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Question: Soaps are sodium salts of fatty acids, \[RCOONa\] ; e.g.\[{C_{17}}{H_{35}}CO{O^ - }N{a^ + }\]. It gi...

Soaps are sodium salts of fatty acids, RCOONaRCOONa ; e.g.C17H35COO−Na+{C_{17}}{H_{35}}CO{O^ - }N{a^ + }. It gives an insoluble precipitate/layer with
A. Ca2+(aq)C{a^{2 + }}\left( {aq} \right)
B. Mg2+(aq)M{g^{2 + }}\left( {aq} \right)
C. HCl(aq)HCl\left( {aq} \right)
D. All of these

Explanation

Solution

The fatty acid is a long chain carboxylic acid. The long chain consists of aliphatic hydrocarbons which are saturated or unsaturated in nature. Some of them are insoluble in water.

Complete answer:
Soaps are fatty acid salts represented by the general formula(RCO2−)nMn+{\left( {RC{O_2}^ - } \right)_n}{M^{n + }} , where RR is an alkyl chain, MM is a metal ion, and nn is the number charge of the cation. If the metal present (MM) is sodium or potassium leads to preparation of toilet soaps. If the metal ion is a dictation like Mg2+M{g^{2 + }} or Ca2+C{a^{2 + }} it leads to formation of metallic soaps.
Soaps are used in different purposes for households and industries. They are excellent surfactants in cleaning, bathing and other housekeeping. Their ability to solubilize particles and grime make it easier to remove the stains from an object. They also kill germs or microorganisms by denaturing the proteins present in it.
They are effective in lubricating and are used as thickeners for industrial purposes. They are used as lubricating greases.
The use of soaps suffers a disadvantage when used in the presence of acid or solvent medium containing calcium and magnesium salts. The properties of soaps are destroyed in the presence of acids due to formation of insoluble fatty acids. The reaction with aqueous HClHCl is:
C17H35COO−Na+(aq)+HCl(aq)→C17H35COOH(s)↓+Na+(aq)+Cl−(aq){C_{17}}{H_{35}}CO{O^ - }N{a^ + }(aq) + HCl(aq) \to {C_{17}}{H_{35}}COOH(s) \downarrow + N{a^ + }(aq) + C{l^ - }(aq)
When used in hard water which contains excess calcium and magnesium ions it leads to formation of scum. The scum is the insoluble precipitate of calcium or magnesium formed with the fatty acid anion.
2C17H35COO−Na+(aq)+Ca2+(aq)→[C17H35COO−]2Ca2+(s)↓+2Na+(aq)2{C_{17}}{H_{35}}CO{O^ - }N{a^ + }(aq) + C{a^{2 + }}(aq) \to {[{C_{17}}{H_{35}}CO{O^ - }]_2}C{a^{2 + }}(s) \downarrow + 2N{a^ + }(aq)
Hence option D is the correct answer.

Note: The properties of soaps are excellent in fresh water but are limited in hard water. Esters of fatty acids are prepared with alcohols and used as emollients in cosmetics.