C18.7 Checking Purity

• a pure substance is a substance that is made up of one element or compound
• two ways to check if a substance is pure:

  1. find the melting and boiling points
  2. carry out paper chromatography
• melting point - temperature at which solid turns into liquid
• boiling point - temperature at which liquid turns into gas
• pure substances have sharp, distinct melting/boiling points
• when a substance contains an impurity, its melting point falls and its boiling point rises. it melts and boils over a range of temperatures, not sharply
• the more impurity there is in a substance the bigger the change in melting and boiling points and the wider the temperature range over which it changes state
• melting and boiling points change in an impure substance because the particles in a solid are held in a lattice and to melt a solid, heat is needed to break up the lattice. when there are particles of an impurity in the lattice, it is easier to break up and so the solids melts at a lower temperature and less sharply. the more impurity, the bigger the effect. in the liquid the impurity makes it harder for particles to leave the liquid and form a gas so the boiling point is higher and less sharp.
• the product of a chemical reaction will have particles of other things mixed in it, even if only in tiny amounts
• increasing the purity means separating more unwanted substances.
• rinsing a solid product with distilled water will wash away some impurities
• repeating a separation technique will help remove impurities. you could dissolve crystals in a pure solvent, and then recrystallise it
• each pure substance has a unique pair of melting and boiling points
• the boiling and melting points of a substance can help you identify what it is