This document discusses material balances in chemical reactors. It defines key terms like matter, mass, and the different states of matter. It explains the law of conservation of mass - that mass is neither created nor destroyed in chemical reactions. A material balance tracks the mass flowing into and out of a reactor. The mass in must equal the mass out. The mass out includes accumulated mass in the reactor, products, wastes, and byproducts. Two example calculations are shown to determine waste produced given feed, product, and byproduct masses.