Many people would say the C in words like "scent" and "science" is silent, but when you look into the history of English spelling, you find multiple reasons "sc" can be pronounced different ways.
A listener named Edwin called in with this question:
At the beginning of the words like "scent" and “science," which letter is actually the silent letter? I want to say it's a C, but I know in “science” the C at the end of that also sounds like S.
The short answer is that in words like "science" and "scent," SC is pronounced like an /s/ phoneme, or a unit of sound. Some phonemes come are represented by single letters, like the S-sound in “sat,” and others are represented by combinations of letters like the SH-sound in “shin.”
The long answer is that SC-words have a tricky history in English. If they’re old enough to have been around in Old English, they were probably pronounced like our modern SH-words: "shin" was spelled S-C-I-N, and a "sheath" for your sword would be spelled S-C-E-A-T-H. Every now and then, some of them kept their C’s but also sound like SH-words now: “omnscience” and “prescient,” or sometimes they sound like CH: “conscience” and “conscious.” A few added an H after the SC and made it just sound like SK: “schizophrenia” and “schism.” And a very few words make it sound just like S: “science,” “rescind,” “descent,” and the Massachusetts coastal town called “Scituate.”
Every now and then, you get a group of words where you can see quite a bit of variation: "fish" and "pisces" both came from a root related to Latin “piscis” (which is spelled with an SC in the middle) "Pescatarian" comes from the same root. That’s the word for someone who eats fish but not other meat. And "porpoise" which literally means “pig fish”comes from “porcus,” the origin of "pork," plus “piscis,” the origin of "fish."
If the SC word is new enough and wasn’t around when Old English was spoken, then the C is there for a very different reason. After Old English had turned into Middle English, we got the word "scent" but it was spelled S-E-N-T as in the related word “sense" (as in “Sense and Sensibility”). The C was added hundreds of years later, after Middle English had turned into Modern English, probably because it was influenced by words like "descent" and “ascend,” which both have C’s in the middle.
So yes, the C of SC is what we usually call a “silent letter” in the case of words like "science" and "scent,” but there’s more to the story once you look into the history of English words. A better answer is to say that the SC in “science“ makes an S sound, like the phoneme /s/. But as you can see, this letter combination can vary depending on the history of each word — as do many letter combinations found in the English language.
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