Virtual Chembook is an online reference for key concepts in general chemistry, biochemistry, organic chemistry, and environmental chemistry.  There are also instructions for many labs.

From site:  Chemistry help is available here for high school or college students. Chemtutor begins with the fundamentals and gives expert help with the most difficult phases of understanding your first course in chemistry. Chemtutor is not necessarily a complete text for your course or a complete outline, but we are proud to offer some insightful help in the parts of primary chemistry that have been, from our experience, the hardest for students to grasp. Chemtutor can be a part of a course of study for independent students, a resource for chemistry teachers, a review, or a tutoring program for students taking high school or basic college chemistry.

From site:  If you’re reading this message, you’re a ZOOMer who’s itchin’ for a challenge. Well, you’ve come to the right place.

For this challenge, you’ll need to use your noggin and your own kitchen to perform experiments. Complete the challenge successfully, and you’ll be rewarded with something you can only get here. That’s right…only those who make it all the way will gain access to The Prize.

What are you waiting for? You want The Prize so badly you can smell it. So let’s get going.

NeoK12 offers a set of YouTube videos explaining and illustrating the states of matter – solid, liquid, gas – as well as plasma, sometimes referred to as the fourth state of matter.  All videos and lessons on this site have been reviewed by K-12 teachers and are safe for kids.

NeoK12 offers a set of YouTube videos explaining and illustrating radioactivity, the history of its discovery, types of radioactive particles, radioactive isotopes and radiation.  All videos and lessons on this site have been reviewed by K-12 teachers and are safe for kids.