National League for Nursing (NLN PAX) Practice Exam

The reasoning behind the selection of carbon dioxide as the correct answer stems from the properties described in the question. Carbon dioxide is more dense than air, which means it will settle at lower levels in the atmosphere compared to lighter gases. This characteristic aligns it with the described unknown gas.

Additionally, carbon dioxide does not support combustion. This is significant since many common gases do facilitate burning, but carbon dioxide, being a product of combustion, actually extinguishes flames by displacing oxygen.

The turning of limewater cloudy is a classic indicator of the presence of carbon dioxide. When carbon dioxide is bubbled through limewater (a solution of calcium hydroxide), it forms calcium carbonate, which is insoluble in water and makes the solution turn cloudy. This chemical reaction is a well-known laboratory test to confirm the presence of carbon dioxide.

In contrast, the other gases listed do not fit all the criteria. Ammonia, while more dense than air, has a strong odor and supports combustion, which does not match the characteristics provided. Methane is lighter than air and supports combustion, thus failing to meet the criteria as well. Hydrogen is highly flammable and is also less dense than air, which makes it an unsuitable option.

Therefore, the properties of