When describing what type of animals to bring the Torah says “Pure and not pure,” why not say “Pure and impure”? 

The Torah is teaching us the importance of clean speech. We were given a mouth, Baruch HaShem, which we use every single day and cannot take for granted. The same mouth we use to talk to our friends and eat food is the same one we use to talk to HaShem. We must make sure that we’re constantly watching over it and keeping it clean.