Why did my mug of plain water explode in the microwave?

Question

I know foods can explode in the microwave, especially if there is fat or oil in the food, so I always cover food when I cook it. Yesterday I was heating a mug of plain water, and after about 90 seconds of heating it I heard a terrific BLAM. I turned off the microwave, waited 10 seconds, and then when I looked inside, most of the water was all over the inside of the microwave with only about an inch left in the mug. What's the scientific explanation behind the explosion of plain water?

Answer 1

Super-heating is correct, but you'd also like to know why it's more likely to happen with a microwave vs. hot plate. In addition to having to be above the boiling point, in order for an actual bubble to form, there needs to be a small particle in the water, or imperfection (such as a scratch) on the surface of the cup for the bubble to form around. This is called "nucleation". If you heat water on a hot plate, the water gets hot enough to boil first on the bottom surface, where there are usually lots of microscopic imperfections, and the bubbles form right away. In a microwave, though, the waves ignore the cup and heat the water directly. If the water has been filtered, the temperature can climb significantly above the boiling point away from the surface, but not have any way to boil. Since the cup itself if not being heated, it acts as a heat sink, absorbing heat from the water near the surface. That means the water at the surface can actually be cooler (below the boiling point) than deeper within, where super-heating occurs. The slightest jostle or thermal convection, though, can transport superheated water along the surface, or excite any incipient nucleation points that do exist in the bulk water, causing bubbles to form. Since the water is so hot there, the bubble forms explosively.

Answer 2

You did not say what kind of mug you are using. Ceramic, porcelain, clay pottery, glass, etc. Always check to see if there is a label or mark on the mug that says microwave safe. Some imported ceramics and glass enamels can also have trace metals and oxides as a color enhancer. These metals may not explode the cup, but will cause little shimmery sparks within seconds of the unit being on. It may also get so hot that it can damage your food and crack the cup surfaces. Any glass can crack in extreme heat. Microwaving is instant high heat and a cold cup can definitely shatter! It would probably be best to use a warmed, plain glass cup. Use a Pyrex container or measuring cup if in doubt. Some dollar store type cheap cups may also have traces of lead in the decorative paints and trims. If you see sparkles coming out of or around the cup, stop the process quickly. The metallic fumes will make your food smell nasty. It will also ruin the glaze or coatings as well as the microwave.

Answer 3

without getting too technical (because if you can't explain something to a 6 year old, then you don't understand it yourself),
the water was "boiling" but none of the "steam" had broken
the surface of the water so it looked calm instead of bubbling as it would on a stove top. as you continued to heat the water, a point was reached where all the steam rapidly broke the surface and condensed outside the cup. because it happened quickly and violently it may have carried liquid water with it.

this is why you should be careful when removing a cup of water from a microwave. it could be in such a state and the motion of you handling the cup or inserting a spoon into the water could trigger the "explosion" giving you a nice burn.

Answer 4

It is caused by a phenomenon known as
super heating. It can occur anytime water is heated
and will particularly occur if the vessel that the
water is heated in is new. What happens is that the
water heats faster than the vapor bubbles can form. If
the cup is very new then it is unlikely to have small
surface scratches inside it that provide a place for
the bubbles to form. As the bubbles cannot form and
release some of the heat that has built up, the liquid
does not boil, and the liquid continues to heat
up well past its boiling point. What then usually
happens is that the liquid is bumped or jarred, which
is just enough of a shock to cause the bubbles to
rapidly form and expel the hot liquid.

Answer 5

Was there any type of writing on the mug? Something metalic in silver or any glitter or something like that. If so that was definetly the culprit. Microwaves can not penetrate aluminum. The heat gets reflected and it bounces around the microwave and creates sparks which creat exposions in the microwave.

Answer 6

Microwaves can superheat water, if an object enters the water it can cause the water to boil over very rapidly. Perhaps a drop of condensation from the ceiling of the microwave fell back into the mug??

Answer 7

Microwaving water can cause it to superheat-- it gets hotter than boiling without actually boiling. When something starts the boiling action (you bump it, something falls into it, whatever), it all boils up at once. That's the explosion you heard, and why the water splattered all over the microwave.

Answer 8

It was probably the material of the mug- if it was made of some kind of clay or something it could have even a slight bit of moisture inside and when this internal water was heated it expanded and caused the structure of the mug to fail.

Answer 9

The water wasn't plain, there must have been some sort of substance in the water or inside the cup, soap residue, sugar, table salt...something must have mixed with the water for that to happen.

Answer 10

H20 IED. Or maybe just a relatively large air pocket in the ceramic mug.