[ELI5] Why alkaline metals explode when they touch water
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They immediately react and start forming NaOH/KOH/etc. which is a highly exothermic reaction (translation: *shit’s hot yo*). It flash boils the water around the metal particles and causes a steam explosion.
Alkaline metals strip oxygen and hydrogen from water, but leave some hydrogen behind. This reaction produces a fair amount of heat.
Flammable hydrogen gas and hot steam are produced quickly, which usually just results in some smoking or a small fire (and much sizzling).
More reactive ones can cause a full-blown BANG.
Some chemical reactions need heat to happen, others create heat when they happen.
These elements are super hungry to be more stable, and water is actually in a constant equilibrium between 3 charged states when a liquid. Negative (OH-), Neutral (H2O), positive (H3O+). So water is plenty ready to react with elements that have a charge.
And when these super hungry metals (which are positively charged) meet water, they find all the right form of water they need (the negative form), easily, rapidly. And the formation of these new molecules with the metal and water releases a ton of heat. It releases so much heat that it actually causes combustion.
And so, kaboom
The metal doesn’t explode. The metal reacts with the water, grabbing up OH- in the water and generating lots of heat (you know how magnets jump together when they get close, similar, but the pushing and shoving of water molecules in the process cause heat). Grabbing up the OH- releases hydrogen gas. It just so happens that hydrogen gas burns – violently – in the presence of oxygen (from the air) and heat (in this case, from the metal contacting water).
When you see the explosion when you toss these metals into water, what is happening is that heat and hydrogen gas are being made (and a metal hydroxide), and if there’s enough heat and enough oxygen, the hydrogen explodes.
I’ve worked on this exact question for a school science project and the answer is actually more complicated than just the sodium reacting with the water (2 Na + 2 H2O -> 2 NaOH + H2) and the hydrogen lighting on fire (2 H2 + O2 -> 2 H2O). That would only produce an angry flame, not an explosion.
What actually happens is that the sodium yeets its electrons into the water and thus becomes positively charged. This static charge makes the lump tear itself apart into needle-like structures which have a much higher surface area, leading to a strongly increased reaction speed which culminates in an explosion. It’s called a Coulomb explosion after the guy who the unit for electric charge is named after.
See [this paper](https://cool-chemistry.ucoz.ua/Articles/P_E_Mason_et_al-Nat-Chem-2015-10.1038-NCHEM.2161.pdf) for a more scientific explanation and cool pictures.
They’re called alkali metals. These are reduction-oxidation reactions, which occur when one reactant (in this case the alkali metal) loses an electron and the other (water) gains. The alkali metal starts as having no charge (we write this as, for example Na(s) or Na with a little zero where the plus or minus usually goes if it’s an ion). It is oxidized and the water is reduced (remember, LEO says GER – lose electron oxidation; gain electron reduction). So in the case of sodium, a sodium atom goes from Na(zero, if I could write that here) to Na+. It loses one electron to become a sodium ion. The reason the alkali metal is so easily oxidized is that its lone electron in its outer shell is unstable. This is true of all alkali metals, which is why they’re all lined up in the left column on the periodic table. Electrons like to at least be paired, and preferably have eight in an outer shell (an octet). Given the opportunity, a lone electron in an outer shell is not going to stay that way. When the alkali metal loses that electron (oxidation), it moves to a lower energy state, giving off energy in the form of heat. In a reaction with water, the energy change is huge, and the reaction is violent, melting and ripping apart the alkali metal. This only speeds the reaction, as a steadily increasing metal surface area is exposed to water.
Reacting with water, the alkali metal donates its lone electron to a hydrogen on a water molecule, stripping it off the water molecule and leaving the electron pair that the hydrogen shared with the rest of the molecule behind, turning the water molecule into OH-, a hydroxide ion. Do this on two hydrogen atoms from two different water molecules, and you immediately put together H2, which is, as you know, flammable. The massive heat from the reaction causes the H2 to combust in the presence of the oxygen in the air. The oxygen for the combustion comes from air, not water.
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