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Sunday, February 6, 2011

Flame Tests

Flame tests may not be the most accurate, nor do they work for gold, but they can be really effective for other elements.  Boron is one that imparts a greenish color to a flame.  This gives rise to the story of how the 20 Mule Team Borax Company got started.  It seems a prospector who was down on his luck was prospecting Death Valley, California with his wife when he tried a flame test on some of the dried salts he found in the dessert.  It burned green so he turned to his wife and said, “She burns green Ma, we’re rich!”

A chemist performing a flame test on copper sulfate.  Notice the blue color of the flame.

Flame tests are conducted on a length of thin platinum wire that is first dipped in a dilute solution of hydrochloric acid then in the pulverized sample of the substance to be tested; then it is held into a hot flame.

Depending on the element contained in the sample it colors the flame various colors.  This technique will present a pretty accurate idea of what king of substance you have, but the accuracy of the test can be improves with a spectroscope.

Table 1 shows the colors of the flame test of various elements:

Symbol
Name
Color
As
B
Ba
Pale/Apple green
Ca
Brick red
Cs
Blue-Violet
Cu(I)
Blue
Cu(II)
Copper(II) (non-halide)
Green
Cu(II)
Copper(II) (halide)
Fe
In
Blue
K
Li
Mn (II)
Manganese (II)
Mg
No colour
Mo
Na
P
Pb
Blue
Rb
Sb
Pale green
Se
Sr
Te
Pale green
Tl
Pure green
Zn
Bluish green

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