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Forensic Science

Forensic Science. 4.7 Testing For Ions. Learning outcomes. Be able to test for positive and negative ions Be able to name a compound after identifying it’s ions. Chemical analysis. Necessary to analyse chemical evidence Illegal drugs Composition of solution Most appropriate method

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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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