In their chemical formula, Calcium Nitrate is Ca(NO₃)₂ while Sodium Hydroxide is NaOH. The reaction between the two are:
Ca(NO₃)₂ + 2 NaOH → Ca(OH)₂ + 2 NaNO₃
As you can see, this is a double replacement reaction. Focusing on the products, in an aqueous solution, NaNO₃ will dissolve in water because all nitrates are soluble. Ca(OH)₂ is insoluble in water so this is the solid that precipitates.
