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Forensic Science

This lesson on forensic science covers the essential testing methods for identifying positive and negative ions in solutions. Learners will understand the significance of chemical analysis in detecting illegal drugs and analyzing chemical evidence. The module focuses on precipitation reactions, where two solutions form an insoluble solid, aiding in the identification of substances. Detailed procedures for testing specific ions using sodium hydroxide and other reagents are provided, along with the interpretation of results, including the color of precipitates and the production of gas.

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Forensic Science

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  1. Forensic Science 4.7 Testing For Ions

  2. Learning outcomes • Be able to test for positive and negative ions • Be able to name a compound after identifying it’s ions

  3. Chemical analysis • Necessary to analyse chemical evidence • Illegal drugs • Composition of solution • Most appropriate method • Quickest • Simplest • Cheapest • Identity of a chemical may provide vital evidence

  4. Ionic compounds • In chemical reactions, ions swap partners AB + CD AD + CB Leadnitrate + potassiumiodideleadiodide + potassiumnitrate • Colourless solutions • Mix together… what is formed • Lead iodide forms as a precipitate • Precipitation reaction

  5. Precipitation reaction • 2 solutions react to form an insoluble solid or suspension • Colour of solid can identify the reactants • Use to identify contents of solution • Polluted river – where’s it coming from • 2 tests needed: • Positive metal ions • Negative non-metal ions

  6. Sodium Hydroxide test • Positive ions based on colour of precipitate • 2-3cm3 of sample • Add few drop of NaOH solution • What colour precipitate is produced • Unique for a particular metal

  7. Testing for non-metal ions • Test varies depending on ions tested for • Carbonate – add dilute acid • Chloride – nitric acid, then silver nitrate • Sulphate – HCl then barium chloride • Results: • colour of precipitate • Production of carbon dioxide

  8. Key points • You can use precipitation reactions to identify ions in solution • A precipitate is an insoluble solid. It’s colour can help to identify a substance • The sodium hydroxide (NaOH) test uses this idea to identify metal ions AB + CDAD + CB precipitate

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