You can't isolate sodium metal from aqueous solution because it reacts with water:
Na(s) + H2
NaOH(aq) + H2
In order to understand this, you should look at the half equation for the reduction of water
O + 2e-
= -0.83 V
= -2.7 V
So, it should follow that any metal with E
> -0.83 V should be thermodynamically stable in water (see data table
) - Ni and Co fall into this category.
Only if the solution is acidic can you compare with the standard hydrogen electrode (0 V). You can use the tables to explain why Zn does not react with water but dissolves in acidic solutions, for example.
Some metals that are thermodynamically unstable in water might not seem to react with it - Mn and Fe are examples. There is a reason for this. Hint: Sugar is thermodynamically unstable in oxygen, so why doesn't it spontaneously combust when exposed to air?