Usually when you think of sodium, you think of salt and anything associated with it, be it seawater or sweat and so on.
You can be forgiven for making that association, given that sodium is what salt is partly made of. But sodium isn’t salt.
Sodium is a highly reactive alkali metal, and in contrast to the sodium chloride compound (i.e. salt), sodium itself reacts very strongly with water.
There are many compounds of sodium in the world, and the reaction caused when water is involved can be explosive. WebElements explains below:
Sodium metal reacts rapidly with water to form a colourless solution of sodium hydroxide (NaOH) and hydrogen gas (H2). The resulting solution is basic because of the dissolved hydroxide. The reaction is exothermic. During the reaction, the sodium metal may well become so hot that it catches fire and burns with a characteristic orange colour. The reaction is slower than that of potassium (immediately below sodium in the periodic table), but faster than that of lithium (immediately above sodium in the periodic table).
The video below, uploaded by EatsTooMuchJam, demonstrates what happens when 0.45kg or 1lb of sodium is thrown into a river. A sizeable amount of hydrogen gas and sodium hydroxide are released, which sadly risks harming the river and the organisms in/around it.
Still, it’s interesting to see what happens, but don’t try this at your local river or lake. It’s extremely dangerous, as you’ll see.