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Chemicool 
A great website where you can search for  for terms and definitions as well as content.
Quizlet
You need to sign up and create a Quizlet account (free). Quizlet has flashcards, on-line quizzes, and other resources created by teachers. You should explore the site yourself because there are resources for all subjects. I have put links to some chemistry vocabulary cards and practice tests below.
​Chemistry for Kids
​Although this website is not for high school chemistry, it has clear explanations of some core concepts.​​