Ask A Scientist Materials Science Archive

name         Natalie
status       student
grade        4-5
location     N/A

Question -   Why does metal bend easily when it is heated?
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 Metals, like almost all materials, soften as the temperature increases.
This happens because the atoms and molecules vibrate faster as the
temperature increases. This causes the material to expand so that the atoms
and molecules are further apart. The lower density means that they can
move around more easily.  Also, because of the increased mobility the
atoms and molecules "hop" from site to site in the solid, which increases
the flexibility of the material.

Vince Calder
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Natalie,

I want you to imagine a row of marbles, all the same size. On top of this 
row place another set of marbles such that each marble that is on the top 
row are sitting directly above a bottom row marble. Now imagine a few rows 
like this. This is the way the atoms in metals are organized. Since the 
atoms within a row can readily slide left and right without upsetting the 
organization (there will still be an identical marble atop), metals are 
said to be malleable (easily shaped by hammering) and ductile (easily 
drawn out by pulling).

Heat has the effect of allowing the atoms to move faster. Thus, while some 
metals can be bent or shaped at average room temperatures, heating these 
same metals makes the bending easier because the atoms are that much more 
mobile.

Conversely, imagine the original set of marbles, but in the gaps between 
the marbles, you were to place a smaller marble that just fits into that 
gap. This is the kind of organization that atoms of materials which are 
not quite as malleable or ductile as metals often take. Table salt is such 
a material, for example. These different sized marbles puts a restriction 
on the mobility of the set of marbles, and so as a result, a material such 
as salt are hard and brittle.

In contrast, molecules of plastic are organized like cooked spaghetti. 
Since there is practically no organization in such materials, we find that 
we can stretch and shape plastics with relative ease.

Greg (Roberto Gregorius)
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