Why alkali metals explode in water

Nature : Why a piece of sodium or potassium explodes when it comes in contact with water has never been precisely understood. Now, using high-speed cameras, Pavel Jungwirth of the Czech Academy of Sciences in Prague and his colleagues took a closer look at what occurs during the very first milliseconds of the process. Their experiment involved allowing a droplet of a sodium–potassium alloy, which is liquid at room temperature, to fall into a container of water. Within 0.3–0.5 ms, metal spikes shot out from the droplet’s surface and the water around the droplet turned blue. Using computer modeling, the researchers have determined that when the metal drop hits the water, each of the atoms on its surface loses an electron. The electrons become solvated in the water, and their ability to absorb light results in the transient blue color captured by the high-speed imaging. At the same time, the positively charged ions remaining in the metal droplet repel each other and fly apart in what is known as a Coulomb explosion. Hence, the researchers show that the runaway, explosive effect exhibited by alkali metals in water is initially caused by electrostatic forces rather than thermal ones.