"ch" is pronounced as /x/ or /ç/ depending on the vowel in front of it, and "s" is pronounced /z/ before a vowel, how come then that the sequence "chs" in the words wachsen and Wechsel is pronounced as "ks"?
"ch" has been developed out of the greek chi. This character has been pronounced in different ways throughout it's development. Even in German, in some words "ch" is pronounced as /k/ (the set of these words depends on the region). So the combination "chs" is pronounced as /ks/. This is a sensible way, as "ch" might be pronounced /k/ and /xs/ or /çs/ are difficult sound combinations for Germans.