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.

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