Starter Match the key words with the correct definitions: SOLUTION a solute dissolved in a solvent SOLUTE a substance that is dissolved in a solvent. SOLVENT a liquid in which chemicals dissolve to make a solution (e.g. water, ethanol) AQUEOUS SOLUTION NONAQUEOUS SOLUTION solution where a solute is dissolved in water solution where the solute is dissolved in a solvent that isn’t water (e.g ethanol) When is chromatography used? Paper chromatography is a technique for separating components of a mixture. Which kind of mixtures ? Water soluble mixtures such as: Inks Dyes/paints Colourings agents The Method How does it work? • The water is drawn up by capillary action (sticky attractive force between water molecules and the paper). • When the solvent reaches the ink spots they are dissolved and carried with the water as it moves up the paper. • The most soluble ink travels the fastest, whilst the less watersoluble inks are left near the bottom of the paper. chromatogram Solvent front Separated dyes Base line (pencil line) Practical Identifying the colours in food dyes Which M&Ms are pure colours Which are made from a variety of colours Carry out an experiment to find out!!!!!!!!! The Method 1. Draw a pencil line 2cms from the bottom edge of the chromatographic paper. 2. Lightly mark 4-5 small crosses along the pencil line (not on the right and left edges of the paper) 3. Draw spots on top of each cross using different colours 4. Write the colour underneath each coloured dot, using a pencil 5. Fold the paper into a cylinder and fasten with a paper clip 6. Add water into a beaker at the depth of ½ cm (make sure that the water line is below the pencil line) and stand the folded paper into the beaker 7. Wait for coloured spots to separate up the paper and then remove from water and leave to dry. Analysing chromatograms Start Finish 1) How many colours in the red dye? 2) Which colours is the purple dye made out of? 3) Which dyes are made using yellow colour? Analysing chromatograms 1) How many dyes were used in Black? 2) Are the yellows in brown and orange the same? How do you know? Calculating Rf values • A chemical can also be identified by its Rf (Retention Factor) • It is the ratio of the distance travelled by the sample (spots) to the distance travelled by the solvent (water). • The formula is: The answer is never greater than 1. Calculating Rf values Calculate Rf for the three spots. Substance E120 E133 E124 Distance moved by sample (cm) 7.2 4.1 9.5 Distance moved by solvent front (cm) 10 10 10 Rf 0.72 0.41 0.95
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