C5- chemical analysis

• Purity (in chemistry)

  Something that only contains one compound or element throughout - not mixed with anything else
• Impurities in a sample

  Lower the melting point and increase the melting range of the substance
• Mixture

  Substances that contain more than one element or compound which aren't chemically bonded together
• Separating a mixture

  1. Filtration

  2. Crystallisation

  3. Simple distillation

  4. Fractional distillation

  5. Chromatography
• Crude oil is a mixture of different length hydrocarbon molecules
• The chemical properties of a substance aren't affected by it being part of a mixture
• Formulation


  Useful mixtures with a precise purpose that are made by following a 'formula' (a recipe)
• It's important that the substances used to make a formulation such as drugs and cosmetics are pure. If impurities get into the formulation, people could be harmed when they use them.
• Chromatography

  An analytical method used to separate the substances in a mixture and identify them
• Chromatography

  • Uses two phases
  • A mobile phase (liquid or gas) where particles can move
  • A stationary phase (solid or thick liquid) where particles can't move
• Chromatography experiment

  1. Substances in the sample constantly move between the mobile and stationary phases

  2. An equilibrium is formed between the two phases

  3. Different chemicals have different distributions between the phases

  4. The mobile phase moves through the stationary phase, carrying dissolved substances with it

  5. Chemicals with different distributions separate into different spots on the chromatogram
• The number of spots on the chromatogram may change in different solvents as the chemical distribution changes
• A pure substance will only ever form one spot in any solvent
• If the chromatogram has more than one spot then it's a mixture
• Paper chromatography

  The stationary phase is the chromatography paper and the mobile phase is the solvent
• Paper chromatography procedure

  1. Draw a line near the bottom of filter paper (make sure to use pencil as its insoluble and wont dissolve in the solvent)

  2. Add a spot of the test substance to the line

  3. Place the paper in a beaker of solvent so the solvent is just below the spot

  4. The solvent seeps up the paper, carrying the test substance with it

  5. When the solvent has nearly reached the top, mark the solvent front and leave the paper to dry
• Chromatogram

  The pattern of spots resulting from paper chromatography
• If the chromatogram has more than one spot, the substance is impure
• If the chromatogram contains only one spot, the substance may be pure (but should be checked in other solvents)
• Chromatogram

  A visual representation of the separation of the components of a mixture
• Chromatograms

  • Can tell you how many components are in a mixture
  • Can tell you what the components might be
• Identifying spots on a chromatogram

  1. If chemicals are coloured, see them as coloured dots

  2. If chemicals are colourless, use locating agents to make them visible (e.g. ninhydrin solution, iodine vapour)
• Rf value

  The distance travelled by solute ÷ distance travelled by solvent
• Calculating Rf value

  1. Measure distance travelled by solute from baseline to centre of spot

  2. Divide distance travelled by solute by distance travelled by solvent front
• Rf values should always be less than 1
• Rf values can change

  If the method used in the chromatography experiment changes (e.g. different solvent, temperature, stationary phase)
• Changing conditions can cause spots that don't separate properly to separate
• Changing conditions can help identify which reference compound is in the mixture if they have similar Rf values in a particular solvent
• Filtration

  Used to separate an insoluble solid from a liquid
• Filtration use

  • Separating impurities from a reaction mixture
• Filtration process

  1. Fold filter paper into a cone shape

  2. Pour mixture into filter paper

  3. Liquid passes through, solid remains in filter paper
• Crystallisation

  Separates a soluble solid from a solution
• Crystallisation process

  1. Pour solution into evaporating dish

  2. Gently heat to evaporate solvent

  3. Remove from heat when crystals start to form

  4. Filter out crystals and dry
• Simple distillation

  Used to separate a liquid from a solution, or to separate liquids with very different boiling points
• Simple distillation process

  1. Set up apparatus with condenser and cold water flow

  2. Heat distillation flask, lower boiling point liquid evaporates

  3. Vapour condenses in condenser and collects as pure liquid
• Simple distillation only separates things with very different boiling points, for similar boiling points use fractional distillation
• Qualitative tests


  Measure physical qualities, such as the colour of a precipitate
• Qualitative tests


  • Have a variety of applications, for example to test chemicals in Industry, to analyse the substances in rocks and soil or to test for potentially dangerous substances in water or waste
• Choosing samples

  1. Take a number of samples at random from the thing you're testing so that you can account for any variations in its composition

  2. Choose representative samples
• Taking just a small sample means you can also carry out other tests on different samples from the substance if you need to