Why can't water or methanol be used as co-solvents for hexane or for petroleum ether?

Answer 1

This all has to do with polarity.

Polar molecules are like magnets, and like magnets they like to have other stuff like them nearby. Water and methanol are both very polar, so they mix with each other really well, as well as other polar things like salt and sugar.

Non-polar molecules are easily forced out by the polar ones, and they frankly don't like polar stuff nearby making trouble either! Alkanes of various stripes are all famously nonpolar, so while they mix with each other just fine, they float to the top of water. Many ethers are nicely polar, but 'petroleum ether' is a group of solvents that do not necessarily actually contain an ether group (go figure). Petroleum ether is a well-known non-polar solvent, so it fits in this group.

So there you go. You can't use water or methanol as a solvent because they've both just too polar. Why not try some benzene instead?

Answer 2

remember like dissolve like so when you mix a polar liquid and a nonpolar liquid it doesn't dissolve you have 2 separate layers

Answer 3

polar molicules are magnets

Answer 4

as above