When you mix soap with water, you are diluting it unto less strong soap.
When you use soap and water mixed togehter to wash your hands, you are using the chemical propertiies of the soap to your advantage. The soap is made of hundreds of thousands of tiny strands that have a "water loving" end and a "oil loving" end. The oil loving ends attach to the oil on your skin, and the water loving ends stick out. This is called a micelle. Thousnds of micelles form and are ready to be rinsed away, with the oil that was on your skin inside of them. We need soap insted of just water because oil does not mix with water, and therfore cannot just be rinsed off with it.
