because of its non-reactive nature and because it is non-narcotic at high pressures, which contrasts with nitrogen's narcotic effects under similar conditions. Professional divers and patients requiring heliox therapy benefit from the use of a helium-oxygen mixture as it avoids nitrogen narcosis, which can lead to a disoriented mental state.
Helium's low density compared to nitrogen also allows for better gas exchange in patients' lungs, which can be particularly beneficial in cases of airway obstruction or in conditions where optimizing the work of breathing is required. In diving, a mixture typically consisting of 79% helium and 21% oxygen by mole is used, which helps divers avoid the effects of the high-pressure underwater environment.
Furthermore, helium's properties make it ideal for various applications such as filling balloons, as an inert atmosphere for welding and for detecting leaks due to its rapid effusion through porous materials compared to heavier molecules like ethylene oxide.