You end up with 5.0 g: 2.0 g + y = 5.0 g y = 3.0 g QUESTION: A compound decomposes (breaks apart) into elements “X” and “Y”. If the decomposition of 5.0 g of the compound produces 2.0 g of element “X”, how much of element “Y” is also produced? A. 2.0 g, B. 3.0 g, C. 5.0 g, D. impossible to tell The Law of Conservation of Mass says that when matter undergoes a change, the total mass remains the same. If you start with 5.0 g. You end up with 5.0 g: 2.0 g + y = 5.0 g y = 3.0 g SCRIPT: A compound decomposes (breaks apart) into elements “X” and “Y”. If the decomposition of 5.0 g of the compound produces 2.0 g of element “X”, how much of element “Y” is also produced? A. 2.0 g, B. 3.0 g, C. 5.0 g, D. impossible to tell PAUSE CLICK. The Law of Conservation of mass says that when matter undergoes a change, the total mass remains the same. CLICK So if we start with 5 grams. We should end up with 5 grams as well. Since we end up with 2 grams of element “X”, we conclude that we also end up with 3 grams of element “Y” 2 + 3 equals 5. The correct answer is B. CONTINUED ON NEXT SLIDE

= (Change in Mass) x (speed of light)2 More general law: Law of Conservation of Mass and Energy. Mass can be converted to energy, and vice versa. : E = mc2 = (Change in Mass) x (speed of light)2 Significant change in mass occurs during nuclear reactions (when very large amount of energy is absorbed or released) This was not known at the time the Law of Conservation of Mass was formulated. For most everyday phenomena, the law of conservation of mass is applicable. Law of Conservation of Mass is also known as the Law of Conservation of Matter: “Matter is neither created nor destroyed.” You should note that the law of conservation of mass is, strictly speaking, not always valid. You could say that it’s a limiting case of a more general law ---- the Law of Conservation of Mass and Energy. Einstein’s equation, E equals m c squared, relates the amount of energy absorbed or released during a process HIGHLIGHT E to the change in the mass. HIGHLIGHT m The c in this equation is the speed of light. CLICK During nuclear reactions, large amounts of energies are involved and significant changes in mass are observed. Of course, this was not known at the time the Law of conservation of mass was formulated. For most everyday phenomena, the law of conservation of mass is quite valid. That’s because the amount of energy released or absorbed corresponds to a change in mass that is typically well beyond the sensitivity of our measuring devices. CLICK The law of conservation of mass is also known as the law of conservation of matter. Matter is neither created nor destroyed. PAUSE END RECORDING.

Video ID: 1-8-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08