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Types of Changes

Physical change: the shape, phase, size, etc. of the material can change the material itself is the same before and after the change The change can be “undone”. Chemical change: The substances present at the beginning of the change are not present at the end new substances are formed.

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Types of Changes

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  1. Physical change: the shape, phase, size, etc. of the material can change the material itself is the same before and after the change The change can be “undone” Chemical change: The substances present at the beginning of the change are not present at the end new substances are formed. The change cannot be “undone.” Types of Changes http://www.learner.org/courses/essential/physicalsci/session4/closer1.html

  2. water evaporates butter melts a can is crushed glass is broken modeling clay is shaped ripping paper boiling water sawing wood breaking a stick dissolving salt in water Review: Examples of Physical Changes

  3. Review: Physical Properties • describe the object or substance • can be observed or measured without changing the identity of the substance

  4. Review: Examples of Physical Properties • color • odor • size • shape • state (solid, liquid, gas, plasma) • texture (smooth, rough, soft, bumpy) • density (measured in g/ml and g/cm³) • melting point/boiling point/freezing point

  5. Review: Chemical Properties • are determined by the arrangement of atoms in the molecules making up the object • describe a object or substance based on its ability to change into a new substance with different properties • cannot be observed with your senses; must be tested • aren’t as easy to observe as physical properties

  6. Key Idea #7 Chemical changes occur when elements and/or compounds react or decompose to produce new substances. http://www.ric.edu/faculty/ptiskus/chemical/

  7. Chemical Properties If the chemical properties of an object or substance have changed, then a chemical reaction has occurred. Examples of Chemical Properties: • pH: a measure of how acidic something is • Flammability: a substances ability to burn • Reactivity: The rate at which a substance reacts

  8. Clues that indicate a chemical change has taken place include: • color change • temperature change • bubbling and fizzing • note: boiling water is NOT a chemical change • heat, light, or smoke is given off • a solid (precipitate) is formed • the change is difficult or impossible to reverse

  9. Compare and contrast the chemical properties of the new substance with the original after a chemical change. Copy the following chart into your notes:

  10. http://www.suwaneedental.com/travel/Vancouver/old%20rusty%20bike.jpghttp://www.suwaneedental.com/travel/Vancouver/old%20rusty%20bike.jpg

  11. http://www.flickr.com/photos/hairport_lisbon/2318561502/

  12. http://cache.lifehacker.com/assets/resources/2008/06/alka-seltzer-diy.pnghttp://cache.lifehacker.com/assets/resources/2008/06/alka-seltzer-diy.png

  13. http://z.about.com/d/chemistry/1/0/_/_/fireworks.jpg

  14. In a chemical change • one or more substances combine or break apart to form new substances. • atoms rearrange themselves during a chemical reaction to form different molecules and compounds. • For example, when hydrogen reacts with oxygen the product is water vapor.

  15. Baking Soda and Vinegar • Acid-base reaction. • Vinegar (acetic acid): C2H4O2 • Baking soda (sodium bicarbonate): NaHCO3 • C2H4O2 + NaHCO3 ---> C2H3O2Na + H2CO3 • The products are sodium acetate and carbonic acid • Carbonic acid quickly decomposes into carbon dioxide and water: H2CO3 ---> H2O + CO2 • The CO2 is what you see foaming and bubbling in this reaction. http://www.newton.dep.anl.gov/askasci/chem99/chem99096.htm

  16. Burning Sugar • Add heat • C12 H22O11 = Carbon + Water (H2O) • Add more heat • Carbon + Water (H2O) = Carbon + Water Vapor (H2O) • Add more heat • Carbon + Hydrogen gas + Oxygen gas

  17. Activation Energy is • the energy needed to start a chemical reaction. Example: • A match won't light spontaneously. • When you strike a match you are providing the activation energy needed to cause a reaction between the chemicals in the match head and oxygen in the air.

  18. All chemical reactions include a change in energy. • Exothermic reactions release energy to their surroundings; usually in the form of heat. • Endothermic reactions need to absorb heat from their surroundings to proceed.

  19. Conservation of Mass • Mass is conserved during chemical changes. • The mass of the reactants is always the same as the mass of the products after the chemical reaction.

  20. In every chemical change • new substances are formed • the starting substances are called the reactants • the new substances are called the products

  21. Natural gas is primarily methane.  • Methane (CH4) is a molecule in which 4 hydrogen atoms are bonded to one carbon atom.  • Lighting a gas stove provides the activation energy that causes the methane to react with oxygen in the atmosphere and release heat. • During the reaction, the atoms recombine to form carbon dioxide and water vapor.

  22. During a chemical change • the number and kind of atoms in the reactants are the same as the number and kind of atoms in the products.

  23. After the chemical change the mass of the reactants equals the mass of the products. • Before the reaction there were 9 atoms • 1 carbon, 4 hydrogen, 4 oxygen • After the reaction the same 9 atoms were there, just in a different combination.

  24. Iron reacts with oxygen in the air to form rust.

  25. After the chemical change • the new substances have different physical and chemical properties than the original elements and/or compounds. • the number and kind of atoms in the reactants are the same.

  26. Pennies get dull over time because the copper in the pennies slowly reacts with the oxygen in the air to form copper oxide.

  27. Chemical reactions • cause chemical changes. • involve a change in substances and a change in energy. • cause atoms to rearrange themselves to form different molecules and compounds. • NOTE: Matter and energy is NOT created or destroyed in a chemical reaction---only changed.

  28. The only sure evidence for a chemical reaction is • the production of new materials. http://www.saskschools.ca/curr_content/science9/chemistry/images/les8images/cakeaft.gif

  29. Chemical Reaction Examples • A sparkler contains magnesium. • When lit, the magnesium reacts with oxygen in the air and produces light and heat.  • The chemicals inside a rocket, when lit, react with the oxygen in the air and produce heat, light and sound.

  30. Alka Seltzer and Water Ingredients: • Water = H2O • Alka Seltzer = Sodium Bicarbonate 3(NaHCO3) + Citric Acid C6H807 Reactants: • Alka Seltzer + Water = [3(NaHCO3) + C6H807] + H2O Products: • Water Vapor + Carbon Dioxide (in air) + Sodium Citrate + Water (in beaker) = 3(H2O)+ 3(CO2) (in air) + C6H5Na3O7+ H2O (in beaker)

  31. Open vs Closed Systems • Open system: matter interacts with its surroundings; can be transferred in and out • Closed system: no matter is transferred in or out of the system

  32. When a substance chemically changes, it becomes a different substance with different properties but the amount of matter remains the same (Conservation of Mass). • Examples of Chemical Changes • Burning sugar • Baking bread • Rusty nail • Fireworks exploding • Raw egg becomes cooked egg • Spoiled milk • Digesting food in your stomach • Cake mix becomes cake • Paper becomes ash

  33. Why is a melting ice cube NOT an example of a chemical change? • Give at least three pieces of evidence in your answer.

  34. Alka Seltzer Lab

  35. Why is baking a cookie an example of a chemical change? • Give at least three pieces of evidence in your answer.

  36. Chemical Properties and Chemical Change • www.elmhurst.edu/~chm/vchembook/105Achemprop.html • http://www.saskschools.ca/curr_content/science9/chemistry/lesson11.html • www.saskschools.ca/.../chemistry/lesson8.html

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