Chemistry

• Insoluble - doesn't dissolve
• solute - dissolves in solvent
• soluble - dissolves
• solvent - liquid you dissolve stuff in
• solution - solute + solvent
• Chromatography - method of separation of a mixture of soluble chemical substances. Works as some compounds in a mixture will dissolve better than others in a chosen solvent
• It can be used to identify additives in food, detecting art fraud and identifying drugs in alcohol
• Complete experiment is called a chromatogram
• Mobile phase - solvent
• Stationary phase - chromatography paper
• Pure sample has only one spot and colour on a chromatogram whereas a mixture will have more than one
• if substance doesn't go up the paper, it is insolube
• don't draw baseline in pen as ink will travel up making results unclear
• don't pour water over the baseline as ink will dissolve in the water and not travel up
• rf value = distance from baseline to spot / distance from baseline to solvent front
• rf value will always be smaller than 1
• temperature has to be the same as solubility is affected by how fast particles are moving. if the temperature increases particles will have more heat energy and move faster which would affect results
• solvent has to be the same as different substance dissolve differently in different solvents
• Rf values can be used as a database to determine the components in a mixture
• The mobile phase moves through the stationary phase carrying the components of a mixture with it. The components with a stronger attraction to the mobile phase compared to the stationary phase will move further up the paper in a given time.
• Those that are more attracted to the stationary phase will move a smaller distance up the paper in a given time. This causes components to be separated out of the mixture. You need to keep the temperature and the solvent the same as the components will travel the same amount each time.
• required practical - Equipment: 250cm3 beaker, wooden splint, chromatography paper, 4 known food colourings A-D, 1 unknown mixture labelled U, glass capillary tubes
• method : 1. Use a ruler to draw a horizontal pencil line 1- 1.5 cm from the short edge of the

  chromatography paper to be the baseline

  2. Mark 2 pencil spots at equal intervals across the line on each sheet. Keep

  at least 1 cm away from each end.

  3. Use a glass capillary tube to put a small spot of each of the known colourings on

  the pencil spots. Then use the glass capillary tube to put a small spot of the

  unknown mixture on the 5th pencil spot. Label each spot in pencil.
• 4. Pour water into the beaker to a depth below your pencil line.

  5. Fold the edge of the chromatography paper over the splint.

  Rest the splint on the top edge of the beaker. The bottom edge of the paper

  should dip into the water (but the water should not touch the coloured spots)

  6. Wait for the water solvent to travel at least three quarters of the way up the

  paper. Do not disturb the beaker during this time.Carefully remove the paper, draw a pencil line to mark where the solvent reached to, and hang the paper up to dry.
• 7. Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent.

  8. For each of the four known colours, measure the distance in mm from the bottom line to the centre of each spot. This has been done for you.

  9. Use the following equation to calculate the R f value for each of the known colours.

  10. Match the spots in mixture U with those from A–D. Calculate the Rf values for each colour in the mixture U.