HAZMAT Technician Practice Exam

What defines the boiling point of a substance?

1. When a liquid becomes solid

2. When vapor pressure equals atmospheric pressure

3. When liquid evaporates completely

4. When temperature reaches absolute zero

The correct answer is: When vapor pressure equals atmospheric pressure

The boiling point of a substance is defined as the temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure. At this point, the liquid transitions into a gas, allowing bubbles of vapor to form within the liquid and escape into the air. This relationship between vapor pressure and atmospheric pressure is crucial; as you increase the pressure, the boiling point also rises because more energy is required for the liquid molecules to overcome the pressure holding them in the liquid state. In contrast, other choices do not relate to the boiling point. The description of a liquid becoming solid refers to freezing, not boiling. Complete evaporation of a liquid does not provide a specific temperature like the boiling point does; instead, it can occur at various temperatures under the right conditions. Lastly, reaching absolute zero is theoretically a state where all molecular motion ceases, which is far removed from any boiling process and also not achievable under current physical laws.