What property of radon makes it difficult to detect without instruments?

The property of radon that makes it difficult to detect without instruments is that it is a colorless gas. This means that radon cannot be seen, which poses a significant challenge for detection in the environment. Without visible indicators, individuals cannot rely on sight to determine its presence in homes or other buildings.

Additionally, while radon does have no odor, which also complicates detection, the defining characteristic of being colorless is what specifically makes it undetectable through sensory perception. The weight of the gas and its effects do not contribute to its detection in the same direct manner. Therefore, being a colorless gas is the most critical factor in recognizing that specialized instruments are often necessary to accurately measure radon levels.