Boil over occurs when which condition is met?

• A. Water at the bottom of the tank boils and expands rapidly
• B. Water at the top of the tank boils and expands rapidly
• C. Steam forms without liquid water
• D. The tank is completely full with no boiling

Answer: (A)

Boil over happens when the heat at the bottom of the tank causes the water there to boil rapidly. The rapid formation of vapor bubbles expands the liquid and drives it upward, so foam and liquid are pushed to the surface and out of the tank. If only the top layer were boiling, there wouldn’t be the same vigorous upward push to overflow. A situation where steam forms without liquid water describes a dry boil, which is a different failure mode and doesn’t explain overflow. And a tank that’s completely full with no boiling obviously won’t overflow.