What would happen to gases at normal atmospheric pressure percentages?

When considering the behavior of gases at normal atmospheric pressure, the correct statement is that they will mix and rise or fall based on density. Gases have the tendency to diffuse and mix together due to their random motion. This mixing is influenced by the density of the gases involved; lighter gases will rise, while denser gases will tend to sink.

Several principles of physics and chemistry govern this behavior, most notably the ideal gas law and concepts related to buoyancy. In a mixture of gases, the different densities cause variations in the vertical distribution of gases. This interaction leads to a dynamic equilibrium where gases will move until they reach a state of homogeneity, but the initial mixing behavior is significantly affected by density differences.

The other options do not accurately reflect these principles of gas behavior in an atmosphere. While gases may remain constant in a closed environment (which does not apply when considering mixing and rising or falling in the atmosphere), and can indeed disperse equally over time, these statements do not capture the nuances of density-driven movement in gas mixtures. Hence, the most comprehensive understanding comes from recognizing that gases mix and stratify based on their density, leading to rising and falling patterns in a given atmospheric pressure context.