In words which have the letters "sc" is the letter "c" always silent? (for example: Fascinate)
"sc" is pronounced as "s" before letters Y, I and E in the beginning or middle of a word, or at the end of a word followed by E:
- scene, descent, scythe, science, convalesce...
In most other cases it is pronounced as sk:
- scanner, scope, scratch, scream, scum...
There are exceptions where "sc" is pronounced as "sh":
- Crescendo, fascist, conscious...
No, "sc" does not always correspond to /s/.
As SovereignSun mentions, usually this pronunciation only shows up in the combinations "sce", "sci" and "scy". A somewhat common exception to this in British English is the word "sceptic", which is pronounced with /sk/. The prounciation /ʃ/ shows up in a few words where "sc" is followed by "i" or "e". Sometimes this is due to palatalization in English, and sometimes it is due to influence from Italian.
In the combinations "sca", "sco" and "scu", we usually have /sk/.
The combination "scr" is usually pronounced /skr/.
The combination "scl" is rare. It occurs at the start of some words from Greek like "sclera", "sclerosis", that are pronounced with /skl/ in educated speech (there many be other pronunciations in uneducated speech). The word "muscle" is pronounced with /sl̩/.
The combination "sch" is very variable and may be pronounced as /sk/, /ʃ/, /stʃ/, or even /s/ depending on various factors. Related ELU question: What effect do neighboring vowel-letters have on the pronunciation of the letters “sc” in a word?
No, it is not always silent. "Scar" rhymes with "car, pronounced "kar."
English orthography is a mess. Virtually any rule you care to make will have exceptions. One reason for the frequency of exceptions is that English is happy to accept words from other languages, which have their own rules of orthography.