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.
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
The "an" in "quand" is nasal and open. The "on" in "con" is more palatal and close.