Select the one word that best substitutes the following description: a substance that easily evaporates at normal temperatures.

Introduction / Context:
This one word substitution question checks understanding of scientific and descriptive vocabulary used to talk about physical properties of substances. The phrase given in the stem describes how a substance behaves at normal temperatures, specifically that it evaporates easily. This type of vocabulary appears in chemistry, environmental science, and general science passages, so knowing the exact term helps in both theory and reading comprehension.

Given Data / Assumptions:
The clue in the stem focuses on evaporation behaviour.

The expression is a substance that easily evaporates at normal temperatures.
Normal temperatures means usual room or atmospheric conditions, not extreme heat.
The correct word should be a standard scientific or semi technical term.
We assume the question is about physical properties, not moral or emotional qualities.

Concept / Approach:
In chemistry and physics, substances that readily change from liquid to gas at ordinary temperatures are called volatile substances. The word volatile can also be used metaphorically for situations that are unstable or likely to change suddenly, but its original sense is about easy evaporation. Our approach is to match each option with its core meaning and see which one naturally describes a physical property related to evaporation. Words like steadfast and enduring describe stability or lasting quality, definite is about clarity, and dense is about thickness or compactness, none of which relate to evaporation.

Step-by-Step Solution:

Verification / Alternative check:
We can check by using the word in typical scientific phrases: volatile organic compounds, volatile liquid, volatile solvent. In each of these, the idea is that the substance evaporates quickly at ordinary temperatures. If we substitute any other option, the phrase becomes meaningless, for example steadfast liquid or enduring solvent, which clearly do not describe evaporation. Standard definitions of volatile in a scientific context also state that it means easily evaporated at normal temperatures, which matches the stem exactly.

Why Other Options Are Wrong:
Steadfast means firm, loyal, or determined, usually describing a person character, not a physical property of matter. Enduring means lasting for a long time, again about duration rather than evaporation. Definite means clear, precise, or exact, which has nothing to do with state changes. Dense describes something that is closely compacted in substance or has high mass per unit volume, not easy evaporation. None of these options communicates the idea of quick evaporation, so they are not suitable substitutes.

Common Pitfalls:
A common mistake is to think of volatile only in its emotional sense, as in a volatile situation, and overlook its original scientific meaning. Another pitfall is to guess based on sound or unfamiliarity, choosing a word that looks technical without checking whether its meaning matches the description. In competitive exams, it is important to connect words like volatile to their domain specific usage, especially in science related passages.

Final Answer:
The correct one word substitute for a substance that easily evaporates at normal temperatures is Volatile.