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Soaps and detergents are cleaning materials that we use because pure water alone cannot remove all types of dirt or any oily substances from our bodies and clothes. Soaps and detergents contain 'surfactants,' which are molecules that line up around water to break the 'surface tension.'
Surface tension is based on cohesive forces that exist between the liquid molecules. The molecules present in the liquid undergo a net inwards force with no force acting above the surface. This causes the surface molecules to contract and resist breaking.
Soaps and detergents are both of a different chemical nature. Soap is a cleaning agent made up of one or more fatty acid salts. Detergent is a chemical compound or a combination of chemical compounds that are also used as a cleaning agent.
Soaps are sodium or potassium salts of some long-chain carboxylic acids, called fatty acids.
Soap generally requires the use of two major raw materials:
  • Fat
  • Alkali
Sodium hydroxide is the most common alkali used in soap production. Potassium hydroxide is yet another alternate. A potassium-based soap is far more water-soluble than a sodium-based soap.
Based on the above features, there are two types of soaps:
  • Hard soap
  • Soft soap
i) Hard soap:
Soaps made by saponifying oils or fats with caustic soda (sodium hydroxide) are referred to as hard soaps. They are typically used for washing.
ii) Soft soap:
Soaps that are made by saponifying oils or fats with potassium salts are referred to as soft soaps. They are employed in the cleansing of the body.
1. Preparation of soap:
a) Kettle process:
Kettle process is the most traditional method. However, it is still widely used in small-scale soap production. In this process, there are primarily two steps to be taken.
  • Saponification of oil
  • Salting out of soap
i) Saponification of oil:
The oil used in this process is transported in an iron tank (kettle). A small amount of the alkaline solution (\(10%\)) is added to the kettle. The above mixture is boiled by passing steam through it. After several hours of boiling, the oil is hydrolysed. This is known as saponification.
ii) Salting out of soap:
To the above boiling mixture, common salt is added. Finally, soap precipitates in the tank. After a few hours, the soap rises to the surface of the liquid as a 'curdy mass'. The clean soap is removed from the top. After that, it is allowed to cool.
2. Effect of hard water on soap:

The presence of calcium and magnesium ions (\(Ca^{2+} and Mg^{2+}\)) in hard water, limits the cleaning action of soap.
When combined with soap, hard water forms a thin layer (precipitates of metal ions) known as scum, which leaves a deposit on the clothes or skin and is difficult to remove.
This can cause the fabric to deteriorate over time and eventually ruin the clothes.
Detergents, on the other hand, are made with chemicals that are unaffected by hard water.
Why ordinary soap is not suitable for using with hard water?
Ordinary soaps precipitate as calcium and magnesium salts when exposed to hard water. They appear as sticky grey scum on the cloth's surface. As a result, the soaps cannot be used in hard water.