Tests for Positive Ions. 1.)Flame tests.

Learning Objectives. By the end of this session: Everyone will have carried out a flame test. Most students will be able to identify Lithium, Sodium, Potassium, Calcium and Barium from their flame tests. A few students will be able to identify an unknown ion using a flame test.

Starter. Watch what happens when I sprinkle some crystals into a Bunsen flame. Describe what you saw. Why do you think this happened? How could we use this? What are the disadvantages of this as an analytical technique?

Exciting Atoms. Whenever we give energy to an atom, we excite it. What this means is that the electrons move faster until they reach the point when they move into some of the higher, unoccupied electron shells. However, they do not stay there very long before they use up this extra energy and then they fall back to the shell they originally occupied. When this happens, any excess energy is released in the form of light, usually coloured light!

Exciting Atoms 2. The most interesting fact about this light is that each element releases light energy of a different wavelength (colour). So, we can use the characteristic colours to identify unknown atoms. That is what we will do today.

Safety. Concentrated Hydrochloric acid gives off fumes of hydrogen chloride and should only be used in the fume cupboard. Be careful with flames and hot wires. Treat all known and unknown compounds as if they were toxic. Goggles must be used.

Flame Tests.

Flame Tests. Using the apparatus on the previous slide, this is the procedure for flame tests. Heat the flame test wire tip until red hot, then dip in Conc. Hydrochloric Acid. Repeat 3 times then place the moist tip in the substance under test, allowing one crystal to stick to the wire. Heat substance in Bunsen flame, note colour. Repeat steps 1-3 for all substances. Clean flame test wire and tidy up.

Colours You should now have colours for the elements you have tested: Li: Na: K: Ca: Ba:

Colours You should now have colours for the elements you have tested: Li: Crimson (Red) Na: Golden yellow. K: Lilac. Ca: Scarlet (Red). Ba: Apple green.

Unknown Element. You have carried out flame tests on the known ion samples and seen the colours they produce. Now perform flame tests on the unknown ion. Note the colour of the flame, then identify the element present. Element X = (I suggest Barium is used)

Learning Objectives. By the end of this session: Everyone will have carried out a flame test. Most students will be able to identify Lithium, Sodium, Potassium, Calcium and Barium from their flame tests. A few students will be able to identify an unknown ion using a flame test.