8th Grade Chemistry Exam Practice

Physical and Chemical Changes

• Physical change: alters the form or appearance of a substance without changing it into a different substance
  • Examples: cutting, crushing, melting, freezing, boiling

Chemical Changes

• Chemical change: produces one or more new substances
  • Examples: burning, rusting, cooking, digestion, decomposition

Reactants and Products

• Reactants: substances that undergo chemical changes
• Products: substances formed as a result of a chemical reaction

Exothermic and Endothermic Reactions

• Exothermic reaction: energy released as products form is greater than energy required to break bonds of reactants
  • Feels warm
  • Examples: campfire, nuclear fission
• Endothermic reaction: more energy required to break bonds of reactants than is released by formation of products
  • Feels cold
  • Examples: melting ice, photosynthesis

Indicators of Chemical Reaction

• Changes that indicate a chemical reaction took place:
  • Color change
  • Odor change
  • Gas production
  • Formation of a precipitate

Factors Affecting Rate of Reaction

• Five factors that affect the rate of a reaction:
  • Temperature: increasing temperature increases rate of reaction
  • Surface area: increasing surface area increases rate of reaction
  • Catalysts: substances that speed up a chemical reaction
  • Inhibitors: substances that slow down reactions

Synthetic Materials and Natural Resources

• Synthetic material: chemicals and resources made by humans
  • Examples: nylon, spandex, rayon
• Natural resource: anything naturally occurring in the environment that humans use
  • Examples: pure elements, simple molecules, complex molecules

Pure Substances

• Characteristics of pure substances:
  • Same type of atom
  • Same molecules
  • Same ionically bonded elements
  • Examples: pure elements, simple molecules, complex molecules

Accidental Discoveries of Synthetics

• Three accidental discoveries of synthetics:
  • Superglue: cyanoacrylate that stuck to everything
  • Non-stick coating: white flakes in a container instead of gas
  • Artificial sweetener: forgetting to wash hands before eating

Polymers

• Polymer: long chains of molecules made up of repeating units called monomers
• Natural occurrence of polymers:
  • Wool
  • Silk
  • Rubber
  • Cellulose
• Properties of polymers:
  • Strength
  • Flexibility
  • Elasticity

Synthetic Fibers

• Four types of synthetic fibers:
  • Acrylic
  • Nylon
  • Polyester
  • Spandex
• Four advantages of synthetic fibers:
  • Less expensive
  • Last longer
  • Dry more quickly
  • Clean easily
  • Strong and flexible
• Three drawbacks of synthetic fibers:
  • Burn more easily
  • Pollutants
  • Can cause allergies
• Four ways in which synthetic fibers are used:
  • Clothes
  • Buildings
  • Materials for goods
  • Plastics

Synthetic Foods

• Three types of synthetic foods:
  • Flavorings
  • Colorings
  • Preservatives
• Four advantages of synthetic foods:
  • Cheaper
  • Foods last longer
  • Safer to consume
  • Obtain in a large scale
• One drawback of synthetic foods:
  • Health risks