Can we cut lithium with a knife?

General information about alkali metals


The elements of the Main group I. of the periodic table. The hydrogen, which is at the top of main group I, is not one of them. And these are the six alkali metals:

  • Lithium li
  • Sodium Na
  • Potassium K
  • Rubidium Rb
  • Cesium Cs
  • Francium Fr

The alkali metals play a major role in chemistry classes because they all have very similar properties, which can easily be explained with the atomic structure of the metals (structure-property concept; substance-particle concept). Experiments with the alkali metals themselves are not suitable as a student experiment because of the high reactivity of the elements, but a lot can be done with the salts of the alkali metals in the student experiment.


The properties of the alkali metals are easy to summarize and remember. All alkali metals are exceptional light (for a metal) and soft (for a metal). Sodium and potassium can be cut with a knife. Lithium is a little too hard for that, and the other three metals (Rb, Cs, and Fr) are too reactive for that. The melting points of the alkali metals are very low (for a metal), cesium melts with the heat of the hand. One could almost think that lithium, sodium and the other elements of the first main group are not metals at all. But that is a mistake. If you cut through sodium with a knife, you can see a shiny metallic surface, which, however, quickly becomes cloudy because the element reacts with the oxygen in the air. The element also conducts electricity, as can be easily demonstrated. All of this suggests that sodium (like the other alkali metals) is a real metal.

Chemically, all alkali metals are very reactive. Lithium is the least reactive and therefore still suitable for careful student experiments. Sodium and potassium can be cut with a knife, and the shiny metallic cut surface quickly becomes matt because the metal reacts with the oxygen in the air to form sodium oxide or potassium oxide. Both metals react violently with water; Potassium even ignites in the process. We will come to the reactions and the reaction equations later.

Rubidium and cesium, like sodium and potassium, cannot be placed on filter paper and cut with a knife, because as soon as they are taken out of the storage container they react with the oxygen in the air and spontaneously ignite. With francium, which is even more reactive than rubidium and cesium, you have completely different problems: Francium is a radioactive alkali metal.


Reaction with oxygen

The alkali metals react very quickly with the oxygen in the air. This should be illustrated using the example of sodium:

$ Na _ {(s)} + O_ {2} {(g)} \ to Na_ {2} O {(s)} $

If this and the following reaction equations are not displayed correctly, then the browser you are using is either outdated or not connected to the Internet to get the MathJax package.

The reaction equation is not yet complete. But it gives the word equation

$ Sodium + Oxygen \ to Sodium Oxide $

pretty good again. However, it is not yet a reactionequation, because the number of atoms shown is different on both sides of the "equation". For example, on the left a Na atom, but on the right are two. You have to "try it out" a bit until you have found the equation:

$ 2 Na _ {(s)} + \ frac {1} {2} O_ {2} {(g)} \ to Na_ {2} O {(s)} $

That looks more like an equation. However, the term $ \ frac {1} {2} O_ {2} {(g)} $ bothers. Many chemistry teachers don't like it at all when you work with half molecules or atoms. So we multiply the whole equation by a factor of 2 and get the final reaction equation for the reaction of sodium with oxygen:

$ 4 Na _ {(s)} + O_ {2} {(g)} \ to 2 Na_ {2} O {(s)} $

Reaction with water
Reaction of sodium with water
Implementation (teacher test in the deduction!):

A large glass tub is filled with water (tap water) to which a few drops of phenolphthalein solution have been added.

Then a small piece of sodium is cut from a larger piece of sodium with the kitchen knife. The sodium piece must then be completely debarked, all sides of the piece must have a metallic sheen.

Then the sodium piece is put into the bowl with the water with the tweezers. Then the pulley of the trigger is pulled down immediately.


The piece of sodium flits back and forth on the water, and you can hear a more or less loud hissing sound. The piece of sodium gets smaller and smaller and forms a ball. Sometimes yellow flames can also be observed when the ball flits back and forth on the water. Often there is a small bang at the end and the rest of the ball bursts. The water in which the sodium floated back and forth felt soapy at the end of the experiment; in addition, it has turned a purple color.

The reaction with water produces a colorless and odorless gas that could easily be identified as hydrogen with the help of the oxyhydrogen gas sample, but we do not do this for safety reasons. The violet color of the phenolphthalein and the soapy feel indicate that an alkali was formed during the experiment. In chemistry lessons only four alkalis actually play an important role: sodium hydroxide solution NaOH, potassium hydroxide solution KOH, lime water Ca (OH)2 and ammonia water (NH4OH). Which of these four alkalis will probably have originated here? Of course, only the sodium hydroxide solution NaOH comes into question.

Screenshot of the YouTube video

The experiment is shown very nicely on the YouTube video "Chemistry Experiment 22 - Sodium and Water", starting with the careful cutting of the sodium piece to the actual reaction. The purple color of the water is also easy to see.

Derivation of the reaction equation

We now want to work out the reaction equation for the reaction of sodium with water. First we set up the simple word equation. Everyone should be able to do that who has observed the experiment closely. What did we use as raw materials? Water and sodium. What did you get out of it? Hydrogen as a gaseous reaction product and sodium hydroxide solution. But caustic soda is nothing more than the compound sodium hydroxide, which has been dissolved in water.

Word equation:

$ Sodium _ {(s)} + water _ {(l)} \ to sodium hydroxide _ {(aq)} + hydrogen _ {(g)} $

We now put this word equation into a reactionU.Nequation around.

Reaction inequality:

$ Na _ {(s)} + H_ {2} O _ {(l)} \ to NaOH _ {(aq)} + H_ {2 (g)} $

Finally, we develop the correct responseequation. There are three H atoms on the right, but only two on the left. So we use two water molecules. Then there are two O atoms on the left, but only one on the right. So we also have to double the NaOH. But now another Na atom is missing on the left side, so we have to insert two atoms here as well.

Correct reaction equation:

$ 2 Na _ {(s)} + 2 H_ {2} O _ {(l)} \ to 2 NaOH _ {(aq)} + H_ {2 (g)} $

That's it then. If you had difficulties understanding the derivation of the reaction equation, please go to the help page "Setting up a reaction equation". Here the whole procedure is explained again in detail.

For experts

Why does the sodium globule flit back and forth on the surface of the water?

The reaction with water produces gaseous hydrogen. This gas cannot completely escape upwards, part of it remains under the sodium ball. This floats, so to speak, on a hydrogen cushion. Due to the recoil that results from the development of gas, the ball flits back and forth on the surface of the water.

Why does a ball even form during the reaction?

The exothermic reaction releases so much energy that the piece of sodium melts - the melting point of sodium is only 98ÂșC, which is very little for a metal. Liquid bodies always take on a shape in which their surface is minimal - the spherical shape. You can see that nicely in science fiction films, when water is spilled or blood is splashed in weightlessness, the liquid then also forms small spheres that float impressively through the picture ...

Reactivity of the alkali metals

The element lithium is by no means as reactive as sodium (see also the YouTube video "Lithium reacts with water"). If you put a small piece of lithium in a sieve spoon, you can hold it in a glass tub filled with water under the opening of a test tube filled with water. The resulting hydrogen then rises into the test tube and can be detected with the oxyhydrogen sample. If you adhere to the appropriate safety measures, you can even carry out this experiment as a student experiment.

If you put a small piece of potassium in a bowl of water, a much more violent reaction sets in immediately than with sodium. Most of the time, spontaneous combustion occurs very quickly (see "Potassium in water (slow motion)" on YouTube).

Rubidium and cesium react even more violently than potassium with water; the experiments may not be carried out in the school. Even cutting rubidium and cesium in the air is not possible because the metals immediately react violently and ignite with the oxygen in the air.

So we see: The reactivity of the alkali metals increases steadily from top to bottom within the element group of the periodic table. If we wanted to represent this increase in reactivity with a simple graph, the result could be:

Reactivity of the alkali metals

But this graphic is not yet perfect! The reactivity of the alkali metals decreases within the group of above to below to. In the graph, however, the reactivity decreases Left to right to. That could confuse one or the other a bit. Hence a second attempt at graphical representation:

Reactivity of the alkali metals

The reactivity of the alkali metals increases from top to bottom within the group. Lithium is the least reactive and francium the most powerful.

In discussing Dalton's atomic model, we said that while this model can explain many chemical phenomena well, it cannot explain others. When explaining the increasing reactivity of the alkali metals, we come across that Limits of this Dalton model. According to Dalton, the increase in the reactivity of the alkali metals cannot be explained.