What is the boiling point of a liquid?

• A. The temperature at which it solidifies
• B. The temperature at which vapor pressure equals external pressure
• C. The temperature at which a liquid starts to vaporize
• D. The temperature at which a gas condenses into a liquid

Answer: (B)

The boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid equals the external pressure surrounding the liquid. This condition allows the liquid to transition into a gaseous state throughout its entirety, not just at the surface, which is a key characteristic of boiling. When the vapor pressure exceeds the external pressure, bubbles of vapor can form within the liquid, leading to the boiling process.

In practical terms, this means that the boiling point can change depending on the atmospheric pressure; for instance, at higher elevations where the atmospheric pressure is lower, the boiling point of water is also lower than the standard 100 degrees Celsius at sea level. Understanding this concept is crucial in various applications, such as refrigeration and HVAC technology, where fluids and their states play a significant role in system performance.