Why does rust not dissociate in water?

[wasteofo2]

From my rhudimentary Chemistry education, I've been led to believe that any bond between a metal and a non-metal is an ionic bond, and that ionic bonds will dissociate in water. Rust is certainly a combination of a metal and a non-metal, and unless I'm horribly mistaken, it is also ionic. And yet, rust does not dissociate in water. Why is that?

Does it have something to do with the unusual way in which rust (Iron (III) oxide, if I'm correct) forms?

Thanks,
Jacob

 

[Borek]

There is no such thing as 100% ionic bond, there is no such thing as 100% covealent bond. Every bond between metal and non-metal is partially covalent - sometimes less, sometimes more. So your first assumption is wrong.

Your second assumption - that every ionic substance should dissolve in water - is also wrong. It happens only if dissolution is favorable - there are at least two processes responsible, one is destruction of the bonds (mostly ionic), second is solvation of ions. Sometimes effects of solvation prevail, sometimes bonding.

 

[redwraith94]

Water is a polar molecule, right? Well each molecule only has so much pull, or only so much force that it can apply to rip an ionic molecule appart.

Look @ it this way, assuming Fe2O3 the Fe ion is in the +3 state, whilst the O is in the -2 state, so for water to dissolve it, the individual water molecules would have to collectively have a stronger pull than both the Fe+3 ions, and the 0-2 ions. Water just doesn't have enough attraction to rip them appart from one another. It really comes down to the strength of the bond(s), the size of the molecules/atoms involved.

For instance, Al2O3 (Aluminum oxide, clear Sapphire) cannot be dissolved in water either (it has a much stonger bond than Fe2O3, btw) however, it CAN be dissolved in molten (ie no water) Na3AlF6 (Sodium Aluminum Fluoride, AKA Cryolite) the Cryolite melts @ ~1000 Degrees C, and this is how Aluminum is produced commercially, using electrolysis with the Aluminum oxide (Al2O3) dissolved in the molten cryolite, because water doesn't have enough strength to rip the bonds between the Al+3, and the O-2 appart.

 

[quetzalcoatl9]

yet another example of an ionic compound that will not dissolve in water is $$Ca(OH)_2$$

There is more of an attraction between the electronegative Ca and OH ions then there is attraction of the ions to the dipoles of water (water is highly polar, which is why some ionic substances dissolve at all, but not quite polar enough to break up everything).