What is the primary characteristic of oxygen as a medical gas?

The primary characteristic of oxygen as a medical gas is its density of 1.429 g/L. This density is significant because it helps medical professionals understand how oxygen behaves in different environments, influences how it is stored, and affects its flow through various medical devices. Understanding the density allows for accurate calculations during the administration of oxygen, ensuring that patients receive the correct dosage and enabling effective delivery via equipment like nebulizers or oxygen concentrators.

In contrast, while oxygen does play a role in combustion (noted in the first choice), it is not itself flammable. The third choice, suggesting that oxygen must always be mixed with nitrous oxide, is inaccurate; oxygen can be administered on its own depending on the clinical situation. The last option, regarding color coding, does not apply as oxygen cylinders are typically color-coded green but not necessarily blue. Thus, emphasizing the density of oxygen underlines its importance in medical use and supports its identification as a primary characteristic.