What are the approximate proportions of oxygen and nitrogen in air by volume?

The composition of the atmosphere is a fundamental aspect of environmental science and is particularly relevant in the study of combustion processes, respiratory systems, and various engineering applications. The approximate proportions of oxygen and nitrogen in air are about 21% oxygen and 79% nitrogen by volume. This mixture is essential for supporting life on Earth; oxygen is critical for breathing in humans and many organisms, while nitrogen acts as a dilutant and plays a significant role in various chemical reactions in the atmosphere.

The ratio of approximately 21% oxygen to 79% nitrogen is consistent across most altitudes within the troposphere, the lowest layer of Earth's atmosphere where we live and where weather occurs. Understanding these proportions is crucial for power engineering, particularly when assessing combustion efficiency and emissions in energy processes.

The other options reflect incorrect proportions, as they do not align with the established scientific consensus on air composition. For instance, a 30% oxygen and 70% nitrogen mix would imply a much higher availability of oxygen than is typically found, which could lead to misconceptions in various engineering applications, such as in the design of combustion engines or respiratory systems.