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10.1 10Acids, Bases and Salts

Identify sodium, calcium, strontium, barium,
copper and potassium ions by flame test.

Date ________________

Flame test is used to identify the presence of some metal ions in a
compound. It is a very quick way to identify these ions. Not all the metal ions
impart colour to the flame.

The alkali and alkaline earth metals have loosely bound valence

electrons that are capable of being excited at the temperature of the Bunsen

flame. In the flame, the volatile salts evaporate and dissociate into ions.

Electrons, which are plentiful in the flame, fall from outside the cations into the

valence sub-shells with the emission of energy in the form of visible light. The

colour of the emitted light is characteristics of the element. The bright orange

spectral lines of sodium in the visible region are associated with the transition of

this outermost electron from the 3p sub-shell (excited state) back to 3s sub-shell.

Chloride salts of metals are usually preferred for performing flame test
because these are generally more volatile than the other salts and hence easily
impart a characteristic colour to the flame.

Spectrum of elements can also be taken when they are enclosed in the
form of a gas in the discharge tube. When the atoms present in the gaseous state
are supplied with energy, an electron is raised from a low energy level to a higher
level. When the same electron drops back, energy equal to the difference
between the two levels is emitted in the form of light radiations. If these
radiations fall in the visible range, they can be seen as colours in the form of
spectrum.

E= hc
l

e- Atomic e-
Excitation

E=-lhc e-
e-

Atomic
De-excitation

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