Superheated Water

March 10, 2004 (Updated December 20, 2007)
Lou Ann Jopp
University of Minnesota Extension Regional Extension Educator, Food Science
Phone: (320) 203-6058 or (888) 241-4591
Email: joppx001@umn.edu

Superheated water is extremely dangerous...people have been severely injured by such water

There have been reports of serious skin burns and scalding injuries around people's hands and face as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Overheating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil. Superheating occurs if water is heated in a container that does not assist the formation of bubbles, which is a visual sign of boiling. Glass containers are the most likely to superheat water because their surfaces have few or no defects. The presence of slight defects, dirt, or other impurities usually help the water boil because bubbles will form on these imperfections. Adding materials such as instant coffee or sugar before heating greatly reduces the possibility of super heating.

Water does not always boil when it is heated above its normal boiling temperature of 100°C or 212°F.

Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave the surface faster than they return, the quantity of liquid water gradually diminishes. That is ordinary evaporation. However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but also from within. If no bubbles are leaving the surface of the water as it is being heated to the boiling point, or above, steam bubbles may be forming inside the hot water. Water molecules then can evaporate into that steam bubble and make it grow larger and larger. When the water is disturbed in some way, it will boil violently. This may happen by inserting a fork or spoon into the water or striking the bottom of the container - and an explosion follows. When water is sufficiently superheated, all that is needed is just a single "seed" bubble to start an explosion and empty the container completely. This situation becomes even worse if the top surface of the water is "sealed" by a thin layer of oil or fat so that normal evaporation cannot occur.

Mild superheating happens fairly often and we rarely think much about it as we sponge up the spilled liquid inside the microwave oven. Severe superheating is less common but is a very dangerous phenomenon.

What Can Consumers Do to Avoid Super-Heated Water?

First: Follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time.

• Do not use excessive amounts of time when heating water or liquids in the microwave oven.


• Determine the best time setting to heat the water just to the desired temperature and use that time setting regularly.

If you really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. When you heat water on the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist steam bubble formation so that boiling occurs soon after the boiling temperature is reached. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave oven may mean that it becomes dangerously superheated. Just a reminder that boiling water is a hazard for children even without superheating.

Second: Handle liquids that have been heated in a microwave oven with respect.

• Do not remove a liquid the instant the oven stops. If the water was bubbling spasmodically or not at all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no indications of superheating, it takes no real effort to be careful. If you cooked the water or any other liquid long enough for it to reach boiling temperature, let it rest for a minute per cup before removing it from the microwave.


• Never put your face or body over the container and keep the container at a safe distance when you add things to it for the first time: powdered coffee, sugar, a teabag, or a spoon. The spontaneous bubbling that occurs when you add something to microwave-heated water is the result of such mild superheating. It is far better to have the liquid boil violently while it is inside the microwave oven than when it is outside on your counter and can splatter all over you.

Finally: This is not meant to scare you away from using your microwave oven or from heating water in it. It is intended to show you that there is a potential hazard that you can avoid. Microwave ovens are wonderful devices as long as you use them properly.

“Using them properly” means not heating liquids too long in smooth-walled containers.

Sources
“Microwave Oven Radiation”, FDA
“How Things Work”, Louis Bloomfield, Professor of Physics, University of Virginia