REQUIRED PRACTICAL 4

Test Tube Reactions

Simple test tube reactions can be done to identify the following ions:

Group 2 ions (M2+)

Ammonium ions (NH4+)

Halide ions (X-)

Hydroxide ions (OH-)

Carbonate ions (CO32-)

Sulfate ions (SO42-)

If the sample to be tested is a solid, then it must be dissolved in deionised water and made into an aqueous solution

Testing for Group 2 Metals

Four test tubes should be placed in a test tube rack

Around 10 drops of 0.1 mol dm-3 barium chloride solution should be added to the first test tube

Around 10 drops of dilute sodium hydroxide solution (NaOH) should be added to the same test tube

Swirl the test tube carefully to mix well

Continue to add sodium hydroxide dropwise to the test tube, until it is in excess

This should then be repeated in the other test tubes, for calcium bromide solution, magnesium chloride solution and strontium chloride solution

Any observations should be noted down in a suitable results table

The same test as above can also be done using ammonia solution and sulfuric acid solution

The positive results testing for the presence of Group 2 ions

Testing for Ammonium Ions

About 10 drops of a solution containing ammonium ions, such as ammonium chloride, should be added to a clean test tube

About 10 drops of sodium hydroxide should be added using a pipette

The test tube should be swirled carefully to ensure that it is mixed well

The test tube of the solution should then be placed in a beaker of water, and the beaker of water should be placed above a Bunsen burner, so that it can become a water bath

As the solution is heated gently, fumes will be produced

A pair of tongs should be used to hold a damp piece of red litmus paper near the mouth of the test tube, to test the fumes

The red litmus paper will change colour and become blue in the presence of ammonia gas

Litmus paper turns red, downloadable AS & A Level Chemistry revision notes

Damp red litmus paper turning blue in the presence of ammonia gas

Testing for Halide Ions

The sample being tested should be added using a pipette to a test tube

The test tube should be placed into a test tube rack

A small amount of nitric acid should be added to the sample using a pipette, followed by a small amount of silver nitrate solution

A precipitate will form, either white, cream or yellow, if a halide ion is present in the sample

Testing for halide ions results

The white, cream and yellow precipitates formed when halide ions react with silver nitrate solution

The white precipitate will form if chloride ions are present in the sample

The white precipitate is AgCl

The cream precipitate will form if bromide ions are present in the sample

The cream precipitate is AgBr

The yellow precipitate will form if iodide ions are present in the sample

The yellow precipitate is AgI

Further Test using Ammonia

AgCl precipitate

AgBr precipitate

AgI precipitate

Results of the test with ammonia to further distinguish between silver halide precipitates

Testing for Hydroxide Ions

A small amount (around 1 cm3) of the solution should be added to a test tube using a pipette

Test the pH of the solution using red litmus paper or universal indicator paper

The presence of hydroxide ions will turn the red litmus paper blue and the pH will be clearly alkaline on the universal indicator paper if hydroxide ions are present

Testing for Carbonate Ions

A small amount (around 1 cm3) of dilute hydrochloric acid should be added to a test tube using a pipette

An equal amount of sodium carbonate solution should then be added to the test tube using a clean pipette

As soon as the sodium carbonate solution is added, a bung with a delivery tube should be attached to the test tube

The delivery tube should transfer the gas which is formed into a different test tube which contains a small amount of limewater (calcium hydroxide solution)

Carbonate ions will react with hydrogen ions from the acid to produce carbon dioxide gas

Carbon dioxide gas will turn the limewater milky

When carbon dioxide gas is bubbled into limewater it will turn cloudy as calcium carbonate is produced

Testing for Sulfate Ions

Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride

If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

Sulfate Ion Test, IGCSE & GCSE Chemistry revision notes

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Exam Tip

HCl is added first to remove any carbonates which may be present and would also produce a precipitate and interfere with the results.