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Conscience has a /ʃ/ sound in the middle of the word.
Science has a /saɪ/ sound, not a /ʃ/ sound.
What rule makes conscience have the /ʃ/ sound and what rule makes science have the /saɪ/ sound?
They are spelled very similarly.
Your help would be greatly appreciated.
The reason boils down to English stress patterns.
Science is stressed on its first syllable and the vowel/diphthong in stressed syllable is strong so it's pronounced /ˈsaɪ.əns/.
Another example is scintillating which is stressed on the first syllable so it's pronounced /ˈsɪntɪleɪtɪŋ/.
On the other hand, the monosyllabic prefix con- bears the primary word stress so when it's added to the word science, the primary stress from the word science moves to the prefix con- therefore conscience is stressed on the first syllable and is pronounced /ˈkɒnʃ(ə)ns/ not /ˈkɒn.saɪ.əns/ (the vowel in unstressed syllable often reduces).
The /ʃ/ sound is due to Yod-coalescence.
Other examples are:
