Sometimes we use the soft sound, and sometimes the hard – but why? Is there any rule?
As I've had occasion to say before,
English spelling does not represent English pronunciation.
Consequently the letter C can represent
- the voiceless alveolar sibilant /s/, as in pace /pes/
- the voiceless velar stop /k/, as in caucus /'kɔkəs/
- (as part of CH) the voiceless palatal affricative /tʃ/, as in church /tʃərtʃ/
as well as other sounds, depending on where their spelling has been borrowed from.
There are etymological rules for pronunciation,
these rules require one to be able to recognize English words borrowed from Latin and Greek, and to distinguish them from English words that come directly from Germanic words. It also requires one to be able to rewind past sound changes and reinsert consonants and vowels that have disappeared in speech. This amounts to studying the history of the language, which is highly recommended and highly rewarding, but is very complex and takes many years to master.
So most people don't ever learn them, and get frustrated at spelling instead.
Once you have, however, you'll see that (if you avoid "CH"), "C" gets pronounced like /s/ before I and E, and like "K" before other vowels. "SC" is /sk/ unless it's a borrowed Italian word with "SCI", which is pronounced like she.
Yes, certainly there is a rule: the rule is to do so whenever the Dictionary tells you to, of course!
Beyond that. . . .
Once you consider coelacanth and czar, chemist and nonchalant, church and arcing, you will understand why the Poet says that “my advice is to give up”. — Dr. Gerard Nolst Trenité
Phonetically, the letter c usually takes a hard sound when it comes before back vowels (a, u and o): car, coal, collect, calendar, curtain; and soft when it comes before the front vowels (e, and i); ceiling, cinder, circuit, certain.
Its not fair to name a few exceptions and say there are no rules. There are rules, and a few exceptions.