Why Do Substances Expand On Heating? Molecular Explanation

[Pranav Jha]

Why do substances expand on heating? Please give an understandable molecular account of the process of expansion.

 

[Mapes]

Try http://books.google.com/books?id=rVVW22pnzhoC&pg=PA397".

 

[Pranav Jha]

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.

 

[Mapes]

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.

 

[sardar]

Substances expand on heating due to the increased kinetic energy of their molecules. At higher temperatures, the molecules have more energy and move faster, causing them to vibrate and collide with each other more frequently and with greater force.

As a result, the intermolecular forces holding the molecules together weaken, allowing them to move further apart. This increase in molecular spacing leads to an overall expansion of the substance.

Additionally, the increased energy also causes the molecules to take up more space themselves. This is because the molecules start to rotate and vibrate more vigorously, taking up more space in their immediate surroundings.

In liquids and gases, the expansion is even more noticeable as the molecules are already in constant motion, and the increased energy further accelerates their movement, causing the substance to expand in all directions.

In solids, the expansion is not as obvious, but it still occurs on a molecular level. As the molecules vibrate more, they push against each other and the bonds between them, causing the solid to expand.

In summary, substances expand on heating due to the increased molecular motion and spacing caused by the higher kinetic energy of the molecules. This molecular explanation helps us understand the process of expansion and its effects on different states of matter.