Question

why is potassium nitrate's solubility at 40 degree celsius 65g but 63.9g at 60 degree celsius?

Answer 1

It is because the solution of water at 60 degree Celsius has very close molecular arrangement(because of anamolous expansion)and hence the molecules of KNO3 GET less space to dissolve among water molecules and hence with only a 63.9g of KNO3 dissolved it gets saturated.
But in 40 degrees the molecular space between the water molecules is more and hence it dissolves more.