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Both Sodium and Potassium react like this in water. I had a science teacher/burned-out stoner in high school that liked to turn off the lights and drop pieces of each into trays of water during class. Since we were in western Washington, I always assumed he was just getting his peak on during 'shroom season.



Alkali metals are famous for their vigorous reactions with water, and these reactions become increasingly violent as one moves down the group. The reaction with water is as follows:

Alkali metal + water → Alkali metal hydroxide + hydrogen gas

With potassium as an example:

2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)

In this reaction, enough energy is produced to ignite the hydrogen, creating a lilac flame above the potassium. If rubidium or caesium react with water though, the subsequent explosion tends to be very violent, a fact not helped by its rapidity in coming about once the metal and water start reacting.

Lithium
Sodium
Potassium
Rubidium
Caesium
Francium

I have a chemistry degree from Cut'n Paste University. But I also actually remember some of this from high school. Burning magnesium metal in oxygen was also a lot of fun... back then.