Why Do Substances Expand On Heating? Molecular Explanation

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Substances expand upon heating due to the increase in kinetic energy of their particles, which causes them to move further apart. The potential energy curve illustrates that as temperature rises, the average distance between particles increases due to the balance of attractive and repulsive forces. Atoms are held together by electromagnetic attraction, but strong repulsive forces from the Pauli exclusion principle come into play at short distances. This results in an asymmetric energy well that dictates the average atomic separation. Understanding these molecular interactions clarifies the process of thermal expansion.
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Why do substances expand on heating? Please give an understandable molecular account of the process of expansion.
 
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Mapes said:
Try http://books.google.com/books?id=rVVW22pnzhoC&pg=PA397".

I don't think I understand the concept. The potential curve got me thinking but still couldn't convince me about the thermal expansion. The book says that when temperature increases, the distance between the particles increases based on the PE curve. I think that was just reiterating the original question. So, I need your help to interpret the explanation in the book.
 
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Are you asking why the pair potential has the certain shape so that average atomic separation increases with increasing energy? Fundamentally, it's because atoms are electromagnetically attracted to each other, but a very strong repulsive force (arising from Pauli exclusion) acts at short distances. The sum of these two forces creates an asymmetric energy well that determines average atomic spacing.
 

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