Flame Tests for Ions

1
Flame Tests for Ions

• Flame Tests Using an Improvised Alcohol Burner

2
Purpose

• When a metal salt solution is burned, the metal
  provides a colored flame.
• Each metal ion gives a different colored flame.
• Flame tests, therefore, can be used to test for
  the absence or presence of a metal ion

3
From the DemonstrationProcedure taken from
Dragojlovic, V. Flame tests using improvised
alcohol burners. Journal of Chemical Education
(76)7, 929.

• Metal salt solutions in a methanol solvent were
  burned in 5mL vials with paper wicks.
• Two unknowns were also tested.
• Each solution provided a different colored flame.

• Li Red
• K Violet
• Ca Orange
• Na Yellow
• Ba Yellow- green
• Unknown A Red
• Unknown B Yellow

4
Important Concepts

• Atomic Orbitals
• Energy States of Atoms
• Ions
• Atomic Emission

5
Atomic Orbitals

• Electrons of atoms reside in concentric spheres
  known as energy shells in which they orbit the
  nucleus of an atom.
• Each shell is assigned a principal quantum
  number, n.
• The value of n is an integer, 1, 2, 3, etc.
• This number determines the relative energy of the
  orbital and relates the distance from the shell
  to the nucleus---the lower the number, the lower
  the energy of the electron and the closer it is
  to the nucleus
• Electrons can be further distinguished according
  to their location in atomic orbitals, specified
  regions in space that depend on their energies

6
Energy States of Electrons

• A ground state refers to the lowest energy of
  system, in this case an atom. All electrons are
  in shells with the least energy.
• As energy is applied to the atom, an electron is
  promoted from its ground state (residence in an
  energy shell) to a higher energy shell or an
  excited state.
• The excited state has a higher n value, has more
  energy, and is less stable.

7
Atomic Emission

• An emission spectra is obtained by adding energy
  to a material.
• The addition of energy promotes electrons of that
  material from the ground state to the excited
  state.
• As the electrons fall from the excited state to
  the ground state, they emit the energy they
  absorbed in the form of electromagnetic radiation
  (heat, light, etc.)

8
Ions

• Ions are charged atoms we use metal salts in
  this demonstration which contain metal cations
  and some non-metal anion
• For example, sodium chloride has neither sodium
  atoms nor chlorine atoms in itrather, it is made
  of sodium ions Na and chloride ions, Cl-
• The sodium ion is the cation and the chloride ion
  is the anion
• Ions undergo atomic emission and give
  characteristic colors
• This is a way to identify what ion might be
  present

9
Conclusions

• Each metal provides a different colored flame by
  which it may be distinguished from other metals.
• The color of the flame is caused by the demotion
  of electrons from an excited energy state to a
  ground state.
• Electrons fall from higher energy states to lower
  energy states which have smaller n values and are
  more stable.

10
Comments

• Atomic emission is used in street lamps,
  fluorescent lights, and neon signs.
• Two common street lamps using this are the
  mercury lamp and the sodium lamp.
• Neon signs frequently implement the emission
  spectra of other gases such as argon and krypton.
• Very sophisticated instrumental techniques such
  as flame photometry and atomic absorption are
  based on the principles in this demonstration

11
Miniquiz on Flame TestsTrue or False

• Answers
• False. A red flame is indicative of Li.
• True.
• True.
• False. The higher the n value, the less stable
  the orbital.
• True.

• The color of the first unknown was red, therefore
  it was calcium.
• The color of the second unknown was yellow,
  therefore it was sodium.
• When an electron falls it emits energy in the
  form of electromagnetic radiation.
• The greater the n value, the higher the energy,
  the more stable the orbital.
• The n value corresponds to the principle quantum
  number.