Question

What happens when sodium hydroxide reacts with:(1)sodium chloride(2)calcium sulphate

Answer 1

Sodium hydroxide and sodium chloride have no visible reaction.
Sodium hydroxide does not react with calcium sulphate.

Answer 2

1)
Nothing happens chemically, but the dissolved NaOH(in water) will reduce the amount of NaCl that can dissolve in solution (because NaOH is more soluble than NaCl, and there's a common ion effect with the sodium ion). What may be happening is that you're causing the NaCl to crash out of solution when you add the NaOH.

2)
\[CaSO _{4}.2H _{2}O+2NaOH \rightarrow Na _{2}SO _{4}+Ca(OH)_{2}+2H _{2}O\]But here is some consideration. It is not sure about calcium hydroxide formation but there is some hydroxide present, it has low solubility as well, but it won't dominate.
Water in Gypsum; Water goes into solution when CaSO4 gets dissolved, but only in stoichiometrci amount (that is 2 moles of water per each mole of dissolved sulfate). As there will be not much sulfate dissolved, there will not much additional water in solution.

Got it Sonia?