chapter 11 science

• physical change - change in which no new substances is formed

  : changes can usually be reversed

  : get the constituents using separation techniques
• chemical change - change in which one or more new substances are formed

  : not easily reversible

  : cannot easily get reactants back

  eg. digestion, cooking, rusting, photosynthesis
• no. of atoms in reactants = no. of atoms in products
• 1.Combustion - chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances

  : candles, sparks

  : carbon + oxygen -> carbon dioxide
• 2. Thermal Decomposition - process in which a substance breaks down into two or more simpler substances upon heating

  : stove, bunsen burner

  : calcium carbonate -> calcium oxide + carbon dioxide
• 3. Oxidation - process where a matter interacted with oxygen and the substance gains oxygen when it undergoes a chemical change

  : browning of an apple

  : glucose + oxygen -> carbon dioxide + water

  eg. rusting: iron reacts with oxygen and water to form rust

  cellular respiration: food molecules broken down in the presence of oxygen to provide cells with energy to survive and reproduce, carbon dioxide and water is produced
• 4. Neutralisation - process where acid is mixed with an alkali

  : acid + alkali -> salt + water
• Acids

  : hydrochloric acid HCl

  : sulfuric acid H2SO4

  : nitric acid HNO3
• Bases - metal oxides and metal hydroxides
• Alkali - soluble bases (bases that dissolve in water, bitter taste)

  : usually end in "oxide", aqueous ammonia special case

  insoluble bases - all other bases
• 1.acid + reactive metals -> salt + hydrogen

  (some metals are unreactive like copper, silver, gold, platinum)

  
  

  -effervescence is observed

  : test for hydrogen -> use a lighted splint

  : if hydrogen is present -> lighted splint extinguished with a "pop" sound
• 2. acid + metal carbonates -> salt + water + carbon dioxide

  -effervescence is observed

  : test for CO2 -> bubble gas into limewater

  : if CO2 is present -> a white precipitate would be formed
• 3. acid + metal oxide -> salt + water

  (neutralisation)
• 4. acid + metal hydroxide -> salt + water

  (neutralisation)

  acidic - battery acid, lemon juice

  neutral - pure water

  alkaline - oven, drain cleaner
• indicators

  • litmus paper

  : acid -> red

  : alkaline -> blue

  : neutral -> no effect

  • universal indicator (paper/solution)
  • pH meter

  : electrical method of measuring the pH of a solution

  : pH electrode dipped into solution, pH value shown on meter

  : more reliable, accurate
• applications of neutralisation

  pH and agriculture

  • acid rain -> soils too acidic (plants grow best at pH 6.5 soil)
  • neutralised by adding calcium hydroxide or calcium oxide (lime)

  toothpaste

  • neutralise acids in the mouth that cause tooth decay

  antacids (medicine that contains alkaline)

  • neutralise excess acids produced by the stomach, relieve discomfort
• how chemical changes occur

  
  

  1)mixing

  2) heating

  : combustion and thermal decomposition

  3) exposure to light

  : photosynthesis -> plants use chlorophyll in leaves, produced glucose and oxygen (CO2, water, sunlight for photosynthesis)

  : photographs turning yellow (UV light)

  4) interaction with oxygen

  : oxidation

  5) electric current

  : electroplating - process in which a substance is coated with a metal with the passage of an electric current

  : medal, watch
• beneficial process

  
  

  1.cooking

  : heat softens food, makes it easier for body to absorb nutrients from food

  
  

  2. respiration

  : glucose molecules react with oxygen, produce carbon dioxide and water

  : energy released allow cells to survive, reproduce
• harmful process

  3. decay (decomposition)

  : process of breaking down organic matter into simpler substances

  : allows nutrients to return to the environment

  4. rusting (oxidation process)

  : prevented by -> painting, oiling, galvanising (coat iron with zinc), drying

  5. combustion

  : produce air pollutants -> carbon monoxide, sulfur dioxide, oxides of nitrogen