The letter "S"
The letter "S" can represent the voiceless fricative /s/ or the voiced fricative /z/ . There's a simple rule to help you know which one it should be:
If the letter "S" is preceded AND followed by a letter representing a vowel (i,e,o,u,a), then it is pronounced /z/. In all other words, it is pronounced /s/.
For example:
/z/ as in prison , oser , désert , aiguiser
/s/ as in chanson , immersion , version , observer
That's really all there is to it! Do the exercises in the next section to make sure you've understood the rule.
Note that there are many francophones who pronounce words ending in -ism as [izm]. However, the standard pronunciation (which follows the rule given above) is [ism]. That said, both are accepted when speaking.
Be sure to pronounce the word immersion as /imɛʁsjõ/ and NOT as /imɛʁzjõ/