Practice Assignment 06
How do they put the colours into fireworks?
This assignment provides practice in performing flame tests, though the purpose of the tests is reversed. The standard procedure is for identifying the cation present in an unknown sample, e.g. a mineral. The practice assignment uses the technique to investigate which cations could be used to impart colours to fireworks, and which types of salt give the brightest flame colours.
The exercise requires group organisation and self-discipline. Students should appreciate that their results will be confused and misleading if samples become mixed-up or cross-contaminated. It is important that each sample is clearly labelled and that the wire loop is returned to its correct sample. Even so, the wire should be cleaned before each test, in case of contamination from fingers or the bench.
Risk of cross-contamination between samples may be reduced by setting up a ‘circus’ of testing stations around the lab; one per salt. In this case it is not necessary for students to clean the watch glass between tests.
Salts should be moistened with water, as stated in PA 06, not hydrochloric acid, as per SP 0006-2. This is because one of the objectives is to compare different salts of the same metal. (Hydrochloric acid is used in the standard procedure to convert whatever mineral salt is present to the more volatile chloride to give a better flame test.)
Health and safety note
Concentrated hydrochloric acid is corrosive. Dropping bottles should minimise risk of spillage, and also facilitate use of minimum amounts. (The procedure states 2 drops.)
Apparatus and Reagents
- Standard Procedure:
- SP 0006-2:2005 Chemical tests for identifying cations and anions in minerals – Part 2: Flame tests
- watch glass
- nichrome wire with small loop at one end
- concentrated hydrochloric acid
Caution: corrosive
• samples of: - calcium chloride
- copper chloride
- lithium chloride
- potassium chloride
- sodium chloride
- • samples of at least one other salt of each of the above metals