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Lesson 5

Lesson 5. Looking for Anions. Learning Objectives: Describe the test for specific negative ions Explain how precipitation reactions can be used as the test for some ions. What is the test for the following substances? Hydrogen Carbon dioxide Ammonium ions.

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Lesson 5

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  1. Lesson 5 Looking for Anions • Learning Objectives: • Describe the test for specific negative ions • Explain how precipitation reactions can be used as the test for some ions

  2. What is the test for the following substances? • Hydrogen • Carbon dioxide • Ammonium ions

  3. Testing for Hydroxide, Carbonate, Sulphate and Sulphite Ions • 1. Universal Indicator – check pH • 2. Add HCL. If a gas is produced, test it with: • Universal Indicator paper • Limewater • 3. Add dilute HCL, then add barium chloride solution

  4. Results

  5. Practical – Testing for Chlorine, Bromine and Iodine A Place 2 cm of each of the halide ion solutions in three different test tubes. B Acidify each solution by adding a few drops of nitric acid solution. C Add a few drops of silver nitrate solution to each test tube. D Observe what happens (note colours carefully).

  6. Reactions with silver nitrate The samples to be tested are first acidified using dilute nitric acid, then silver nitrate solution is added. Bromide Iodide Chloride Cream precipitate of silver bromide Yellow precipitate of silver iodide White precipitate of silver chloride

  7. Equations A white precipitate shows the presence of chloride ions Ag+(aq) + Cl−(aq) → AgCl(s) A cream precipitate shows the presence of bromide ions Ag+(aq) + Br−(aq) → AgBr(s) A yellow precipitate shows the presence of iodide ions Ag+(aq) + I−(aq) → AgI(s)

  8. Ifnitric acid is added to magnesium carbonate (MgCO3), effervescence occurs. The magnesium carbonate slowly dissolves, forming the salt magnesium nitrate, water and carbon dioxide gas. Word equation: nitric acid + magnesium carbonate  ______ + water + carbon dioxide Balanced equation: 2HNO3 + MgCO3 _____+ ____ + CO2

  9. Questions • When barium chloride solution is added to lithium sulphite solution, a white precipitate of barium sulphite is formed, leaving a solution of lithium chloride. • lithium sulphite + barium chloride  ________ + __________ • Balanced equation: • Li2SO3(aq) + BaCl2(aq)  __________ + BaSO3(s) 2. When barium chloride solution is added to potassium sulphate solution, a white precipitate of barium sulphate is formed leaving a solution of potassium chloride. Write a word and balanced symbol equation

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