Here is a worksheet on differentiating the use of the adjectives impassible and impassable in prose. Actually, this isn’t a problem, I expect, most primary and secondary students will encounter. In the event they do, however, let me summarize this full-page document, with a three-sentence reading and ten modified cloze exercises: impassible is a non-standard version of impassive, which means, variously, “giving no sign of feeling or emotion,” “unsusceptible to or destitute of emotion,” and “unsusceptible to physical feeling.”
Impassable, on the other hand, simply means “incapable of being passed, traveled, crossed, or surmounted.” If nothing else, this is a simple usage exercise capable, I think, of helping students understand why good usage makes good writing, and also meets the Common Core Standard: “(L.11-12.1b)-Resolve issues of complex or contested usage, consulting references, (e.g., Merriam-Webster’s Dictionary of English Usage, Garner’s Modern American Usage) as needed.” Professor Brians’ book (which, incidentally, he allows access to at no cost at the Washington State University website) works well for this task or practice, I think, particularly where emergent or struggling readers are concerned.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.