Elements, compounds, mixtures

Elements, compounds, mixtures

1. Element - Something that cannot be broken down into anything simpler

   • Compound - Two or more elements chemically combined
     • Mixture - Two or more substances mixed together without a chemical reaction

2. Pure substances have fixed melting and boiling points, impure substances have varying melting and boiling points

3. Experimental techniques

   • Filtration
     • Used to separate solid from a liquid, such as sand and water
       • Substance left in filter paper is residue, and liquid that comes through is filtrate
   • Crystallisation
     • Used to separate a solute from a solution, e.g sodium chloride
       • 1. Heat solution in evaporating basin to boil off some of water until an almost saturated solution is formed
         • 2. Test by dipping a glass rod into solution and see if crystals form
           • 3. Turn off bunsen burner and crystals will form as more water evaporates and solution cools
             • 4. Remove crystals by filtration
   • Simple distillation - Used to separate components of a solution
     • Could use crystallisation for substances like NaCl, but this allows water to be collected
       • Water boils, and is condensed back into a liquid by condenser. Salt remains in flask
   • Fractional distillation
     • Separates a mixtures of liquids, such as ethanol and water
       • Relies on their different boiling points
         • Ethanol boils at 78 degrees Celsius, whereas for water it's 100 degrees Celsius
   • Paper chromatography
     • Used to separate a variety of mixtures, such as coloured inks or food colourings
       • Can also separate a mixture of colourless substance like sugars, but then spots have to be made visible
         • Rf (retardation factor) = distance moved by a spot from pencil line/distance moved by solvent front (from pencil line)
     • Chromatogram - Visible record of results of separating components of a mixture

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