Chemical changes

• A substance with a pH lower than 7 is acidic.
• A substance with a pH higher than 7 is alkaline.
• What colour does universal indicator turn in an acidic solution?

  Red
• What colour does universal indicator turn in an alkaline solution?

  Blue or purple
• Acids form H+ ions in water.
• Alkalis form OH- ions in water.
• The reaction between acids and bases is called a neutralisation reaction.
• Titrations are a method of finding the concentration of a solution by finding out how much acid is needed to neutralise a given volume of alkali.
• For titrations, you can use the indicator phenolphthalein. It turns pink in alkali and colourless in acids.
• Strong acids ionise completely in water, weak acids ionise partially in water.
• For every decrease of 1 on the pH scale, the concentration of H+ ions increases by a factor of 10.
• Concentration measures how much acid there is in a certain volume of water.
• Acid + metal oxide = salt + water
• Acid + metal hydroxide = salt + water
• Acid + metal carbonate = salt + water + carbon dioxide
• Metal oxides and metal hydroxides are bases.
• Metals that are more reactive than carbon:

  • potassium
  • sodium
  • lithium
  • calcium
  • magnesium
• Metals that are less reactive than carbon:

  • zinc
  • iron
  • copper
• Acid + metal = salt + hydrogen
• Metal + water = metal hydroxide + hydrogen

  Only potassium, sodium, lithium and calcium react with water.
• Lots of metals oxidise. These oxides are often the ores that the metals need to be extracted from.
• Oxidation

  Is

  Loss

  Reduction

  Is

  Gain
• Metals less reactive than carbon are extracted using reduction by carbon.
• Metals more reactive that carbon need to be extracted using electrolysis.
• Metals like gold are so unreactive that they can be found in the earth as its elemental form.
• Oxidation and reduction happen at the same time. This is called a redox reaction.
• During electrolysis, an electric current is passed through an electrolyte ( a molten or dissolved ionic compound).
• The positive ions in the electrolyte move towards the cathode (-ve electrode).
• The negative ions in the electrolyte move towards the anode (+ve electrode) and lose electrons.
• An ionic solid cannot be electrolysed because the ions are in fixed positions.
• Positive metal ions are reduced to the element at the cathode.
• Negative non-metal ions are oxidised to the element at the anode.
• Extracting metals with electrolysis is very expensive as lots of energy is required to melt the ore and provide the current required.
• Aluminium is extracted from the ore bauxite.
• Aluminium oxide has a very high melting point, so it's mixed with cryolite to lower it.
• In the electrolysis of aqueous solutions, there will be H+ and OH- ions from the water.
• If H+ and metal ions are present, hydrogen gas will be produced at the cathode (if the metal is more reactive than hydrogen).

  If the metal is less reactive, a solid layer of the pure metal will be produced instead.
• If OH- and halide ions are present, molecules of chlorine, bromine or iodine will be formed at the anode.

  If no halide ions are present, then the OH- ions are discharged and oxygen is formed.
• Half equations show the reactions at the electrodes.