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In the making of soap, a vegetable oil such as palm oil is boiled with a concentrated alkali solution in a reaction known as saponification. The alkali reacts with the oil and breaks it down into glycerol and fatty acids. These fatty acids react with the alkali to form sodium or potassium salt of the fatty acids.
The Composition of SoapSoaps are cleansing substances which are widely used in our daily life. Soaps are usually made from plant oils or animal fats. For example, the plant oils used are coconut oil, olive oil, palm oil and others. Each molecule of fat consists of a combination of one molecule of glycerol and three molecules of fatty acids.
Different fats have different fatty acids. The fatty acids can be separated from the glycerol through the hydrolysis process using an alkali as a catalyst. The fatty acid that is produced reacts with alkali to form a fatty acid salt. The alkali used is sodium hydroxide or potassium hydroxide.
The Making of Soap
Image via Wikipedia
Image via Wikipedia
In the making of soap, a vegetable oil such as palm oil is boiled with a concentrated alkali solution (sodium hydroxide solution) in a reaction known as saponification. The alkali reacts with the vegetable oil and breaks it down into glycerol and fatty acids. These fatty acids react with the alkali to form sodium or potassium salt of the fatty acids.
Soap is the sodium or potassium salt of a fatty acid formed. Common salt is then added to the mixture and boiled. Common salt will lower the solubility of soap in water. Hence, the soap is precipitated out. Glycerol is a by-product of the soap-making industry which is very useful. Examples of fatty acids are palmitic acid and oleic acid.
Characteristics of the Component of a Soap Molecule
A soap molecule is made up of two parts a) a hydrocarbon chain which is long and bonded by covalent bonds. It forms the tail of the molecule b) a head part at the end of the hydrocarbon chain which is ionic.
The head part of the soap molecule is ionic and is soluble in water (hydrophilic). The tail part of the molecule is only soluble in organic substances such as oil or grease.. It is hydrophobic. Thus, the soap molecule is soluble in both water and oils. When soap is mixed with water, the soap molecules decompose to form sodium ions which are positively charged and soap ions which are negatively charged.
The Cleansing Action of Soap Molecules
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The grease that has been pulled off is surrounded by soap molecules and forms oil droplets. Lather (foamy bubbles) produced help to float the grease or oil droplets in water. The oil droplets in the soap water form an emulsion solution. As each ionic head is charged (negatively), the oil droplets repel each other. This prevents the droplets from combining again. The oil droplets are then removed with the soap solution.
When soap dissolves in hard water, the soap molecules react with calcium and magnesium ions in hard water and form a grey, sticky insoluble ‘scum’. The action of hard water on soap weakens the cleansing action of soap and causes more wastes of soap. Synthetic detergents are made by mixing oil with concentrated sulphuric acid. Detergents form lather in hard water. Modern washing powders contain detergents and soap. Sodium carbonate (washing powder) is added to powders to soften hard water. Carbonate bonds with the calcium or magnesium, leaving soap free to remove the dirt and grease.
However, the use of synthetic detergents can cause a pollution problem because synthetic detergents are not biodegradable.