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Criteria of Purity

Criteria of Purity . a) Describe paper chromatography and interpret chromatograms b) Describe how chromatography techniques can be applied to colourless substances. c) Identify substances and test their purity by melting point and boiling point determination and by paper chromatography.

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Criteria of Purity

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  1. Criteria of Purity a) Describe paper chromatography and interpret chromatograms b) Describe how chromatography techniques can be applied to colourless substances. c) Identify substances and test their purity by melting point and boiling point determination and by paper chromatography. d)Explain the importance of purity in substances in everyday life. Eg. in foodstuffs and drugs.

  2. Chromatography • method of separating and identifying mixtures; • used to separate the solutes in a mixture • Eg. to identify artificial dyes in food  can be used to identify very tiny amounts of substances

  3. Chromatography

  4. Chromatography

  5. Chromatography • principle involved depends upon the different solubilities of the substances in the mixture in the solvent used – the most soluble solute will be carried quickly up the paper with the solvent while the least soluble solute will be left behind. • Steps involved

  6. Chromatography • a dye strongly adsorbed onto paper, and not very soluble in solvent will be left behind • a dye weakly adsorbed onto paper, and very soluble in solvent will be carried furthest • simulation

  7. Chromatography solvent front ensure that the spots are above the solvent; and the solvent just touches the tip of the paper Start line drawn with pencil

  8. Chromatography - uses • To separate (a) pigments from plants; (b) dyes from ink; (c) amino acids from proteins for analysis. • To identify poisons and drugs, or to detect traces of unlawful dyes or other additives in foodstuffs. • To determine the purity of a given substance.

  9. Criteria of Purity • Pure substance - substance with no other substances mixed with them • has fixed (sharp) melting point and boiling point • Mixture • A mixture does not have definite properties. • It melts or boils over a range of temperatures

  10. Checking the Purity • Checking its melting point • Checking its boiling point No two substances have the same pair of melting and boiling pts. Hence melting and boiling pts. can be used to identify a substance. Using chromatography

  11. Determination of boiling point

  12. Checking purity - with mpt. & bpt • Impurites will lower the melting point and raise the boiling point • For an impure substance, the temperature does not stay constant during boiling/melting.

  13. Checking purity - using chromatography • A pure substance will give only one spot in the chromatogram. Mixture Pure substance

  14. Produces only one spot Produces more than one spot Chromatography Can be differentiated by Mixtures Pure substances Has sharp melting/boiling point Melts/boils over a range of temperatures Melting/Boiling pt.

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