4.8 CHEMISTRY

• A pure substance is a single element or compound not mixed with any other substance.
• A formulation is a mixture that has a fixed composition, designed for a useful product
• pure substances melt and boil at specific temperatures and have a fixed melting and boiling point. This data can help us distinguish pure substances from mixtures
• For formulations

  • Each chemical has a particular purpose
  • Formulations are made by mixing the components in carefully measured quantities to ensure that the product has the required properties
  • examples of Formulations: mineral water, fuel, specific medicines, cleaning agents, paints , alloys , fertilisers
• Note: The every day definition of a pure substance is a substance that has nothing added to it
• Paper chromatography is used to separate different dyes in an ink or is used to separate mixtures
• 6 MARK ANSWER FOR PAPER CHROMATOGRAPHY:

  1. Take a piece of filter paper and use a pencil to draw a line towards the bottom of the filter paper. This is called the baseline
  2. Add your samples of ink to the pencil line( 1 or multiple)
  3. Fill a beaker with shallow amount of solvent (e.g. water) or ethanol.
  4. Place the filter paper into the beaker, ensuring the pencil line is not submerged into the water.
  5. Place a lid or cling film on top so the solvent does not evaporate.
  6. Wait for the solvent to seep up the paper.
  7. The pattern we have left is the chromatogram
• Each of the dyes in the ink travel up at different rates and so they end up separating out.
• The mobile phase are the molecules that can move (liquid), in this case it is the water
• The stationary phase is where the molecules don't move(solid or thick liquid), in this case it is the filter paper
• Rf value = Distance travelled by substance / Distance travelled by the solvent
• A higher Rf value means the substance has moved further than the solvent
• A lower Rf value means the substance hasn't moved as far as the solvent
• Paper Chromatography is used to separate mixtures of compounds based on their polarity
• In Paper Chromatography, the more polar a compound is, the closer it will stay to the starting point
• Chromatography is used to separate mixtures into their individual components, which allows us to identify them
• Paper chromatography is used when we want to see what substances are present in something like soil samples, blood or urine
• A spot of ink that has moved higher up the page is more soluble and spends more time in the mobile phase than the stationary phase, so it moves faster. Also, it has the least affinity to the paper
• A spot of ink that is closest to the baseline is the least soluble and spends more time in the stationary phase than the mobile phase and also has a stronger affinity to the paper
• When a single dot appears on the chromatogram, this means the substance is likely to be a pure substance
• The Rf values change dependant on the type of solvent and the type of paper used
• The test for hydrogen is to use a burning splint held at the open end of a test tube of the gas and if hydrogen is present it will make a squeaky pop
• to test for oxygen, uses a glowing splint inserted into the open end of the test tube and if oxygen is present, the splint will relight
• to test for carbon dioxide, bubble the tested gas through limewater and if carbon dioxide is present, it will go cloudy/milky
• To test for chlorine, use damp litmus paper and when chlorine gas is present the damp litmus paper will bleach and will turn white
• colours for the flame tests:

  • Lithium : Crimson
  • Sodium: Yellow/Orange
  • Potassium: Lilac
  • Calcium: Brick red/ orange red
  • Copper: Green
• If a sample containing a mixture of ions is used, some flame colours can be masked
• Metal hydroxides and their precipitates

  • Aluminium, calcium, and magnesium ions form a white precipitate with NaOH
  • Only aluminium's precipitate dissolves when excess NaOH is added
  • To distinguish between Calcium and magnesium, do a flame test as magnesium doesn't produce a flame and calcium produces a brick red flame
  • Copper(II) produces a blue precipitate
  • Iron(II) produces a green precipitate
  • Iron(III) produces a brown precipitate
• Equations: e.g. Cu2+ + 2OH- -> Cu(OH)2
• You need as many OH- ions as the charge on the metal ion
• The Na from the NaOH and whatever the metal ion was bonded to will react to form a compound together: e.g. CuCl2 + 2NaOH -> Cu(OH)2 + 2NaCl
• Reactions with carbonates

  • Carbonates react with dilute acids (HCL) to create carbon dioxide this is why we see fizzing
  • If the gas bubbled through limewater makes it go cloudy, the gas is CO2
  • If carbonate ions are present we would see fizzing
• Halides identification

  • First add dilute nitric acid, followed by silver nitrate solution
  • Dilute nitric acid is added to remove impurity carbonate ions
  • Chloride gives a white precipitate
  • Bromide gives a cream precipitate
  • Iodide gives a yellow precipitate
• Carbon dioxide test

  1. Carbon dioxide is created by reacting calcium carbonate with dilute hydrochloric acid

  2. The gas is bubbled through limewater, if the limewater goes cloudy, the gas is CO2
• Halides


  • Chloride gives a white precipitate
  • Bromide gives a cream precipitate
  • Iodide gives a yellow precipitate
• Sulfates

  • To test for sulfate ions, add dilute HCL to remove impurity carbonate ions
  • Then add Barium Chloride
  • So if sulfate is present then the compound would be Barium Sulfate which is a white ppt
• Elements and compounds can be detected and identified using instrumental methods
• Instrumental methods


  Accurate, sensitive and rapid methods of detecting and identifying elements and compounds
• Instrumental methods

  • Flame emission spectroscopy
  • Chemical tests
• Flame emission spectroscopy

  1. Sample is put into a flame and the light given out is passed through a spectroscope

  2. Output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations

  Different ions emit different wavelengths of the visible light spectrum