Expt:Testing for negative ions

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Apparatus and chemicals

The exact concentrations of the test solutions are not important. Use approximately 0.1–0.5 mol dm^–3 for salt solutions and 0.5–1.0 mol dm^–3 for acid solutions, except for nitric acid, which is corrosive at such concentrations (use 0.4 mol dm^–3 instead).

• Eye protection
• Test-tubes
• Dropping pipettes (these can be used just for the carbon dioxide testing or also for dispensing solutions; if the latter, far more pipettes will be required)
• Nitric acid 0.4 mol dm^–3 (Irritant)
• Silver nitrate solution 0.1 mol dm^–3
• Barium chloride solution 0.1 mol dm^–3 (Harmful)
• Hydrochloric acid
• Aluminium powder (Highly flammable)
• Sodium hydroxide solution less than 0.5 mol dm^–3 (Irritant)
• Limewater
• Red litmus paper
• Ammonia solution 0.4 mol dm^–3.

For the initial observations

• Sodium chloride or potassium chloride solution
• Sodium bromide or potassium bromide solution
• Sodium iodide or potassium iodide solution
• Sulfate solution, eg sodium sulfate
• Carbonate solution, eg potassium carbonate
• Nitrate solution, eg potassium nitrate.

For testing unknowns The number of unknowns required depends on the time available. It is a good idea to use at least four solutions to ensure students are challenged. Label the solutions A, B, C etc and make sure you know which is which.

Technical notes

Barium chloride solid is toxic; the 0.1 mol dm^–3 solution is harmful. Wash your hands after use and warn students to do the same.

Ammonia solution is an irritant when concentrated but not at the concentrations used by students in this activity. However, it can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use. Nitric acid is an irritant. Silver nitrate solution can stain skin and clothes.

Student worksheet - making observations

This activity is in two parts. In the first part you observe the reactions of various negative ions and in the second you use those observations to identify unknown solutions.

Use the table 'Tests for negative ions' to record your observations during each test. Use a clean test-tube each time or wash up thoroughly between tests using distilled or deionised water to avoid contamination. Use a small portion of the test solution each time (no more than 1 cm³).

Write balanced symbol equations for the reaction that occurs in each of the tests (except the test for a nitrate).

Health & safety

Wear eye protection. Take extra care when dealing with unknown solutions.

At the concentrations used in this experiment:

• Barium chloride solution is harmful; wash your hands after use.
• Sodium hydroxide is an irritant.
• Ammonia solution can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use.
• Nitric acid is an irritant.
• Silver nitrate can stain skin and clothes.

Tests for negative ions

Student worksheet - identifying unknowns

Using the observations chart you made in Testing for negative ions – making observations, test the unknown solutions provided and identify the negative ions present. Make careful observations, including any negative results. You may need to try a number of tests before you get a positive result. Design a table to record your observations. You may wish to use the headings: Unknown sample; Test tried; Observations; and Conclusion.

Health & safety

Wear eye protection. Take extra care when dealing with unknown solutions.

At the concentration used in this experiment:

• Barium chloride solution is harmful; wash your hands after use.
• Sodium hydroxide is an irritant.
• Ammonia solution can give off ammonia vapour, which can irritate the eyes and lungs. Keep the lid on the bottle when not in use.
• Nitric acid is an irritant.
• Silver nitrate can stain skin and clothes.

Reference

This experiment has been reproduced from Inspirational Chemistry, Royal Society of Chemistry, London, p.177-179 and Index 7.3.1

Further problem solving ideas

There are several suggestions in C. Wood, Creative Problem Solving in Chemistry, London: Royal Society of Chemistry, 1993.