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Comments/guidance mock exam performances 2015

Common errors:
  • Students are still explaining a covalent bond as a shared pair of electrons rather than the attraction between the shared pair of electrons and the nuclei of both atoms.
  • Atoms lose/gain electrons to become ions. Some students are writing about an ion needing to lose/gain electrons.
  • Cracking was confused with fractional distillation. In the mark scheme the term long/short chain molecules is preferred to long/short chain hydrocarbons.
  • Reading the burette scale the wrong way.
  • Not putting the 0 in 1.80.
  • When in doubt use 2 d.p.
  • Not taking the average of the concordant results in a titration. Trying to average all results except the rough titre value.
  • Students stating that water has a lower melting point than sodium oxide because the covalent bonding in water is weak rather than the intermolecular forces between water molecules are weak.
  • Observations are what you can see. Specifically, students stating carbon dioxide gas as an observation rather than bubbles/fizzing.
  • Do not provide two different answers to a question. For example, when asked for an observation that can be made when dilute hydrochloric acid is added to calcium carbonate, one response was, "The calcium carbonate will shoot around the surface and gradually disappear". Often one is correct and one is not and by putting both no marks will be given. First response is taken.
  • Use formulae given in the question when applicable. Some students did not have the correct formula for Lead Sulphate even though it was given.