LiNaKCa flame tests

Group 1 metals react with water to produce hydroxides. For example, sodium reacts to produce sodium hydroxide and potassium would give potassium hydroxide, as shown in the equations below:

sodium + water -> sodium hydroxide + hydrogen
2Na + 2H2O -> 2NaOH + H2
potassium + water -> potassium hydroxide + hydrogen
2K + 2H2O -> 2KOH + H2

These hydroxides dissolve in the water as they are produced and the water becomes alkaline. Hence the Group 1 metals are called The Alkali metals.

Most simple alkali metal compounds are white, crystalline solids which are soluble in water. The alkali metal chlorides, LiCl, NaCl and KCl are good examples, which appear as identical white, crystalline salts when encountered in the school laboratory.

One way to tell the difference between these Group 1 chlorides would be to carry out flame tests on them.

Here a small sample of a each salt is introduced into a blue Bunsen burner flame. Can you identify the metal in each of the flame tests on the basis of its colour? And which of the four samples is the odd one out?

Answers are given in the second set of four pictures, which follow in the same sample order.

Crimson red

Crimson red

Yellow

Yellow

Lilac

Lilac

Orange red

Orange red

Of course, the colours that we see in these images depend on a number of things including the camera used to record the images and computer screen used to view them. The colours we actually see with our eyes during such experiments may differ in shade and intensity from those shown here.

The four animated .gif images above were assembled from images taken with a Casio FH100 digital camera (which has a 10.1MP 1/2.3-inch backlit CMOS sensor).

By way of contrast the still images below were recorded using the same samples under the same conditions with a Ricoh Caplio GX100 digital camera (10.01 MP CCD 1/1.75-inch primary-color sensor).

Lithium chloride flame test

Lithium chloride flame test

Sodium chloride flame test

Sodium chloride flame test

Potassium chloride flame test

Potassium chloride flame test

Calcium nitrate flame test

Calcium nitrate flame test

The differences between the two image sets are quite striking.

In school textbooks the colours are recorded as; lithium = crimson red, sodium = yellow, potassium = lilac and calcium = red (sometimes as orange red or brick red).

Rubidium chloride and caesium chloride also produce characteristic flame test colours. What are they?

The Group 1 alkali metals all react vigorously with cold water. Calcium reacts a little less vigorously and is in Group 2 of the Periodic Table, The Alkaline Earth metals.

As such is the odd one out of the four flame tests shown above.

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One thought on “LiNaKCa flame tests

  1. I’m liking the fact that the gif plays in my inbox without having to go to the site. I think I’ll use this method for short tech how to videos; one less click is always a good thing. Do you know of software that converts video into gif?

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