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This chapter explores the fundamental concepts of physical and chemical changes. A physical change, such as crushing ice, only alters physical properties like size and shape. In contrast, a chemical change involves a transformation during a chemical reaction, resulting in new substances formed from reactants. It covers examples of endothermic and exothermic reactions, factors affecting reaction rates such as temperature, concentration, surface area, and catalysts, as well as the role of polymers in various applications, distinguishing between natural and synthetic types.
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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 • 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.
Evidence of a chemical change include energy being released or absorbed, color change, formation of a precipitate (solid) and a gas being produced.
Chemical Reactions NaCl + H2SO4 NaHSO4 + HCl (Reactants) (Products) Reactants: Substances you start with. Products: New substances formed.
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.
Factors that affect reaction rates: Reaction rates – a measure of how quickly reactants turn into products.
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.
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.
Polymers large molecules composed of many separate molecules bonded together in a chain-like fashion
Natural Polymers 1. rubber 2. silk, cotton, and wool 3. proteins and starches
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.
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.
