C is pronounced /s/ when followed by an i, e, or y. We call this a soft c.
Our language is Latin based. In early Latin there was no soft c, but in late Latin the two sounds of c developed because of how the language was being spoken, much like slang words evolve today.
There are exception words of course like:
- Cello (from Italian like many musical words)
- Celtic (the Irish language has no letter K so all C’s are pronounced hard
- Soccer (the word soc was a shortened slang word for Association football then the suffix er was added)
and a few others...
The rule “soft c before e, i, and y “ has to be learned to be able to decode many words.