LINK TO SITE
This free site offers an extensive video collection of science-related materials. It includes a personalized learning dashboard, more than 100,000 exercise problems, and more than 4,000 micro-lectures via video tutorials stored on YouTube. Science subjects include healthcare, medicine, physics, general chemistry, biology, astronomy, cosmology, and organic chemistry.
According to Wikipedia, Khan Academy is a nonprofit educational Website whose mission is to provide "a free, world-class education for anyone, anywhere."
An app is available for iOS.