Question

(a) Describe the mechanism of cleansing action of soaps. (b) Why do soaps not work in hard water?

Answer 1

the soap the chain of hadrocarbons one end of this chain is water soluble and other end is fat soluble hence when the soap and water combines water soluble part soluble to water and grease removed from it

Answer 2

soap has a tadpole like structure which has a hydrophobic(water repelling) tail and a hydrophilic(water attracting) head containing ionic element like sodium ...
when a cloth full of dirt and grease is dipped in a soapy water the hydrocarbon end reacts with the dirt and grease and dissolves into it whereas the ionic end dissolves in the water . 
thus the total charge of the chain becomes negative and they repel each other forming a  structure in which the dirt or grease is at the centre and the chain pointing outwards .
later when the cloth is rinsed the dirt along with the micelle comes out of the cloth inside the water which now appears dirty .
thus this is a cleansing action of soap.. 

soap doesn't work in hard water because in hard water mg and ca ions are present which reacts with the chain and does we need large amount of soap in hard water and many times ,it doesn't work in the hard water....