What you actually see
Drop a pea-sized piece of sodium into a trough of water and the show starts instantly: the metal skates across the surface on a cushion of hydrogen, hisses violently, and often ends in a flash and a bang. Teachers love the demo precisely because it looks impossible — a soft, silvery metal behaving like it is made of fireworks. The French word for sodium's group literally encodes this: the alkali metals are the most reactive metals in the whole table.
The outward facts are simple enough to write in an exam: sodium floats (density 0.97 g/cm³, lighter than water), melts into a shiny ball (the reaction is strongly exothermic), turns the water alkaline as sodium hydroxide forms, and releases hydrogen gas — the pop test proves it. The interesting question is the one the demo begs: why is it so violent?
The equation behind the bang
Chemically, sodium hands one electron to water. Water's hydrogen atoms happily accept electrons, so sodium hydroxide and hydrogen gas come out:
+ → + ↑
Balance it and you see the two keys to the violence. First, the reaction is exothermic — it releases serious heat. Second, the hydrogen gas produced has to go somewhere fast, and it mixes with air right at the hot surface. Hot hydrogen + oxygen + a freshly cleaned metal surface is a textbook ignition recipe.
Why sodium gives its electron away so fast
Look at sodium's position: Group 1, Period 3, electron configuration [Ne] 3s¹. It carries exactly one electron outside a stable neon core, and that electron sits far from the nucleus. Sodium is one valence electron away from being "noble" — losing it costs little and yields the ultra-stable Na⁺. That single fact explains the whole Group 1 story: reactivity increases down the group (the outer electron sits even further out), which is why potassium reacts harder than sodium and caesium harder still.
The 2015 surprise: a Coulomb explosion
For a century, chemists told the story above and stopped at "the hydrogen ignites". But high-speed camera experiments reported in Nature Chemistry (2015) caught something faster: within a tiny fraction of a second, electrons spray out of the sodium into the water so quickly that the metal surface is left crowded with positive ions that all repel each other. The surface literally tears itself apart into a spike-forest of flying charged particles — a "Coulomb explosion" — multiplying the contact area between metal and water thousands of times over. Only after that do you get the heat, the hydrogen burst and the flame.
So the honest modern answer has two layers: the fast, invisible Coulomb explosion spreads the metal into the water, and the exothermic + chemistry supplies the heat and fuel for the bang you actually hear.
Why you should never try this at home
Every year school and university labs suffer injuries from stolen sodium. The metal is stored under oil for a reason: even moisture in air attacks it, and a larger piece escalates from fizz to genuine explosion — flying molten sodium and caustic spray can blind. On this page you can replay the chemistry as many times as you like; in the lab, watch your teacher do it from a safe distance, behind a screen.
You asked
1Why does sodium float and potassium catch fire?
Sodium floats because its density (0.97 g/cm³) is just below water's. Potassium sits one period lower — its single valence electron is further from the nucleus and even easier to lose, so the reaction is fast enough to ignite the hydrogen immediately, burning with the famous lilac flame.
2Is the water turning pink from phenolphthalein proof of NaOH?
Yes — that is the standard classroom confirmation. Phenolphthalein is colourless in neutral solution and pink above about pH 8.2, so a pink trail behind the sodium ball shows the hydroxide forming in real time.