Soap: Soap is a metallic salt of saturated (C_nH_2n+1COOH) or unsaturated (C_nH_2n+1COOH) Higher Fatty Acid. There may be pb, Mg, Ca or other metallic salts.But as detergents only Na or K salt of higher fatty acids, soap is made on a commercial scale by boiling natural oil or fatty oil which is used for soap manufacturing.

The following oils are used in soap manufacturing:

• Cotton seed oil
• Coconut oil,
• Oliver Oil
• Soya bin Oil
• Palm Oil
• Ground nut Oil

Soap Manufacturing Process: Soap is made on a commercial scale by boiling natural oil or fatty oils with aqueous solution of sodium or potassium hydroxide.

Sodium soaps are sparingly soluble in strong sodium chloride solution. The mixture of soap and glycerol in aqueous solution obtained by saponification in therefore saturated with common salt when the soap is precipitated and rises to the surface where it can be skimmed off. It is washed with cold water and is cast or bars and dried.

Most exception for k-salt because addition of  NaCl salt cause precipitation of the Na-salt of fatty acid.

C₁₇H₃₅COOK+NaCl = C₁₇H₃₅COONa + KCl

KCl made the fatty acid into K-fatty acid. Thus the glycerol are not separated.

Properties of Soap :

• Soap dissolve very slowly in cold water but rapidly in hot water.
• Soap dissolves in hot alcohol but is only sparingly soluble in other organic solvents such as acetone, ether or petroleum.
• In extremely dilute solutions soap ionizes:

RCOONa === RCOO^–+ Na⁺

RCOO^–+ H₂O == RCOOH + OH^–

Sometimes there is free fatty acid molecules in soap to form what is known as acid soaps.

xRCOOH + yRCOONa ——à (RCOOH)_x ( RCOONa)_y Acid Soap

• Soap react with hard water

2C₁₇H₃₅COONa + CaSO₄ ————à ( C₁₇H₃₅COO)₂ Ca + Na₂SO₄

• The no. of  carbon atom in the aliphatic chain of Soap is very important.

Less Then 9 C – No detergency power

Less then 17 C – Extremely good detergent

Above 17 C — Poor solubility

Selection of Soap :

1. Soap should contain at least 62% of fatty acid and between 6.5% and 8.5% combined alkali expressed as Na₂O
2. No free fat, unsaponifiable oil or free caustic should present free caustic alkali denotes careless in manufacture but sodium carbonate or silicate are often added purposely to make the soap harder or to improve its detergent action.
3. Salts such as sodium chloride and sodium sulphate should only be present in small quantity.
4. It should show a high degree of surface affinity.
5. The most important factors which determine the properties of soaps in the number of carbon atoms in the aliphatic chain. If there are less than 9(CH₂) group, the soap will have virtually no detergent power.

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