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• Types of questions


  • Evaluate questions (need advantages, disadvantages, and a justified conclusion)
  • Compare questions (need similarities and differences)
  • Explain questions (need as much scientific detail as possible)
• The exam won't remind you to include these things, so you need to remember them to help structure your answers
• Things to brain dump at the start of the exam

  • Equation: concentration = mass / volume
  • Conversion: 1 cm^3 = 0.001 dm^3
  • Equation: uncertainty = range / 2
• Required practicals for Combined Science

  • Making salts
  • Endo and exothermic reactions
  • Electrolysis
• Making salts


  1. Add solid reactant in excess

  2. Filter to collect excess solid

  3. Heat solution to evaporate some water

  4. Allow to cool slowly to form crystals
• Endo and exothermic reactions

  • Endothermic reactions: temperature decreases as they take in heat from surroundings
  • Exothermic reactions: temperature increases as they give out heat to surroundings
• Improving endo/exothermic reaction experiments

  1. Use a polystyrene cup to minimize heat transfer

  2. Put a lid on the cup

  3. Surround with insulation like cotton wool
• Electrolysis


  Using electricity to split an ionic compound
• Electrolysis of molten ionic compounds

  1. Positive ions attracted to negative electrode

  2. Negative ions attracted to positive electrode
• Electrolysis of aqueous solutions

  • Positive electrode: halide ion or hydroxide ion will be oxidized
  • Negative electrode: least reactive positive ion will be reduced
• Electrodes in electrolysis need to be regularly replaced as the carbon can react with oxygen to form CO2
• 20% of the chemistry exam is maths, including things like averages, uncertainties, and quantitative chemistry calculations
• Relative atomic mass


  Average mass of an element's isotopes, calculated by multiplying the mass of each isotope by its percentage abundance and summing
• Relative formula mass


  Total mass of all the atoms in a chemical formula
• Percentage by mass


  Mass of a component divided by total mass, multiplied by 100
• Brackets


  Multiplied by the number outside the bracket
• Magnesium has just one of the brackets
• The 24 is just from the number outside the bracket
• The contents of the bracket need to be multiplied by two
• Need to add two lots of 14 for nitrogen and three lots of 16 for oxygen
• This gives a total relative formula mass of 148
• Calculating percentage mass


  1. Divide mass of component by total mass

  2. Multiply by 100
• Percentage mass of nitrogen in magnesium nitrate is 18.9%
• Percentage mass of carbon in calcium carbonate is 12%
• Concentration


  Mass over volume, measured in g/dm^3
• Concentration of 2g of sodium chloride dissolved in 200cm^3 of water is 10g/dm^3
• Balancing chemical equations


  1. Count atoms on each side

  2. Adjust coefficients to balance
• Balancing ensures conservation of mass - no atoms are lost or gained
• Key topics for exam

  • Topics 1-5 in revision guide
• Protons and electrons


  Relative mass of 1, protons positively charged, electrons negatively charged
• Neutrons


  No charge
• Number of protons = number of electrons in an atom</b>
• Electrons fill shells, starting with inner shell of 2, then outer shells of up to 8
• Determining atomic structure


  1. Proton number = number of protons = number of electrons

  2. Neutrons = total mass - proton number
• Plum pudding model of atom

  • Positive sphere with electrons embedded
• Nuclear model of atom

  • Positive nucleus with electrons orbiting at specific distances
• Mendeleev left gaps in periodic table for undiscovered elements
• Groups in periodic table

  • Group 1 (alkali metals)
  • Group 7 (halogens)
  • Group 0 (noble gases)
  • Transition metals
• Reactivity of group 1 elements

  Increases down group as outer electrons further from nucleus
• Reactivity of group 7 elements


  Decreases down group as outer electrons further from nucleus