Why are conscience and science pronounced differently?

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.

Solution 1:

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:

• Extension -> /ɛkˈstɛnʃ(ə)n/.
• Social -> /ˈsəʊʃ(ə)l/.