Written by Edwin Tunggawan
on August 11, 2020

Revisiting Alkali Metals

This is a note on alkali metals. I actually haven’t visited chemistry much since graduating high school back in 2009, but recently when I look for science experiment demonstration videos on YouTube I got some experiments with explosions. Those experiments got me interested in revisiting chemistry, and given that I’ve been following the Periodic Videos from the University of Nottingham for some time right now I think I should just jump into it.

As a reference, the following is an image of the periodic table of elements I grabbed from Wikipedia.

The Periodic Table

The alkali metals belong to the group 1 of the periodic table along with hydrogen (H). They are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).

The one that got my interest in particular is sodium, due to its reactive capacity with water which allows it to cause explosions as the result of the chemical reaction as demonstrated in many experiment videos I’ve seen. But while the sodium is the most widely used alkali metal for us to demonstrate the chemical reaction of the metal with water, in general all of the alkali metals will react with water.

Alkali metals are so reactive that in nature they’re generally found being combined with other elements, as said in this article from Encyclopedia Britannica. But in this note, I’m concerned mostly with their reaction with water.

Lithium (Li)

Lithium is commonly used on batteries, namely the lithium-ion rechargeable batteries. When reacting with water, lithium metal will form a colorless solution of lithium hydroxide and hydrogen gas as said in this article on WebElements.

The flame color of lithium is red, as mentioned in the flame test section of the Chemistry LibreTexts.

\[2Li_{(s)} + 2H_{2}O_{(l)} \to 2LiOH_{(aq)} + H_{2(g)}\]

Sodium (Na)

Sodium is usually found in common salt, bonded with chloride as NaCl. When reacting with water, sodium metal will form a colorless solution of sodium hydroxide and hydrogen gas as mentioned in this article.

Given that the sodium metal is heated enough in the chemical reaction, it will burn with orange fire,

\[2Na_{(s)} + 2H_{2}O_{(l)} \to 2NaOH_{(aq)} + H_{2(g)}\]

Potassium (K)

Potassium is also common in salt, bonded with chloride as KCl. This potassium chloride salt is said to be able to lower blood pressure so people with high blood pressure often got recommendation to use the potassium chloride salt instead of the sodium chloride salt.

As lithium and sodium do, potassium metal will form a colorless solution of potassium hydroxide and hydrogen gas if it reacts with water. It’s also mentioned that the fire burning from the reaction will have a pale lilac color.

\[2K_{(s)} + 2H_{2}O_{(l)} \to 2KOH_{(aq)} + H_{2(g)}\]

Rubidium (Rb)

Rubidium is said to not being used much in daily life according to the Royal Society of Chemistry. It’s slightly radioactive and can be used to detect brain tumors, and rubidium nitrate is used in fireworks to give purple color.

It’s not mentioned in WebElements what color the rubidium fire will be if reacting with water, and in some videos I’ve seen the reaction seems to happen pretty fast without any flame. But this video from the Royal Society of Chemistry shows that the color is reddish purple as the color they’re said to add to fireworks.

\[2Rb_{(s)} + 2H_{2}O_{(l)} \to 2RbOH_{(aq)} + H_{2(g)}\]

Caesium (Cs)

According to the Royal Society of Chemistry, caesium is used on atomic clocks and is found in the pollucite mineral which is commonly found at Bernic Lake at Manitoba, Canada. It’s also highly reactive to water, and is said to react very fast.

The caesium flame is said to be bluish purple.

\[2Cs_{(s)} + 2H_{2}O_{(l)} \to 2CsOH_{(aq)} + H_{2(g)}\]

Francium (Fr)

According to WebElements francium is very rare and expensive that it’s unlikely that anybody has reacted it with water. I’d like to be an optimist and say that someone has definitely done that, but after checking it on the Royal Society of Chemistry’s website it’s said that francium has no use so far due to its half life being only 22 minutes and is very toxic due to its radioactivity. So I guess chances are that nobody dares to react it with water.

But since it still belongs to the family of the alkali metals due to having common properties with them, we expect it to react similarly with water as the other alkali metals. Since it’s very hard to produce and very radioactive, it seems that the flame color of the element is still unkown at this point.

\[2Fr_{(s)} + 2H_{2}O_{(l)} \to 2FrOH_{(aq)} + H_{2(g)}\]

Closing Notes

So in general, for alkali metals we can model their reaction with liquid water as follows with the element X as a placeholder for the alkali metal used in the reaction.

\[2X_{(s)} + 2H_{2}O_{(l)} \to 2XOH_{(aq)} + H_{2(g)}\]

The reaction between the solid alkali metal with liquid water will form hydroxide solution of the said element and hydrogen gas. The hydrogen gas is highly flammable, and the chemical reaction also produces heat which may ignite the hydrogen gas or the metal itself. When the metal got ignited, we can see visible flame that may have different colors depending on the metal. The ignition of the hydrogen gas can cause explosion.

Due to its highly reactive nature, pure alkali metal chunks need to be handled very carefully as it will react to even the air around it. Heavier alkali metals such as rubidium and caesium will even spontaneously ignite when exposed to air at room temperature.

It seems that alkali metals react best with water in its liquid form, as this experiment shows that sodium doesn’t react with ice when there’s no liquid water layer in the surface of the ice. Alkali metals do react with water vapor in air, but it seems that for the lighter alkali metals not enough heat is produced to cause any ignition as with the case with the sodium metal shown in this video.