URI components question

Hi all,
I have a question about URIs.

I was wondering if is correct thinking that an uri can be decomposed in 
components
that can be stored in unescaped form mantaining the uri semantics, so the 
possibility
to reconstruct from the components the (same or equivalent) uri they were 
composing.

Perhaps better said the question is: can we always build an URI from 
unescaped components ?

Many programming apis offer the possibility to build an uri from unescaped 
components.
For 99% of the cases, for me, it worked good. But considering the following 
URI:

http://a/b?p1=R%26D&p2=q

the unescaped query component, orignally containing 2 parameters becomes:

p1=R&D&p2=q

loosing its meaning since now we have 3 parameters.
My conclusion is that (at least) query component cannot be unescaped.
Is this right, does it apply only to query or unescaped components should 
not exist at all ?

Regards
Michele Vivoda