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 + CDAD + CB Lead nitrate + potassium iodidelead iodide + potassium nitrate Colourless solutions Mix together… what is formed Lead iodide forms as a precipitate Precipitation reaction

5. 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: 1.Positive metal ions 2.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