How are volatile substances characterized?

Volatile substances are characterized by their ability to evaporate easily at room temperature. This property is due to the high vapor pressure of volatile liquids, which allows them to transition from the liquid to the gaseous state with minimal heat input. As a result, volatile substances can quickly disperse into the air, making their presence noticeable through odor even in small quantities.

The other options do not accurately describe the nature of volatile substances. For instance, substances that evaporate only at high temperatures do not fit into the volatile category, as volatility implies a capacity for evaporation at lower temperatures. Additionally, the weight of a substance in relation to air does not determine its volatility. Lastly, the requirement of chemical reactions for evaporation is not a characteristic of volatile substances; evaporation is a physical change, not a chemical one. Therefore, the defining feature of volatile substances is indeed their ability to evaporate easily at room temperature.