I was recently reviewing a page over at Span¡shD!ct and though I had read the following (or something similar) numerous times before, I have never stopped to really think about why the "z" must undergo the change described as follows:
To maintain the /k/, /g/, and /s/ sounds in the first person singular, the [c], [g], and [z] change to [qu], [gu], and [c] respectively. All other persons and all endings are regular.
For context, the page was about the preterite tense and specifically used "cruzar" as an example, explaining that the "z" in "cruzar" changes to "c" for 1st person singular -- crucé (not cruzé).
When I think of all the Spanish words I know that contain a "z," I can't think of any examples where "z" is not pronounced with an "s" or "soft c" sound. Did the "z" have a hard sound at one point in time? Or is it possible that there is another reason for this spelling change?
Thanks in advance for any insight you may have on this topic.