Dissolving on a molecular level .

[Cheman]

Dissolving on a molecular level...

How do things dissolve eg in water? Thinking in terms of molecules, ions, etc, why does dissolving occur? Why do covalent molecules not dissolve in water but do in other covalents solvents, etc?

As much help as possible would be appreciated. :-)

Thanks.

 

[chem_tr]

Dissolving needs another term to be used, this is solvation. Solvation is briefly surrounding of solvent molecules around solute molucule; so a solvent "film" occurs around the molecule, in other words,a solvent-like micro-environment is formed. This helps the solvated molecule be soluble in the solvent.

In molecular nature, similar solute molecules are surrounded by similar solvent molecules; therefore, nonpolar molecules are best dissolved by nonpolar solvents. Highly hydrophobic hydrocarbon chains escape water, thus forming their own solvent films, by dissolving each other.

 

[so-crates]

Two substances A and B dissolve because of the A-B interactions. Let's take an example, an NaCl crystal in water. The H20 molecules will "collide" with an Na or Cl atom. Water is a polar molecule, meaning the O side has a partial negative charge and the H groups have a partial positive charge. Since NaCl is an very polar ionic solid, these two charges will interact with one another. Multiple water molecules will surround a Na+ ion on the oxygen side and "shield" it from the outside. Similiarly, multiple water moleules will also surround a chlorine on the hydrogen side. So in the case of the sodium you wind up with something like this:

 

[pack_rat2]

Why do covalent molecules not dissolve in water...

What you mean is non-polar, not covalent. For instance, sugar is covalent, but it dissolves in water.