STUDENT INSTRUCTIONS - PDST



Mandatory Experiment 1.1

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

Student Material

Theory

The characteristic colours given off when salts of certain metals are heated in the flame of a Bunsen burner are noted. The colours obtained can then be used to identify the metals present in unknown compounds.

Method 1 – using a platinum wire

Chemicals and Apparatus

Lithium chloride [pic]n

Sodium chloride

Potassium chloride

Barium chloride [pic]n

Strontium chloride

Copper(II) chloride [pic]

Concentrated hydrochloric acid [pic]

Platinum (or nichrome) wire held in glass rod

Bunsen burner

Six small beakers

Test tubes

Pestle and mortar

Procedure

NB: Wear your safety glasses.

1. Using the fume cupboard, place some concentrated hydrochloric acid in a test tube. Dip the platinum wire into the concentrated hydrochloric acid in the test tube. This helps to clean the wire.

2. Hold the platinum wire at the tip of the blue cone of a Bunsen flame. If the wire is clean there should be no colour from it when it is placed in the Bunsen flame. If there is a colour, repeat the cleaning process.

[pic]

Fig. 1

3. Crush a little of the salt to be tested with a pestle and mortar.

4. Again dip the platinum wire in concentrated hydrochloric acid and then in the salt to be tested. Some of the salt should stick to the wire.

5. Hold the platinum wire in the flame of the Bunsen burner (Fig. 1) and note the colour given off.

6. Repeat the experiment for each of the other salts given to you by your teacher. (To avoid cross contamination, it is best to have a separate platinum wire for each sample of salt. If this is not possible, leave the sodium salt until last, as its colour is very persistent.) Again, note the colour in each case. If you are given an unknown salt, you should be able to identify the metal in the salt from the results of your experiment.

Method 2 – using a soaked wooden splint

As platinum wire is expensive, many schools may not possess it in sufficient quantities for this experiment. Wooden splints (e.g. lollipop sticks) soaked overnight in water work quite well in place of the platinum. You will need a wooden splint for each sample to avoid cross contamination.

Chemicals and Apparatus

Lithium chloride [pic]n

Sodium chloride

Potassium chloride

Barium chloride [pic]n

Strontium chloride

Copper(II) chloride[pic]

Bunsen burner

Six small beakers

Test tubes

Pestle and mortar

Wooden splints (e.g. lollipop sticks)

Procedure

NB: Wear your safety glasses.

1. Crush a little of the salt to be tested with a pestle and mortar.

2. Label 6 beakers, and place a small quantity of the corresponding salt in each.

3. Dip the soaked splint in the salt to be tested. Some of the salt should stick to the splint.

4. Gently wave the splint in the flame of the Bunsen burner and note the colour given off. Be careful not to hold the splint too long in the flame, as charring may occur and give a false colour.

5. Repeat the experiment for each of the other salts. To avoid cross contamination, use a separate splint for each salt. Again, note the colour in each case.

6. If you are given an unknown salt, you should be able to identify the metal in the salt from the results of your experiment.

Questions relating to the experiments

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?

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

3. What procedure is used to avoid cross contamination?

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