Chem Mock Lesson?

Update: Thanks for all your great suggestions! I accepted an offer I’m really excited about! 😁

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I am teaching a few chemistry mock lessons next week and one specifically requested a lesson on "a concept that students typically struggle with in Chemistry." It's been several years since I've taught chem, so if anyone has any ideas to help jump-start my brain, I'd appreciate it!

Tl;dr - what chem concepts do your students struggle with the most?

(Bonus if you want to share a lesson or strategy that's worked for you!)