CHEMICAL REACTIONS 8.5E – Investigate how evidence of a chemical reaction indicate that a new substance with different properties has formed Properties and Changes in Matter Matter can undergo both physical change and chemical change. Chemical “Change” vs. “Reaction” Let’s make it clear… In a chemical reaction • A chemical change produces one or more new substances. • There is a change in the composition of one or more substances. The (chemical) reaction is what causes the (chemical) change!! CHEMICAL REACTIONS: Elements can combine to form new substances Substances can be broken down into simpler substances Atoms are re-arranged NOT created or destroyed CHEMICAL REACTIONS: Formulas and equations express what happens What is written is a recombination of those atoms Ex. O2 + 2H2O 2H2O2 This “rearrangement” shows that new substances with new properties has been formed Remember: Law of Conservation Mass of Reactants = Mass of Products CHEMICAL CHANGES A change that produces new substances Properties of the substances change Signs (Evidence) that a Chemical Change has occurred: a. Gas is formed/released – usually seen as bubbles b. Change in odor c. Change in temperature – rising or falling 1. A chemical reaction that gives off heat is called “exothermic” (the temperature goes up) 2. A reaction that needs heat is called “endothermic” – the temperature goes down (during the reaction at least) Signs (Evidence) that a Chemical Change has occurred: d. A precipitate forms – a solid forms from mixing 2 or more liquids together e. Light is produced (flame, bioluminescence, glow sticks, explosions) f. Change in color (unexpected, ex. rust) g. Sound (boom, crackling sounds) Specific examples that a chemical change has occurred: This is a precipitate: SOLID is formed- lead nitrate + potassium iodide = lead iodide (a precipitate) a precipitate is an insoluble solid that forms when two solutions are combined and react chemically. Insoluble means that the solid will not dissolve. Common Misconception Alert! Temperature change due to adding or removing heat does not represent a chemical change. For example, melting or freezing is a change of state which is a physical change. Is this still chocolate? State of Matter Change Label on your page: Sublimation: the transition of a substance directly from the solid to the gas phase without passing through the liquid phase. Now you know… The difference between physical and chemical changes And that chemical reactions are a change in energy and that there is evidence of them occurring But what TRIGGERS chemical reactions? Chemical Reactions What chemical reactions CAN look like! Picture and Video examples: Digestion Gas is Formed Elephant’s Toothpaste Gas Production Flammable Water Water = H2O. The batteries split the H2 and the O. H2 = hydrogen. Change in Color Steve Spangler – Color Change Change in Odor Rotten Egg Lab A Precipitate is Formed Golden Rain Light is Produced Surprize Fire Change in Temperature X on Fire! Thermite Reaction These are fast paced! See if you can determine the evidence for each of these reactions! Stations Lab! Get your notebooks and title a new clean page: “Evidence of Chemical Reactions” You will rotate stations and determine if what you observe is a chemical reaction. There are instructions for each station – read them first, the follow them! Safety is key – gloves, goggles and aprons are required – no exceptions!! Make this chart now: Chemical Reactions Chart Copy this onto your page: Station Number Change Change in in color temperature Production of gas Precipitate forms New substance made Other observations
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