Many people believe that water can explode out of a container after being microwaved.
Is this true?
Superheating and microwave ovens
There have been many reports of injury to people using microwave ovens to heat water to make hot drinks. Water heated in a microwave oven may be superheated and when objects (e.g. a spoon) or granulated materials (e.g. instant coffee) are put into it, the water may boil very vigorously or even appear to explode out of the container. The vigorously ejected boiling water can cause serious burns. Sometimes even the act of taking the container out of the oven and or putting it on the bench can cause the boiling.
Physclips, School of Physics, University of New South Wales, Sydney Australia
Referring to FDA review on reports of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven.
This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.
This phenomenon of overheating in microwave ovens is known to occur to a greater degree in microwave ovens than in saucepans or kettles by the following reason
In a microwave oven, the water is usually hotter than the container, whereas parts of the kettle or saucepan are usually hotter than the water. Further, the surfaces of some containers used in microwave ovens may be very smooth, almost at a molecular scale, whereas this is not true for kettles or saucepans.
Microwave ovens heat the water directly: the microwaves pass through the container and the water, and the water itself absorbs energy from them. The container absorbs little energy directly. In a kettle or saucepan, the container itself (saucepan) or a heating element (some kettles) is hotter than the water. The hottest points cause a small amount of local superheating, boiling is initiated here, and this then stirs the water.
Mythbusters have demonstrated and confirmed this phenomenon in their 2003 season episode 4 show 'Microwave Madness'.
Superheated liquids are used to trace the track of energetic particles in so-called "bubble chambers" and similarly container surface imperfections or any added particulates can serve as nucleation sites for spontaneous boiling of superheated water. Since microwave heating introduces no convection in the liquid it readily causes superheating. Any subsequent disturbance, such as picking up the cup, placing a spoon, tea bag or particulates into the superheated liquid causes instant bubble nucleation and spontaneous "bumping" of the liquid. The water vapor (steam) has a >1200 fold larger volume than the liquid water and easily displaces the superheated liquid water out of the container.