Teacher Material



Mandatory Experiment 1.1

Flame tests (Li, Na, K, Ba, Sr, Cu)

Teacher Material

• Potassium salts are often contaminated with sodium. For this reason the yellow colour of sodium may interfere with the lilac colour of the potassium.

• Each metal gives off a characteristic colour. These colours are lithium (deep red), sodium (yellow), potassium (lilac), barium (yellow-green), strontium (red) and copper (blue-green).

• If wooden splints are being used, do not leave them in the flame for too long. Gently pass through the flame and observe the colour.

• If too much salt adheres to the splint, there is a danger of some falling into the Bunsen burner. This is likely to cause a prolonged colour tinge in the flame, making the Bunsen burner unsuitable for further use in this experiment.

• An alternative to soaking wooden splints in water is to soak them overnight in solutions of the salts to be tested. This eliminates the potential problem of contamination of the Bunsen burner by solid material.

• Nichrome wire may be used instead of platinum wire in Method 1.

• If a platinum or nichrome wire not held in a glass rod is used, then a tongs should be used to hold the wire in the flame.

• The wooden splint method has two advantages: The problem of cross contamination is eliminated, and there is no need to use concentrated hydrochloric acid for cleaning.

Safety considerations

Safety glasses must be worn.

Chemical hazard notes

Concentrated hydrochloric acid [pic]: Very corrosive to eyes and skin, and its vapour is very irritating to lungs.

Barium chloride [pic]n is harmful by ingestion and inhalation.

Copper(II) chloride [pic] is toxic if swallowed, and is an eye and skin irritant.

Lithium chloride [pic]n is harmful by ingestion, and is a severe eye and skin irritant.

Potassium chloride is an eye irritant.

Disposal of wastes

Dilute with excess water, neutralise (if necessary) with anhydrous sodium carbonate, and flush to the foul water drain.

Solutions to student questions

1. What colour is observed in each case when the following salts are heated in a flame: copper sulfate, lithium carbonate, sodium sulfate, strontium nitrate, barium nitrate, potassium sulfate?

Copper sulfate: Blue-green

Lithium carbonate: Deep red

Sodium sulfate: Yellow

Strontium nitrate: Red

Barium nitrate: Yellow-green

Potassium sulfate: Lilac

2. Why do metals give off a characteristic colour in a flame?

The energy differences between energy levels in metal atoms vary from metal to metal. Using energy from the flame, electrons in the metal atoms move to higher energy levels, and then return to lower energy levels, emitting light whose energies in each case is equal to the energy difference between the higher energy level and the lower energy level.

3. What procedure is used to avoid cross contamination?

If platinum wire is used, either a separate platinum wire is used for each test, or the wire is thoroughly cleaned with concentrated hydrochloric acid after each test.

If wooden splints are used, a separate splint is used for each test.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download