Most people know that water boils at 212°F (100°C) at sea level. They also know that if the pot is open (i.e., not a pressure cooker), the bubbling, steam-water mixture will not exceed 212°F. While this observation is generally true, there is another aspect of boiling that many people are not familiar with – “superheating”. In order to form a steam bubble in a pool of liquid water, the water temperature must actually exceed 212°F in a thin film near the heated pot bottom. This “superheating” phenomenon is usually limited to a few degrees at most, and generally diminishes to 0.1°F or less when vigorous boiling begins.

This superheating is the driving force for the rapid phase-change that is called boiling. Without excess thermal energy available in the liquid molecules, their conversion to gas is a slow process – evaporation. Evaporation occurs only at the upper surface of a hot liquid when individual molecules are sufficiently energetic to break the weak intermolecular bonds they share with their neighbors and travel into the air space above.

In contrast, bubble formation involves a huge number of molecules simultaneously. Because of the excess energy of the molecules in the superheated film near the heated pot bottom, they can expand and change phase (from liquid to gas) very rapidly. Thus, the rate of vapor formation in bubbles is many times higher than the rate of vapor formation by evaporation alone. When the vigorous motion of the rising bubbles begins, the superheated liquid in the film and the balance of the 212°F liquid above it in the pot are very effectively stirred together and we measure an average temperature of 212°F essentially everywhere in the pot.

By contrast, when a container of water is heated in a microwave oven instead of through the wall of a pot, bubble formation in the film next to the pot bottom does not occur. Accordingly, dangerous amounts of energy can be deposited into the interior of the water mass creating a large body of superheated liquid water. If superheated liquid water accumulates in the vessel, it can expand explosively when a “nucleation site” (e.g., a fork or dry food particle) makes contact with the energetic liquid.

This author investigated one such injury-causing explosion involving an 8-cup container of homemade chicken soup heated in a microwave oven. Unfortunately, the water was heated about twice as long as it should have been, and it vaporized explosively when a fork was inserted, scalding the woman who was simply trying to spear a piece of chicken. Recommendation: Don’t heat water for long periods of time in a microwave oven and do check it frequently to be sure it doesn’t approach the boiling temperature.

The purpose of “Investigation Anecdotes” is to inform our readers about the intriguing field of engineering investigations. We hope you are instructed by this content, and we encourage you to contact us if you seek additional information. Copyright Martin Thermal Engineering, Inc. (2013)