why do you need to use more soap to wash ur hands if u use hard water?

Answer 1

Hard water contains limestone particles that corrode soap

Answer 2

Because soap is (mostly) the sodium salt of a long-chain fatty acid like stearic: CH3(CH2)16COO-Na+. The long CH2 chain solubilizes grease and oil. The -COO-Na+ head solubilizes the whole thing in water. But hard water has Ca++ and Mg++ ions in it, whose stearates are water insoluble. These are responsible for soap scum and "ring-around-the-bathtub. So first you have to precipitate those ions, then the rest of the soap is available for washing your hands. Detergents with -SO3-Na+ heads do not have insoluble Mg++ or Ca++ salts, and so you can use them for washing in hard water.

Answer 3

You get more lather with soft water (because you have less minerals or something like that), so it feels like you need more soap with hard water.