Question

under ground water or water with Mg salts will produce more scum when soap solution is added to each and shaken well

Answer 1

Hard water contains calcium and magnesium ion which reacts with soaps ( sodium salt of fatty acid ) forms scum.
2C₁₇H₃₅COO^-Na⁺ + Mg⁺² (or Ca⁺²) $\to$ (C₁₇H₃₅COO)₂Mg or ​(C₁₇H₃₅COO)₂Ca + 2Na⁺

Scum formation is dependent on concentration of Ca⁺² or Mg⁺² .
As ground water contains higher amount of these ions, therefore more amount of scum is produced as compare to tap water(where concentration of Mg²⁺ ions are comparatively lower)