C2 - Identify the ions in an ionic compound using flame tests

• An ionic compound contains a positive ion (cation) and a negative ion (anion). These both need to be identified to work out the identity of an unknown ionic compound.

• Cations can be identified with flame tests.

Some metal cations have distinctive colours when held in a blue Bunsen burner flame.

A nichrome wire is used to hold the test solid – nichrome has no flame colour of its own and it has a high melting point.

• Bunsen burner, heatproof mat

• Nichrome wire (in holder)

Ionic compounds to test

• Sodium chloride, calcium chloride, lithium chloride, potassium chloride, copper(II) sulfate (1 g of each salt)

• Concentrated hydrochloric acid

Safety: Follow the safety advice given by a teacher.

1. Take a piece of nichrome wire with a loop at one end.

2. Firstly clean the nichrome wire by dipping the loop in the concentrated hydrochloric acid and placing it in the hot blue flame of the Bunsen burner.

3. Repeat until the flame is no longer coloured, i.e. impurities are removed.

4. Dip the clean nichrome wire in concentrated hydrochloric acid and then into the solid ionic compound which is sitting in a watch glass.

5. Hold the loop with the solid ionic compound in the hottest part of the flame and record the characteristic colour.

6. Repeat for all the solid ionic compounds available, cleaning the wire loop each time.

Table of results