Elements compounds and mixtures

• All substances can be classified into

  Elements, compounds and mixtures
• Element

  • A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler
• Compound

  • A substance made up of two or more elements chemically combined
  • There are an unlimited number of compounds
  • Compounds cannot be separated into their elements by physical means
• Mixture

  • A combination of two or more substances that are not chemically combined
  • MIxtures can be separated by physical methods such as filtration or evaporation
• A pure substance may consist of

  a single element or compound which contains no other substances
• Pure substances melt and boil at specific and sharp temperatures
• Melting point analysis is routinely used to assess the purity of drugs

  • done using a melting point apparatus which allows you to slowly heat a small amount of sample, making it easier to observe the exact melting point
  • Compare to data tables
  • The closer the measured value is to the actual melting or boiling point the purer the sample is
• Simple distillation

  • Used to separate a liquid and soluble solid from a solution or a pure liquid from a mixture of liquids
  • Solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask
  • Vapour passes through the condenser where it cools and condenses, turning into the pure liquid that is collected in a beaker
  • After all the water is evaporated from the solution, only the solid solute will be left behind
• Fractional distillation

  • Fractional distillation is used to separate two or more liquids that are miscible with one another
  • Solution is heated to the substance with lowest boiling point
  • This substance will rise and evaporate first, and vapours will pass through a condenser where they cool and condense, turning into a liquid that will be collected into beaker
• Crystallisation

  Crystallisation is used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent rather than in cold
• Filtration

  • Filtration is used to separate an undissolved solid from a mixture of the solid and a liquid / solution
  • A piece of filter paper is placed in a filter funnel above a beaker
  • A mixture of insoluble solid and liquid is poured into the filter funnel
  • The filter paper will only allow small liquid particles to pass through as filtrate
  • Solid particles are too large to pass through the filter paper so will stay behind as a residue
• How to do crystallisation

  • The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind
  • Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution
  • If the solution is saturated, crystals will form on the glass rod
  • The saturated solution is allowed to cool slowly
  • Crystals begin to grow as solids will come out of solution due to decreasing solubility
  • The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry
• Paper chromatography

  Paper chromatography is used to separate substances that have different solubilities in a given solvent
• How to do paper chromatography

  • A pencil line is drawn on chromatography paper and spots of the sample are placed on it
  • The paper is then lowered into the solvent container
  • The solvent travels up the paper by capillary action, taking some of the coloured substances with it
  • Different substances have different solubilities so will travel at different rates (This causes the substances to separate and those substances with higher solubility will travel further than others)
  • This will show the different components of the ink/dye
• Why is pencil used in paper chromatography

  Pencil is used as ink would run into the chromatogram along with the samples

  ADD
• The pencil must sit above the level of the solvent so the samples don’t wash into the solvent container
• Rf value

  distance moved by substance / distance moved by solvent