Sodium

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few sodium rich non water soluble compounds (analog to the high water solubility of nitrides on the complementary oxoacid side)
use of sodium to make insoluble compounds soluble (in mining)

Misc

Chemical gardens: [1]
Adding metal salts to an aqueous solution of sodium silicate (otherwise known as waterglass) or potassium silicate.
In a double displacement reaction the newly formed metal silicate falls out while the newly formed sodium salt becomes even more suluble and thus stays in solution.
Sodium silicate [2]