HAZMAT Technician Practice Exam

At what point does boiling occur for a liquid?

1. At absolute pressure

2. When the liquid's temperature exceeds 100°C

3. When pressure equals temperature

4. When vapor pressure equals surrounding pressure

The correct answer is: When vapor pressure equals surrounding pressure

Boiling occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. At this point, the liquid can transition to the gas phase throughout the entire body of the liquid rather than just at the surface. As a liquid is heated, its vapor pressure increases. When this vapor pressure matches the external pressure acting on the liquid, bubbles of vapor can form within the liquid. This is the boiling point for that specific pressure. For instance, under standard atmospheric pressure (1 atm), water boils at 100°C. However, in higher altitudes where the atmospheric pressure is lower, water will boil at a temperature lower than 100°C. The other choices do not accurately describe when boiling occurs. The concept of absolute pressure does not directly pertain to the boiling process, as it is the relationship between vapor pressure and surrounding pressure that determines boiling. While high temperatures can lead to boiling, exceeding 100°C alone does not ensure that boiling will happen unless the vapor pressure has reached the level of the surrounding pressure. Thus, understanding the relationship between vapor pressure and surrounding pressure is crucial in determining the boiling point.