Soaps And Detergents - Practice Questions & MCQ

Sodium or potassium salts of higher fatty acids such as lauric acid(C₁₁H₂₃COOH), palmitic acid(C₁₅H₃₁COOH), etc. are called soaps. Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acids are soft soaps. Shaving creams and liquid soaps are soft whereas others are hard. Soaps are generally obtained by the hydrolysis of oils and fats with aqueous alkali. The process is called the saponification of oils and fats. Fats and oils are triesters of higher acids and glycerol. Hard soaps are prepared from cheap oils and fats and sodium hydroxide. They contain free alkali and are used for washing purposes. Soft soaps are prepared from good oils and potassium hydroxide. They do not contain free alkali and are used as toilet soaps, shaving creams and shampoos. Transparent soaps are prepared by dissolving the soap in ethanol and then evaporating the excess solvent. Medicated soaps are soft soaps and are prepared by adding some antiseptics like Dettol, savlon etc. In some soaps, deodorants are added. Toilet soaps are prepared by using better grades of fats and oils and care is taken to remove the excess alkali. Colours and perfumes are added to make them attractive. Shaving soaps contain glycerol to prevent rapid drying. A gum called rosin is added while preparing these soaps. It forms sodium rosinate which leathers well. Laundry soaps contain filler like sodium rosinate, sodium silicate, borax and sodium carbonate. 

Why do soaps not work in hard water?
Hard water contains calcium and magnesium ions. These ions form insoluble calcium and magnesium soaps respectively when sodium or potassium soaps are dissolved in hard water.

            Soap                                                         Insoluble calcium stearate(Soap)

These insoluble soaps separate as scum in water and are useless as a cleansing agent. In fact, these are hindrances to good washing because the precipitate adheres onto the fibre of the cloth as a gummy mass. Hair washed with hard water looks dull because of this sticky precipitate. Dye does not absorb evenly on cloth washed with soap using hard water, because of this gummy mass.

These are also called synthetic detergents or syndets or soapless soaps or just detergents. They have cleansing power as good or better than ordinary soaps and can be used for washing even with hard water and do not precipitate in presence of Ca²⁺/Mg²⁺ or in acidic solution. 
A synthetic detergent is the sodium salt of a long chain alkyl hydrogen sulphate or the sodium salt of a long chain benzene sulphonic acid. Like soap, they contain both hydrophilic and hydrophobic parts in the molecule.

Detergents are mainly classified into three categories, namely:

1. Anionic Detergents: Anionic detergents are sodium salts of sulphonated long chain alcohols or hydrocarbons. Alkyl hydrogensulphates formed by treating long chain alcohols with concentrated sulphuric acid are neutralised with alkali to form anionic detergents. Similarly, alkyl benzene sulphonates are obtained by neutralising alkyl benzene sulphonic acids with alkali.
   
   In anionic detergents, the anionic part of the molecule is involved in the cleansing action. Sodium salts of alkylbenzenesulphonates are an important class of anionic detergents. They are mostly used for household work. Anionic detergents are also used in toothpastes.
2. Cationic detergents: These are mostly acetates, chlorides or bromides of quaternary ammonium salts containing one or more long-chain alkyl groups. Being more expensive than the anionic detergents, they find limited use. But these detergents are also used in hair shampoos and hair conditioners. Such detergents, however, possess germicidal properties and are used quite extensively as germicides. Examples are trimethyl stearyl ammonium bromide, cetyl trimethyl ammonium bromide or N-benzyl quaternary ammonium chlorides.
3. Non-ionic detergents: They are neutral surface active detergents and obtained from long-chain alcohols by treatment with ethylene oxide in the presence of a base. One such detergent is formed when stearic acid reacts with polyethyleneglycol.