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---
  layout: post
  title: Can coffee explode in the microwave oven
  author: Joe Schwarcz
  source: McGill Blogs
---
  HYPERLINK
"http://blogs.mcgill.ca/oss/2016/05/06/you-asked-can-coffee-explode-in-t
he-microwave-oven/"  You Asked: Can coffee explode in the microwave
oven? 

  A sensational sounding e-mail about “exploding coffee” has been
making the rounds. It describes the misadventures of an unfortunate soul
who heated up water for coffee in a microwave oven. When he picked up
the mug, it “exploded!”

Explode is probably too strong a term, but spurting and frothing is a
real possibility. This is due to a phenomenon known as superheating.
First, we have to understand what boiling is all about. At the surface
of a liquid molecules are always evaporating. If we leave a glass of
water out, it will eventually disappear.  If we heat the liquid, its
molecules move faster, become more energetic and more molecules go into
the vapour phase. As a consequence, the liquid disappears more quickly.
At the boiling point, molecules all over the liquid, not only at the
surface are energetic enough to go into the vapour phase. They do this
most readily by evaporating into airspaces that exist in the container.
All containers have imperfections where air gets entrapped when a liquid
is introduced. As these air pockets fill with vapour, they expand and
begin to rise. That is why we see streams of bubbles which originate at
the sides or the bottom of the container.

In a microwave oven, the container is not heated, only the water.  So
the container actually cools the liquid in contact with it, meaning that
the liquid in the center is always hotter, sometimes by as much as 10
degrees C. But the liquid in the center cannot boil, because there are
no air bubbles for it to evaporate into.  By the time the liquid near
the edge of the container reaches the boiling point, the liquid in the
middle is considerably hotter; it is superheated.

The addition of sugar or a tea bag now can spur vigorous boiling. This
is because the surface imperfections introduce trapped air bubbles into
which the superheated liquid vaporizes. Sometimes just picking up the
container can have an explosive effect as the superheated liquid comes
into contact with air bubbles on the periphery. Accidents can be
prevented by putting a plastic spoon into the mug or glass while it is
heating in the microwave. In this case the scare-mongering note about
“exploding coffee” may actually has some basis in fact.

Joe Schwarcz PhD – May 6th/2016