Reactions of alkali metals with water

All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group. This shows that the reactivity of the alkali metals increases as you go down Group 1.

Note that you are expected to know some of the key safety points for these reactions. These include:

  • use of a plastic safety screen
  • using small pieces of the metal
  • using a large volume of water
  • using tweezers to hold the metal
  • wearing thick rubber gloves

Lithium

When lithium is added to water, lithium floats. It fizzes steadily and becomes smaller, until it eventually disappears.

lithium + water → lithium hydroxide + hydrogen

2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)

Sodium

When sodium is added to water, the sodium melts to form a ball that moves around on the surface. It fizzes rapidly before it disappears.

sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Potassium

When potassium is added to water, the metal melts and floats. It moves around very quickly on the surface of the water. The metal self-ignites, which also ignites the hydrogen gas. This results in sparks and a lilac flame. There is sometimes a small explosion at the end of the reaction.

potassium + water → potassium hydroxide + hydrogen

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)

Potassium reacting and showing a lilac flame in a bowl of water

Test for hydrogen gas

To identify the gas given off by these reactions as hydrogen, it is necessary to test the gas:

  1. Collect the gas in a test tube.
  2. Put the flame from a burning splint into the top of the test tube.
  3. The gas will burn quickly and give off a squeaky ‘pop’.
Diagram showing a test for hydrogen gas, using a lighted wooden splint and a test tube.

Strong alkalis

The hydroxides formed in all of these reactions dissolve in water to form alkaline solutions. These solutions turn universal indicator purple, showing they are strongly alkaline. Strong alkalis are corrosive. Care must be taken when they are used – goggles and gloves should be worn.