What is a boil over?

• A. Water at the bottom of the tank boils and causes rapid expansion
• B. Water at the top of the tank boils and creates a mist
• C. The tank temperature increases without boiling water
• D. The pump stalls and pressure builds

Answer: (A)

Boil over happens when the water at the bottom of a tank reaches boiling point and rapidly converts to steam, causing a quick expansion that pushes liquid up and out of the vessel. The bottom layer boils first because it’s in direct contact with hot surfaces, and if agitation is limited or venting is insufficient, the steam and entrained water surge can overflow the tank. That’s why the best description is water at the bottom boiling and expanding rapidly, leading to overflow. Boiling at the top would produce a visible steam mist but wouldn’t drive the entire liquid level upward in the same overflow manner. A temperature rise without boiling shows heat increasing without a phase change, so no boil over. A pump stalling and pressure buildup describes a mechanical issue, not the rapid vaporization and overflow caused by bottom boil over.