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Name: Meryl
Status: student
Grade: 9-12
Location: NJ
Country: USA
Date: Fall 2009



Question:
What is actually happening to the bonds of a metal or glass when expanding during heating? Why do some metals expand more than others?


Replies:
Meryl,

Although it is very tempting to think of a chemical bond as an actual physical object, we have to be very careful not to do so. A chemical bond is really just how we conceptualize, how we imagine, atoms are connected to each other in a molecule or in a metal.

If, instead of imagining a chemical bond as a physical object, we imagine the chemical bond as an electrostatic force, much as the way two opposite poles of a magnets tend to be attracted to each other, then we would focus our attention to what is happening to the magnets (or atoms) rather than the bond.

When we do so, we realize that what heat does - if it is manifested as a change in temperature - is make the atoms vibrate in place at a greater speed and with a higher offset from a central point. Thus, if all the atoms tend to vibrate with a much greater offset from center, macroscopically, this will appear as though the bulk material has expanded.

So, what happens to the chemical bond in this conceptualization? Well, they are still there. When magnets vibrate, the magnetic attraction does not really change, except now the distance between the two magnets changes periodically and faster.

Greg (Roberto Gregorius)
Canisius College


This is not easy to answer that will fit all cases, so be aware there are exceptions. Visualize a solid, and to some extent liquids, as composed of masses (atoms/molecules) connected by a collection of springs (chemical bonds within an (atom/molecule) ). As the temperature increases the (atoms/molecules) rattle around, requiring more space -- they bounce against one another harder. The tighter the (atoms/molecules) are held together the less they are affected by this increase in temperature. If (atoms/molecules) are connected differently in different directions they will "rattle" in some directions more than in others. Even without even knowing the details, I think you can understand why I said above that the answer is not easy!! Taking water as a weird example -- as a solid just around 0 C. it is less dense than liquid water -- so ice floats. That is the exception, not the rule. So ice floats. Most solids are more dense than their corresponding liquids. Glasses which are more or less randomly connected are more like the liquids, that is to say less compacted, than the same substance as a crystal form of the same substance.

I realize this is a bit rambling, but your question is tough to answer satisfactorily, because there are many exceptions to any rule one might put forth. Keep those kinds of questions coming. They are a challenge!!

Vince Calder



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