C5 - Chemical Changes

• Metals


  • Can be placed in order of reactivity by their reactions with water and dilute acid
• Hydrogen gas


  • Given off if metals react with water or dilute acids
  • The gas 'pops' with a lighted spill
• A more reactive metal

  Will displace a less reactive metal from its aqueous solution
• Positions in the reactivity series

  • Hydrogen
  • Carbon
• Oxidation

  Loss of electrons
• Reduction


  Gain of electrons
• Metal ore


  • Contains enough of the metal to make it economic to extract
  • Ores are mined and might need to be concentrated before the metal is extracted and purified
• Gold and some other unreactive metals can be found in their native state
• Reactivity series


  • Helps decide the best way to extract a metal from its ore
  • The oxides of metals below carbon in the series can be reduced by carbon to give the metal element
  • Metals more reactive than carbon cannot be extracted from their ores using carbon, they are extracted by electrolysis of the molten metal compound
• Salt


  A compound formed when the hydrogen in an acid is wholly or partially replaced by metal or ammonium ions
• Making salts


  1. Reacting a suitable metal with an acid

  2. The metal must be above hydrogen in the reactivity series, but not dangerously reactive

  3. The reaction produces hydrogen gas as well as a salt

  4. The salt can then be crystallised out of solution by evaporating off the water
• Metal and acid reaction

  • An example of a redox reaction
  • The metal atoms lose electrons and are oxidised, and hydrogen ions from the acid gain electrons and are reduced
• Neutralisation reaction


  1. Occurs when an acid reacts with a base

  2. Produces a salt and water
• Salt formula


  The sum of the charges on the ions in a salt add up to zero, enabling you to work out the formula
• Crystallising a salt


  A pure, dry sample can be crystallised out of solution by evaporating off most of the water, and drying with filter papers if necessary
• Indicator


  • Needed when a soluble salt is prepared by reacting an alkali with an acid
• Titration


  Can be repeated without the indicator to make a salt, then a pure, dry sample of its crystals prepared
• Carbonate and acid reaction

  Produces a salt, water, and carbon dioxide gas
• Acid


  Substances that produce H+(aq) ions when added to water
• Base


  Substances that will neutralise acids
• Alkali


  A soluble hydroxide that produces OH-(aq) ions when added to water
• pH scale


  • Shows how acidic or alkaline a solution is
  • Solutions with pH values less than 7 are acidic, pH values more than 7 are alkaline, and a pH value of 7 indicates a neutral solution
• Weak acid solutions


  Have a higher pH value than solutions of strong acids with the same concentration
• As pH decreases by one unit


  The hydrogen ion concentration of the solution increases by a factor of 10 (i.e., one order of magnitude)