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Presentation transcript:

represented by a pair of filled circles, QUESTION: Consider the pictorial representation shown below for a process at the atomic/molecular level. Which process involves a chemical change? A. 1 only, B. 2 only, C. both, D. neither Consider the pictorial representation shown below for a process at the atomic/molecular level. Which process involves a chemical change? A. 1 only, B. 2 only, C. both, D. neither PAUSE CLICK A chemical change involves the transformation of one or more substances into other substances. Let’s examine case 1. We start with a collection of identical molecules. Each molecule is represented in the picture by a pair of circles --- one filled and one unfilled. HIGHLIGHT Since the atoms that make up the molecule are not the same, what we have have a sample of a pure compound. What happens after the change occurred is shown on the right side of the arrow. We can see that the atoms have rearranged. We now have molecules represented by pairs of identical circles. That is… we have molecules represented by a pair of filled circles, HIGHLIGHT filled circle molecules and molecules made of two unfilled circles. HIGHLIGHT unfilled circle molecules This means our original pure substance… a compound ….. has been transformed into a mixture of two different substances… two different elements. Therefore, process 1 involves a chemical change. CALLOUT “chemical change” pointing to process 1 Let’s examine process 2. Again we start with a pure compound… in fact, the same compound as in process 1. HIGHLIGHT left picture But we end up with the same pure compound. HIGHLIGHT right pictgure The only thing that happened here is that the molecules moved closer together. Nothing happened to the molecules. No new substances were formed. This is what we would call a physical change. CALLOUT “physical change” pointing to process 2. CLICK PAUSE END RECORDING

Video ID: 1-29-1 © 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