Question

Why can't sodium or calcium be precipitated from aqueous solutions?

Answer 1

You mean sodium or calcium-based ionic compounds?

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Sodium and calcium solid can never be precipitated because those are metals, which could never be dissolved cleanly in the first place.

Sodium would just react and form `"NaOH"`, and calcium would slightly react to form `"Ca"("OH")_2`:

`2"Na"(s) + 2"H"_2"O"(l) -> 2"NaOH"(aq) + "H"_2(g)`

`"Ca"(s) + 2"H"_2"O"(l) -> "Ca"("OH")_2(s) + "H"_2(g)`

However, calcium-based ionic compounds can be precipitated just fine...

`"Na"_2"CO"_3(aq) + "Ca"("NO"_3)_2(aq) -> 2"NaNO"_3(aq) + bb("CaCO"_3(s))`

Sodium-based ionic compounds generally won't. There isn't a single sodium-based ionic compound I can think of that is insoluble in water.