I know that there is a liaison between the final "d" in "quand" and a following vowel, but is there a difference for each word in isolation?
These words don't mean the same thing at all. In pronunciation, quand [kɑ̃] et con [kɔ̃]
Some examples :
- J'ai 20 ans = I am 20. ("Ans" has the same pronunciation of an in quand)
- Il faut qu'on aille chez Pierre = We need to go at Pierre's home. ("qu'on" has the same pronunciation as con)
But please, don't use "con", I don't think you will have to in a translation. It is a vulgar word.
Yes, quand and con (also qu'on) are pronounced with distincts (at least for native ears) nasalisations, \ɑ̃\ for the former and \ɔ̃\ for the latter.
The "an" in "quand" is nasal and open. The "on" in "con" is more palatal and close.
I would say that the pronouciation is not that different, but if you have a good level in french you will distinguish between the two just from the context, for example: "C'est quand on va y aller à la piscine?" and " Mais t'es con !"
You should try using a phonetic dictionnary to hear the differences, it's important to be able to make the difference. http://www.larousse.fr/dictionnaires/francais-anglais/on/553284 (number 4, qu'on) and http://www.larousse.fr/dictionnaires/francais-anglais/quand/64737