Obviously, sodium is an extremely reactive metal. When sodium comes into contact with water, it displaces hydrogen gas as follows:
2 Na(s) + 2H2O() 2 NaOH(aq) + H2(g)Sodium has a density of only about 0.97 g/mL, so it floats on water. The reaction produces jets of hydrogen gas below the waterline; this is what propels the sodium around the surface. The reaction releases heat, and as the sodium and the solution warm up, the reaction goes faster and faster. If the sodium gets trapped on the water's edge or against some other obstacle, enough heat can be generated to boil the water around the metal and actually melt the sodium.
This is a dangerous reaction. If the sodium gets hot enough, the hydrogen jets can ignite and burn. That's what caused the sparks you saw. Burning hydrogen makes the temperature rise even faster. The rate of the reaction increases so quickly that an explosion may occur! Molten sodium can be thrown out of the dish.
Wear goggles and stand behind a safety shield when you're anywhere near this reaction. (I put the dish into a fume hood and close the window after I drop the sodium into the water.) Use a piece of sodium that is no larger than a grain of rice. Fill the dish with ice water rather than tap water to prevent the reaction from ending with a bang.
Author: Fred Senese firstname.lastname@example.org
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/inorganic/faq/print-sodium-and-water.shtml