What is the net ionic equation of sodium hydroxide when it dissolves in water?
When you write down the net ionic equation, you have to consider that the educt and the products are in different phases, a solid and a liquid phase. Undissolved $\ce{NaOH}$ is a solid, denoted by the index $\ce{(s)}$, while the dissolved $\ce{Na+}$ and $\ce{OH-}$ ions are solvated in the aqueous phase (index $\ce{(aq)}$ in the equation):

$$\ce{NaOH(s) ~-&gt;~ Na+(aq) + OH^{-}(aq)}$$