Understanding the normal composition of external air is fundamental in environmental science and chemistry. Air is a mixture of various gases, and their proportions are relatively stable, though trace elements and water vapor can vary.

Nitrogen Composition Analysis

The question asks to identify the statement that is not the correct normal composition of external air by volume. Let's examine each option presented:

• Nitrogen - 80%: The actual percentage of Nitrogen in the atmosphere is approximately 78.09%. Therefore, stating Nitrogen as 80% is not entirely accurate and is higher than its true proportion. This makes this statement the incorrect one among the choices provided.

Oxygen Composition Details

• Oxygen - 20.93%: The normal composition of Oxygen in the atmosphere is very close to 20.95%. The given value of 20.93% is extremely close to the actual value, making it a correct representation for practical purposes.

Carbon Dioxide Composition Facts

• Carbon dioxide - 0.03%: The normal percentage of Carbon dioxide in the atmosphere is about 0.04%. While 0.03% is a slightly lower figure, it is often cited as an approximate value and is very close to the actual concentration, especially considering natural fluctuations. This statement is considered reasonably accurate in the context of general knowledge.

Other Gases Composition Overview

• Other Gases - 0.94%: This category typically includes Argon (approximately 0.93%) and other trace gases like Neon, Helium, Methane, Krypton, and Hydrogen, which make up very small percentages. When combined, Argon and these trace gases sum up to roughly 0.94% or slightly more. This statement is largely accurate, primarily accounting for Argon.

Based on the analysis, the statement "Nitrogen - 80 %" is the one that does not accurately represent the normal composition of external air by volume, as nitrogen's actual percentage is closer to 78.09%.