CHEMISTRY

• Substance


  Elements are made up of the same atoms (there are about 100 different elements)
• Relative atomic mass


  Mass number - atomic number
• Atomic number


  Number of protons in the nucleus
• Nucleus


  • Contains protons and neutrons
• Electron shell


  • Electrons are spread around the nucleus
• Properties of protons, neutrons and electrons

  • Mass
  • Electrical charge
• Number of protons = atomic number
• Number of neutrons = mass number - atomic number
• First shell can hold 2 electrons, second shell 8 electrons, third shell 8 electrons
• Electron configuration


  Arrangement of electrons in shells
• Isotopes


  Atoms of the same element with the same number of protons but different numbers of neutrons
• Isotopes have the same chemical properties but different physical properties
• Relative atomic mass is calculated by knowing the abundance of each isotope
• If an atom gains 2 electrons, it forms a -2 ion
• Mixture


  Two or more elements or compounds not chemically combined together
• Solvent


  The liquid in which a solute dissolves
• Solute


  The substance that dissolves in a liquid to form a solution
• Solution


  The mixture formed when a solute is dissolved in a solvent
• Soluble


  Describes a substance that will dissolve
• Insoluble


  Describes a substance that will not dissolve
• Physical processes to separate mixtures

  1. Filtration

  2. Crystallisation

  3. Simple distillation

  4. Fractional distillation

  5. Chromatography
• Rf value


  Distance travelled by ink spot / distance travelled by solvent
• Chromatography relies on the stationary phase and mobile phase
• Paper chromatography uses uniform, absorbent paper as the stationary phase
• The mobile phase is the solvent that moves through the paper, carrying the dissolved substances
• The different dissolved substances are attracted to the two phases in different proportions, causing them to move at different rates
• A pure substance produces one spot on the chromatogram, an impure substance or mixture produces two or more spots
• The most soluble ink travels the fastest, the less soluble inks are left near the bottom
• Chemicals with strong bonds to the paper travel further up the paper
• The Rf value is used to compare the components of various samples
• Filtration 


  Separates substances that are insoluble from those that are soluble
• Crystallisation 


  Separates a soluble substance from a solvent by evaporation
• Simple distillation 


  Separates a liquid from a mixture by evaporation followed by condensation