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Chapter 20

Chapter 20. Sections 1 and 2. Physical Changes. A physical change is a change that affects only the physical properties of a substance (size, shape, state). Example: You can change the physical properties of ice by crushing it or by simply letting it melt. Chemical Changes.

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Chapter 20

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  1. Chapter 20 Sections 1 and 2

  2. Physical Changes • A physical change is a change that affects only the physical properties of a substance (size, shape, state). • Example: You can change the physical properties of ice by crushing it or by simply letting it melt.

  3. Chemical Changes • A chemical change is the result of a chemical reaction. • A chemical change will take a substance and change it into something new. • Atoms are rearranged when chemical changes occur.

  4. Evidence of a chemical change include energy being released or absorbed, color change, formation of a precipitate (solid) and a gas being produced.

  5. Chemical Reactions NaCl + H2SO4 NaHSO4 + HCl (Reactants) (Products) Reactants: Substances you start with. Products: New substances formed.

  6. Two major types of chemical reactions: 1.      Endothermic Reactions – A chemical reaction in which energy is absorbed. ü      Can feel warm to the touch. ü      Examples include photosynthesis and eating food. 2.      Exothermic Reactions – A chemical reaction in which energy is released. ü      Can feel hot to the touch. Examples include fireworks going off and striking a match.

  7. Factors that affect reaction rates: Reaction rates – a measure of how quickly reactants turn into products.

  8. 1.      temperature – an increase in temperature generally increases the rate of reaction. 2.      Concentration – a measure of the amount of solute in a solvent. Generally, most chemical reactions proceed at a faster rate if the concentration of the reactants is increased. For example: A glass of milk with 6 teaspoons of chocolate is more concentrated than a glass of milk with 2 teaspoons of chocolate.

  9. 3.      Surface area – how much of a material is exposed. When one of the reactants in a chemical reaction is a solid, the rate of reaction can be increased by breaking the solid into smaller pieces. This increases the surface area of the reactant. 4.      Catalysts – a substance that increases the rate of a reaction but is not itself changed by the reaction.

  10. Polymers large molecules composed of many separate molecules bonded together in a chain-like fashion

  11. Natural Polymers  1.  rubber  2.  silk, cotton, and wool 3. proteins and starches

  12. Synthetic Polymers 1.      Plastics – easily molded, durable, and often not biodegradable. Uses include storage containers, rain-proof gear, etc. 2.      Nylon – strong polymer that will stretch but not absorb water or rot. Uses include fibers for clothing, making parachutes and ropes.

  13. 3. PVC (polyvinyl chloride) – a hard polymer used to make records, pipes, and for wire insulation. • 4. Acrylic – often used as a glass substitute. Used to make many outdoor signs and fake fingernails. • Polyester – found commonly in boat sails and fibers for clothing.

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