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.

Alkali metals in water

All together now!

All together now!

The reactions of the alkali metals with water are amongst the more spectacular demonstrations carried out at school. Lithium, sodium and potassium are commonly used, rubidium and caesium are not because they are too reactive.

Lithium, sodium and potassium are all stored in oil to prevent them reacting with air. Even so, a thick layer of oxide accumulates with time as shown in the pictures here:

Lithium

Lithium

Sodium

Sodium

Potassium

Potassium

Being in the same Group of the Periodic Table they all react in the same way. For example, when lithium reacts with water the products are lithium hydroxide and hydrogen gas.
lithium + water -> lithium hydroxide + hydrogen
2Li + 2H2O -> 2LiOH + H2

Lithium at the top of Group 1 is the least reactive alkali metal. As for the others reactivity increases with each successive member as you go down the Group with Francium the most reactive at the bottom.

Lithium with water

Lithium with water

Sodium with water

Sodium with water

Potassium with water

Potassium with water

Experimental observations are often recorded in a table like this:Snap 2014-05-16 at 16.27.59All three metals disappear as they react, forming soluble hydroxides as products.
Lithium, sodium and potassium added to water

Lithium, sodium and potassium added to water

Rubidium and caesium are even more reactive. Videos showing rubidium and caesium reacting with water can found on the internet, but not francium as it is too reactive to be isolated.

Here are some questions students are often called upon to answer at school:
1. Why are the Group 1 metals called the alkali metals?
2. What colours do lithium, sodium and potassium compounds produce in a Bunsen burner flame?
3. Write equations for the reactions of sodium and potassium with water.