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Some time ago I read a warning that microwaved water can "explode" under certain circumstances. There was one case as an example where someone heated water in the microwave, took it out and then it "exploded" causing him to be burned.

Can something like this really happen, and if yes, under which exact circumstances?

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    Just to add the opposite can happen too. If you chill water in the freezer (it happened to me with the long plastic freeze-at-home ice lollies) and then subject it to a sudden shock, instant freezing to ice can occur. Alternatively there are commercial reusable hand-heaters which work on this principle. – Nick Jul 26 '12 at 16:39
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    I've had this happen with coffee when inserting a spoon or sugarcubes after microwaving. – Pieter B Aug 1 '13 at 8:28
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    Just for your knowledge: A microwaved apple or egg would. – Samuel Liew Aug 1 '13 at 9:42
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Yes, this can happen with water, and in fact other liquids as well.

According to the UNSW School of Physics, this is called superheating, and it occurs when a liquid is heated to a temperature slightly above its boiling point without the liquid starting to boil.

The result is an unstable liquid that can boil (steam) violently when a foreign object (i.e. instant coffee, tongue, spoon, etc.) contacts the liquid (I'll explain how/why later in this answer). This violent boiling is the "explosion" referred to in your question, and it can be quite dangerous. (Note that this is not, by definition, an actual explosion, but colloquially, it may be referred to as such. See the comment by dm.skt below.)

Superheating occurs when a liquid is heated in a smooth container*, such as a brand-new mug or bowl, for a long time in a microwave. The smooth container prevents steam bubbles from forming on the surface of the container as the liquid is heated. The formation of steam bubbles is a crucial step of boiling; without steam bubbles, the liquid cannot boil, and this is why superheating is possible.

Introducing a non-smooth object into superheated water allows the water to form steam bubbles on the irregular surface of the object, leading to a potentially rapid boil of the water.

*Note: As a container is used and/or washed, microscopic abrasions and scratches will accumulate on the interior surface of the container, whether from stirring with a spoon or scrubbing. Thus, the likelihood of superheating occurring with any given container decreases each time the container is used and superficially scratched, because steam bubbles will form more easily the more scratches are present.

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    Are you sure the purity of liquid is unimportant and that container smoothness is the only factor? In other words, don't impurities in the liquid also cause it to boil more readily? – JYelton Mar 2 '11 at 16:51
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    @JYeltion The important factor is the presence of nucleation sites, those can be either particles in solution or present at the surface of the container. – Mad Scientist Mar 2 '11 at 16:54
  • Interesting, thanks for the clarification. – JYelton Mar 2 '11 at 22:49
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    Seeing is believing? youtube.com/watch?v=1_OXM4mr_i0 – endolith Mar 4 '11 at 18:59
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    It's absolutely true that this can happen, although "explode" is a kind of inaccurate terminology. An explosion is a rapid expansion with a massive release of energy. Here, no energy is released (the water does not combust), it is merely a liquid that assumes gaseous form very, very suddenly due to external energy having being applied. – dm.skt Mar 11 '11 at 22:35
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Mythbusters showed that this is most easily achieved by using distilled water (with no impurities). Distilled water will fail to boil and therefore be more likely to cause an explosion. Tap water, on the other hand, will generally boil.

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    It is not necessary to use distilled water, I often microwaved milk and tap water. For a mug of water/milk, 2 minutes are sufficient (may differs depending on your microwave) to cause the liquid to explode up when you pick them up. If you microwave for too long though, the liquid may explode inside the microwave, and it will be messy to clean up. – Lie Ryan Mar 13 '11 at 15:41
  • For best result though, try to get them out of the microwave every 30 seconds and stir them (take care, use a long spoon to protect the hand from those hot liquids). Do this a few times, and the bubbling will get progressively worse (I did this once since I wanted a very hot milk, and the bubbling ended spilling half the mug). – Lie Ryan Mar 13 '11 at 15:52
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Apparently, yes.

The page quotes New Scientist, reproduced here for convenience:

A portion of the water in the cup is becoming superheated — the liquid temperature is actually slightly above the boiling point, where it would normally form a gas. In this case, the boiling is hindered by a lack of nucleation sites needed to form the bubbles . . . I imagine that by keeping the cup still and microwaving for a long time, one could blow the entire contents of the cup into the interior of the microwave as soon as you introduced any nucleation sites. It is this sometimes explosive rate of steam production that means you should take great care when using a microwave oven.

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protected by Community Jan 12 '14 at 22:55

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