Chemistry-OCR/MODULE 3

Qualitative Analysis

Carbonate test: test based on a gas

This test forms the basis for a test for the carbonate ion -

Carbonates react with acids to form carbon dioxide gas.

1.     In a test tube add dilute nitric acid to the solid or solution to be tested

2.     If bubbles are present, then the unknown compound could be a carbonate

3.     To prove the gas is carbon dioxide:

a.     Bubble the gas through limewater-

b.     It reacts to form a fine white precipitate of calcium carbonate, which turns limewater cloudy

Tests based on precipitates:

Sulphate test

Barium sulphate is not soluble in water, whereas most sulphates are

The ionic equation for the reaction is below:

The barium ions are added as aqueous barium chloride or barium nitrate.

 If you intend to test for a halide, then use barium nitrate- with barium chloride, you are introducing chloride ions to your solution

Halide Tests:

Most halides are soluble except silver halide

Aqueous silver ions react with aqueous halide ions to form a precipitate of silver halide.

1.     Add aqueous silver nitrate to an aqueous solution of a halide

2.     The silver halide precipitates are different colours

3.     Add aqueous ammonia to test the solubility of the precipitate.

a.     Chlorine- soluble in dilute ammonia

b.     Bromine- soluble in concentrated ammonia

c.     Iodine-insoluble in concentrated ammonia

Sequence of tests:
For anions the correct order of tests is:

1.     Carbonate

2.     Sulphate

3.     Halides

-In the carbonate test, you add dilute acid and are looking for effervescence from carbon dioxide gas

Neither sulphate or halides form bubbles with dilute acid. If no bubbles are present, then there is no carbonate ion

-You add a solution containing barium ions and are looking for a white precipitate of barium sulphate.

Barium carbonate is white and insoluble in water so if you carry out a sulphate test on a carbonate you will get a while precipitate.

Therefore, it is important that carbonates are tested for first and only proceed to the sulphate test if no carbonates are present

-Add a solution of silver ions to produce a coloured precipitate

Silver carbonate and silver sulphate are both insoluble in water and will produce precipitates. So a halide test must be done last after carrying out the carbonate and sulphate tests.

A mixture of ions in one solution:
1. Carbonate test:

-If bubbles are seen continue adding dilute nitric acid until bubbles stop forming

-All the carbonate ions would have reacted so none left to interfere with the other tests.

-If you are going to test for sulphate or halide ions use dilute nitric acid instead of hydrochloric acid or sulphuric acid.

-Sulphuric acid contains sulphate ions and hydrochloric acid contains chlorine ions which will show up for the halide and sulphate tests.

2.Sulphate test:
-Add an excess barium nitrate solution

-any sulphate ions will precipitate out as barium sulphate

-don’t use barium chloride as the chlorine ions affect the halides test

-filter the solution to remove the barium sulphate

3.Halide test:
-add silver nitrate

-any carbonate or sulfate ions have been removed so any precipitate formed must involve halides.

-add ammonia to confirm which halide

Test for cations:
Test for ammonium ion

-heat together ammonium ions and aqueous hydroxide ions to form ammonium gas

-aqueous sodium hydroxide is added to a solution of an ammonium ion

-ammonia gas is produced- likely to smell the gas rather than see it as ammonia is very soluble in water

-the mixture is warmed and the ammonia gas is released.

-easy to test for the gas by using a moist pH indicator paper. –Ammonia alkaline and a presence will turn the paper blue.

Summary Questions:
1. You could add silver nitrate to each of the solutions and then add aqueous ammonia to see their solubility. NaCl will form a white precipitate and will be soluble in dilute ammonia. NaBr will form a cream precipitate and will be soluble in concentrated ammonia. NaI will form a yellow precipitate but will be insoluble in concentrated ammonia.

2. A carbonate test is done first because barium nitrate can react with the carbonate ions to form barium carbonate which is also white and insoluble in water.

3. It’s important to use nitric acid rather than hydrochloric acid or sulfuric acid as they introduce sulfate and chloride ions which will then intervene with the halide and sulphate tests to form precipitates.