The word that completes the sentence "A substance that evaporates at room temperature is described as ________." is "volatile." A volatile substance readily evaporates at room temperature because it has a high vapor pressure and low boiling point, allowing it to change from liquid to gas easily without needing to be heated to a boiling point. Examples of volatile substances include ethanol, acetone, and ether. This property is characteristic of substances that can turn into vapor quickly under normal conditions, making "volatile" the accurate term to describe such substances.