DM misdirection for meta gaming..?

is it wrong/is there a better way to go about, basically lying to my players about the type of damage they take in certain situations as long as i know what their resistances and such are? for instance, if i have a illusory cave thing and theres a fake pool of lava and they fall in and they would take psychic dmg but i say its fire damage so they dont realize its an illusion unless they actually have that thought and try to discern if its real?