Question

What is the difference between soaps and detergents? State, in brief, the cleansing action of soaps in removing an oily spot from a fabric. Why are soaps not very effective when a fabric is washed in hard water? How is this problem resolved?

Answer 1

The differences between soaps and detergents are:

Soaps	Detergents
1. Soaps are the sodium salts of long-chain carboxylic acids.	Detergents are sodium salts of long-chain benzene sulphonic acids.
2. The ionic group in soaps is  –COO-Na+.	The ionic group in soaps is SO3-Na+ or SO4-Na+
3. Soaps are not useful when water is hard	Detergents can be used for washing purposes even when water is hard
4. Soaps have relatively weak cleansing action.	Detergents have a strong cleansing action

When soap is dissolved in water, its hydrophobic ends attach themselves to dirt and remove it from the cloth. 
First, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in water like particles in a colloidal solution. The various micelles present in water do not come together to form a precipitate as each micelle repels the other because of the ion-ion repulsion.
Thus, the dust particles remain trapped in micelles (which remain suspended) and are easily rinsed away with water. Hence, soap micelles remove dirt by dissolving it in water.
When soap is added to hard water, the Ca²⁺ and Mg²⁺ ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding calcium and magnesium salts which are precipitated as scum. The insoluble scum sticks on the clothes and so the cleaning capacity of soap is reduced.
\[\ce{Ca^2+ RCOONa->\underset{\text{(Scum)}}{(RCOO)2Ca} + 2Na^+}\]                                    
This problem can be solved by:  

• using soft water instead of hard water.
• using detergents instead of soaps.
• treating the hardness of water and then using it for washing purposes.