Chemical Changes (Riha)

• Physical change

  A change in which no new substance is formed
• Chemical change (Chemical reaction)


  A change where new substances are formed
• Representing a chemical change


  1. Reactants (starting substances) on left side of arrow

  2. Products (new substances formed) on right side of arrow
• Physical changes


  • Boiling
  • Melting
  • Freezing
  • Dissolving salt in water

  
• Physical change vs Chemical change

  • Physical change is easy to reverse
  • Chemical change is difficult to reverse
• Two types of chemical changes

  1. Two or more substances chemically combined to form new substance(s)

  2. A substance breaks down to form two or more new substances
• During a chemical reaction, atoms of reactants rearrange and combine chemically to form the new products
• The total number of atoms before and after a chemical reaction remains the same
• Mass is conserved during a chemical reaction
• Combustion


  A chemical reaction in which a substance is heated in the presence of oxygen to form new substances
• Combustion

  • Burning of petrol in cars
  • Burning of charcoal
• Thermal decomposition


  A process in which a substance breaks down into two or more simpler substances upon heating
• Oxidation


  When a substance gains oxygen to form a new product
• Oxidation


  • Rusting
  • Respiration
• Neutralisation


  When an acid reacts with an alkali
• How do chemical reactions occur

  1. Mixing

  2. Heating

  3. Exposure to light

  4. Interaction with oxygen

  5. Using electric current
• Cooking causes chemical reactions in food
• Respiration


  Glucose reacts with oxygen to produce carbon dioxide and water
• Decay or decomposition is the process of breaking down organic matter into simpler substances
• Vacuum packaging reduces amount of oxygen and limits growth of bacteria and microorganisms
• Rusting


  An oxidation reaction that occurs in the presence of oxygen and water
• Preventing rusting


  • Coating iron rod with paint
  • Oiling bicycle chain
  • Galvanising with zinc
• Traditional slash-and-burn leads to production of more carbon dioxide gas -> Global warming
• Combustion of fuels produces air pollutants such as carbon monoxide, sulfur dioxide and oxides of nitrogen
• Air pollutants have harmful effects on human health and environment
• Large amount of carbon dioxide are released due to human activities and combustion
• When carbon dioxide dissolves in seawater, the seawater becomes more acidic
• Acidic seawater reacts with calcium carbonate in shells and skeletons of marine organisms
• Fish and plants in the oceans die as they cannot survive under conditions of lower pH, affecting ecosystems
• Chemical reactions can be exothermic (release energy) or endothermic (absorb energy).
• In a chemical reaction, atoms are rearranged to form new substances with different properties.
• A chemical change is when the properties of a substance are changed, such as its colour or smell.
• In a precipitation reaction, two solutions react to form an insoluble solid called a precipitate.
• Combustion reactions involve a substance rapidly reacting with oxygen to produce heat and light.
• An indicator can be used to determine whether a solution is acidic or alkaline by changing color based on its pH level.
• Air pollutants


  Unwanted chemical substances in the air which can cause harmful effects to the environment and living things
• Sources of air pollution


  • Natural process such as volcano or lightning
  • Human activities
• Carbon Monoxide (CO)


  Colourless, odourless and poisonous
• Carbon Monoxide (CO)


  • Incomplete combustion of fuels containing carbon
  • Combines with haemoglobin in blood, preventing absorption of oxygen and resulting in asphyxiation (difficulty in breathing)
  • Lack of oxygen causing brain damage and death of cell tissues
  • Causes breathing difficulties
• Nitrogen dioxide (NO2)


  Reddish brown, pungent and noninflammable