Lesson 5

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 What is the test for the following substances?substances?

• Hydrogen• Carbon dioxide• Ammonium ions

Testing for Hydroxide, Testing for Hydroxide, Carbonate, Sulphate and Carbonate, Sulphate and

Sulphite IonsSulphite Ions1. Universal Indicator – check pH2. Add HCL. If a gas is produced, test it with:

•Universal Indicator paper•Limewater

3. Add dilute HCL, then add barium chloride solution

ResultsResultsAnion present

Hydroxide ion,

Carbonate ion

Sulphite ion

Sulphate ion

effect onuniversalindicator

effect of addinghydrochloric

acid

effect of addingbarium chloridefollowed byhydrochloric

acid

Practical – Testing for Practical – Testing for Chlorine, Bromine and Chlorine, Bromine and

IodineIodineA 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).

halide ion chloride

ionbromide ion iodide ion

effect of adding acidified silver

nitrate solution

Chloride

IodideBromide

The samples to be tested are first acidified using dilute nitric acid, then silver nitrate solution is added.

White precipitate

of silver chloride

Cream precipitate

of silver bromide

Yellow precipitate

of silver iodide

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)

Word equation:nitric acid + magnesium carbonate______ + water + carbon dioxide

Balanced equation:2HNO3 + MgCO3 _____+ ____ + CO2

If nitric acid is added to magnesium carbonate (MgCO3), effervescence occurs. The magnesium carbonate slowly dissolves, forming the salt magnesium nitrate, water and carbon dioxide gas.

1. 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