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Offline new guy

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« on: 20/02/2006 18:29:05 »
Im trying to understand the chemistry of why some things dissolve and some things dont.

Lets start of with a simple hydrocarbon. In order for a alkane to disolve, we would need to break its van der waals forces first, then break its intramolecular forces such as its covalent bonds, and then break the hydrogen bonds that holds water together. If this is achived, then the substance will dissolve happily in water.  Is that correct


Offline daveshorts

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Re: dissolving
« Reply #1 on: 21/02/2006 10:11:55 »
You have half the story there

The interesting thing is not why do things dissolve but why some don't. If there are no intermolecular forces then everything will just mix up as everything vibrates thermally. The hotter it is the stonger this tendancy is as everything is shaken harder.

There are three interactions you need to worry about
solvent - solvent
solute - solute (the thing you are dissolving)
solvent - solute

if the solvent-solvent interactions are very strong they will tend to expel a solute - think CO2 when you open a can of coke.

If the solute-solute interactions are strong, if two molecules meet they will stick together and precipitate out.

If the solvent-solute bonds are strong the dissolved state is more stable so more wil dissolve. Sometimes loads of energy can be released from these bonds - if you add conc sulphuric acid to water you can actually boil the water.

so if you try and dissolve an alkane in water the water-water bonds are much stronger than alkane-water bonds,  so it will not dissolve.