Expert: Daniel Mazur - 4/16/2008

Question
why does metal expand when heated. my projet is to heat a knitting needle with a candle. Why does the knitting needle move when heated? Because it expands why do the molecules get denser?

Answer
Hello Rachel,
yes, metals expand, when heated. You wrote nothing about your age or level, and that is important, because the answer to "why metals expand when heated" is quite involved. I am guessing you might be at a junior high-school level (please take no offense, if my estimate is wrong) so I will try to answer suitably to that level.
Well, the first thing to understand is that metals are made of atoms and the observable physical properties of metals are given by the properties of individual atoms and by the interaction forces (attraction and repulsion) between them. Specifically the "distance between atoms" (or "interatomic distance") is due to a balance between the attracting and the repelling interatomic forces. Expansion of material comes from an increase of the interatomic distances and heating is one way to bring it about.
Secondly, the thing we perceive as "temperature" of a material is certain tiny vibrations of atoms. The more the atoms vibrate (at a very high frequency), the hotter the material appears to us. Owing to these vibrations we need to start speaking about the "average interatomic distance", where the average is taken over time that spans many vibrations.
As a third step you need to acknowledge that the average interatomic distance, at which "balance between attracting and repelling forces" happens, changes with temperature. Why this is so, that is a university topic, the important thing to know now is that atoms feel much more freedom in "jumping away" from each other than in "approaching" one another during the vibrations cycles. So, when the vibrations increase with higher temperature, the atoms will not be getting much closer to each other on the "approach" part of their vibrations, but they will jump considerably farther apart on the "depart" part of their vibration cycle. The result is that the "average interatomic distance" rises with rising temperature and this is observed as thermal expansion.
Your knitting needle may move during heating for several reasons. Most likely, the heating is not the same along the length and perimeter of the needle. Some bits expand, while other bits stay colder. This leads to bending movements. Also it is possible that the needle material is not very homogeneous (like a poorly mixed alloy) and the bending is a result of "bimetallic effect". This effect means that thermal expansion of different materials is different for the same temperature differences. You have not described the "movements" of your needle in any detail, so this is as much as I can explain now.
Lastly, the metal "molecules" do not get denser with expansion. Metals have no molecules to start with, they exist as atomic crystals. Precisely this makes it sure, that metals always expand with rising temperature and shrink on cool-down. Expansion corresponds to the material getting *more dilute* (density drops) with rising temperature and contraction corresponds to the material getting *denser* (density rises) with dropping temperature.
This is it. I know it is a lot to take in on the first read, so please take your time. I tried to explain it in an accessible way, while not giving up any part of the scientific truth. You will have to judge, how successful I was in making things clearer to you.
All best with your project!
Daniel