Which of the following best defines boil over in this context?

• A. Water at the bottom of the tank boils and causes rapid expansion
• B. Water at the bottom of the tank boils and causes rapid cooling
• C. The entire tank floods with water from the roof
• D. Boil over is the same as a steam explosion

Answer: (A)

Boil over happens when the bottom layer of water in a tank gets heated to the point of boiling. Water converts to steam, and steam takes up a lot more volume than liquid water. That rapid expansion creates enough pressure to push the hydrocarbon liquid upward and out of the tank, causing a violent overflow. This is driven by bottom-water boiling, not by cooling or by water flooding from above. It’s not the same as a steam explosion, which is a sudden, high‑pressure rupture inside a confined space; boil over is specifically the rapid overflow caused by bottom water turning to steam and forcing the liquid to spill over the rim.