Ask A Scientist© Environmental Earth Science Archive

name         Sherly S.
status       educator
age          30s

Question -   Why do glass tumblers break on pouring hot water
suddenly?
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When hot water is poured into a tumbler there is an uneven increase in the
thermal expansion of the walls of the vessel. If the expansion is uneven
enough and the strength of the glass is not strong enough, the tumbler
breaks.

Vince Calder
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This is a simple problem that is not simple to explain.   There are two things
to keep in mind.  1) Materials like glass are brittle and cannot bend very
far without breaking.  2) Materials like glass expand when they get warmer.


Simply stated, when the hot water is poured into the tumbler, some parts of
the glass warm up quickly (like the sides which are thin.)  These parts
expand.  However, they are still attached to the thicker parts of the
glass (like the bottom), which have not expanded.  This difference in
expansion sets up strong forces in the glass which can break it.


If you put a tumbler into an oven and warm it up slowly, you do not get this
problem.  As the glass warms up, it gets large in all directions.  No
stresses.


Thermal stresses are a problem not only for glass, but for thick pieces of
metal, masonry and so on.  Almost anything that is fairly stiff and brittle
can have this problem.  Large metal parts sometimes require days to cool
down or they will crack.


You may wonder, how can a little bit of heat crack a large, strong metal  or
glass object?  It is because thermal expansion in solids is a very strong
force.


If you put a solid piece of glass in boiling water, it will expand about
0.02%.  Maybe thousandths of an inch.  That is not a lot.  But if you tried
to prevent it from expanding by putting a large clamp on it, you would need
to squeeze it with a force of 2000 pounds per square inch to completely
eliminate the expansion.  The piece of glass does not expand far, but if you
try to prevent it from expanding, it will push VERY hard.


Clearly, if part of the tumbler is warm and part is cold, thousands of
pounds of force can occur, which will break the glass.

Bob Erck
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When glass gets hotter, it expands.
But glass is also a very poor conductor of heat.
If you pour hot water into a glass vessel, the interior surface will get hot 
instantly,
but the bulk and exterior surface will temporarily remain the same temperature.

The expanded glass inside pushes outwards on the outer layers of glass, forcing 
them to be stretched.
Glass has very little tolerance for being stretched.
Any existing microscopic crack on a stretched surface will grow all the way through 
the thickness of the glass, abruptly.

"Heat resistant" glasses such as Pyrex are tolerant partly because they are treated 
to have better surfaces,
but mostly because the glass has a  chemical composition with substantially lower 
thermal expansion coefficient.

Jim Swenson
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