The etymology of 'chemical'

'Chemical' as a word has been around since the sixteenth century, but did not pertain to chemistry until the seventeenth. It comes from either French's 'chimique' or modern Latin 'chymicus', in the case of 'alchemic' according to the OED. The spelling of 'che-' came into use in the mid-seventeenth century, and is based on the Greek 'khemia'.

Incidentally, 'alchemical' and 'alchemist' both came into use in the sixteenth century as well. Alchemy itself was the chemistry of the Middle Ages, and the word 'alchemy' pre-dates 'alchemical' by a two or so centuries, coming into use in the fourteenth century. the Latin 'alchemista' was translated to 'alchemister' between the fourteenth and sixteenth centuries, but this is now obsolete.