Chemical Changes in Matter

Chemical changes/reactions in matter result in new substances being formed. In a chemical reaction, molecules can come apart and the atoms get used to make different molecules. But we can’t see atoms moving around….. so how do we know a chemical reaction has happened?

Magnesium metal in hydrochloric acid Gas is Produced Q: What physical change does this seem like? A: Vaporizing Q: How is it different from vaporizing? A: No thermal energy is added to the system to make this happen Magnesium metal in hydrochloric acid

Precipitate Forms Q: What physical change does this kind of seem like? A: Freezing Q: How is it different from freezing? A: Thermal energy is not removed from the system to make this happen. Two liquids are mixed and a solid substance is formed

Color Change… …and light and sound produced Q: Is putting green food coloring in water an example of a chemical reaction?

Changes in Temperature Q: Which part of this picture shows chemical change? A: The burning of natural gas (or anything else) The temperature change can be an increase or decrease

Changes in Properties Changes in other properties not previously mentioned How would you described the properties of bread dough vs. a loaf of baked bread?

The tricky part—some observations made during a physical change can be the same as some observations in a chemical change. What do I know about HOW the change happened? What do I know about WHAT the substances are made of?

Spawn of SPuD Revisited: Is change of state an example of physical change? Explain. Spawn of SPuD Revisited: Is ‘Chemistry in a Bag’ an example of physical change? Explain.

The chemical reaction that occurs during Chemistry in a Bag: 2NaHCO3 + CaCl2 CaCO3  + 2NaCl  + H2O + CO2 Reactants: the starting substances Products: substances made in a chemical reaction What observations can you make about the reactants compared to the products?

SPuD: After completing “Money Matters” and “Chemistry in a Flask Part 2,” what results would you expect to see when measuring the mass of the flask/water/balloon/tablet before and after mixing the Alka-Seltzer and water? a. I would expect the mass after mixing to increase. b. I would expect the mass after mixing to decrease. c. I would expect the mass after mixing to stay the same. Explain your choice.