2. Learning Objectives: Describe and explain the tests for ions using sodium hydroxide solution Explain how precipitation reactions can be used to test for some ions Lesson 3

3. Silver nitrate and sodium chloride dissolve in water. The solutions are colourless. Mix the solutions and a white precipitate forms.

4. A solution of sodium chloride...... contains sodium ions, Na +, and chloride ions, Cl -.

5. A solution of silver nitrate...... contains silver ions, Ag +, and nitrate ions, NO 3 -.

6. Mixing the two solutions puts all the ions together. The silver and chloride ions immediately form a white precipitate of solid silver chloride. The sodium and nitrate ions remain in solution.

7. AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq) Silver nitrate + sodium chloride  silver chloride + sodium nitrate A chemical equation summarises this precipitation reaction.

8. Practical – Testing for Cations A Put 1 cm depth of each of the cation solutions into six different test tubes. BAdd a few drops of sodium hydroxide solution to each test tube, and observe what happens. CIf a precipitate forms, add a further 1 cm of sodium hydroxide. Again observe any changes that might occur.

9. CationSymbolPrecipitate AmmoniumNH 4 + (aq)none AluminiumAl 3+ (aq)White CalciumCa 2+ (aq)White Copper(II)Cu 2+ (aq)blue Iron(II)Fe 2+ (aq)Green Iron(III)Fe 3+ (aq)brown 1. In the cation column what does the number in the bracket correspond to? 2. Which of the cation(s) need further testing ? Reactions of adding Sodium hydroxide

10. Further testing Ammonium Ions Heat the sample with concentrated sodium hydroxide. A smelly alkaline gas (ammonia) will be produced. It turns universal indicator paper blue/purple.

11. Aluminium and Calcium Ions If you add excess (more!) sodium hydroxide to a calcium precipitate and aluminium precipitate, different things will happen. Aluminium precipitate Calcium precipitate no change, will remain white Will dissolve Excess NaOH

12. Task Worksheet – C3.3 3b ‘More about Cations’ Only questions 1, 2 and 3. DO NOT WRITE ON THE SHEET!