Analytical Methods - Ion Tests (SQA National 5 Chemistry): Revision Note

Flame tests

• A flame test is a simple laboratory technique used to identify a metal ion based on the characteristic colour it produces when heated in a Bunsen flame

How to carry out a flame test

1. Clean a wire loop (usually made of unreactive nichrome) by dipping it in dilute hydrochloric acid

2. Heat the loop in a roaring blue Bunsen flame until no colour is produced

3. Dip the clean loop into the solid sample to be tested

4. Place the end of the loop back into the hot flame and observe the colour

Flame test method

Flame test results

Precipitation tests for ions

• A precipitation reaction occurs when two different solutions are mixed and they react to form an insoluble solid

  • This insoluble solid is called a precipitate

• Essentially, you are mixing two soluble compounds to create one insoluble one

Predicting when a precipitate will form

• To work out if a precipitate will form when two solutions are mixed, you need to know the solubility of the potential new products

• The solubility rules for common ionic compounds are given on page 8 of the SQA Data Booklet

• You can use this table to predict whether a substance is soluble or insoluble

The method

1. Identify the ions present in the two starting solutions

2. Work out the two new "swapped partner" compounds that could form.

3. Use the table on page 8 of the Data Booklet to look up the solubility of these two new compounds

4. If one of the new compounds is listed as insoluble (or very slightly soluble), then it will form a precipitate

Using precipitation to identify ions

• Precipitation reactions are very useful as chemical tests to identify the presence of specific ions in a solution

• If you know that a certain ion forms a distinctively coloured precipitate with a particular reactant, you can use this to test for it.

Testing for chloride ions (Cl^-)

• Add a few drops of silver nitrate solution (AgNO₃ (aq))

• The formation of a white precipitate (silver chloride) indicates that chloride ions are present

Testing for sulfate ions (SO₄^2-)

• Add a few drops of barium chloride solution (BaCl₂ (aq))

• The formation of a white precipitate (barium sulfate) indicates that sulfate ions are present

Test for carbonate Ions (CO₃^2-)

• Add a small amount of any dilute acid, such as HCl (aq)

• Fizzing / effervescence suggests that carbon dioxide gas is produced

  • This can be confirmed by bubbling it through limewater, which would turn cloudy

  • For more information about testing for carbon dioxide, see the Gas Tests revision note