Question

Both soap and detergent are some type of salts. What is the difference between them ? Describe in brief the cleansing action of soap. Why do soaps not form lather in hard water ? List two problems that arise due to the use of detergents instead of soaps.

Answer 1

Soap molecules are sodium or potassium salts of long-chain carboxylic acids whereas detergent molecules are ammonium or sulphonate salts of long-chain carboxylic acids. Cleansing action of soaps: The dirt present on clothes is organic in nature and is insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water and, thus, the dust particles are easily rinsed away by water. Soaps do not form lather in hard water because soap molecules react with calcium and magnesium ions present in hard water to form insoluble substance, called scuum which remains after washing in water. Problems that arise due to the use of detergents instead of soaps are as follows: a) Being non-biodegradable in nature, they get accumulated in the environment and cause soil and water pollution. b) Their entry into the food chain leads to bio-accumulation in living beings and causes several health issues.

Answer 2

Answers

The molecules of soap are sodium or potassium salts of long-chain carboxylic acids.

Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.

Cleansing action of soap can be described as follows:

 A soap molecule has a tadpole-shaped structure.

 At one end (long non-polar end) of the soap molecule is a hydrocarbon chain which

is insoluble in water but soluble in oil.

 At the other end (short polar end) of the soap molecule, there is a carboxylate ion

which is hydrophilic, i.e. water soluble but insoluble in oil.

 Soap on mixing with water forms a concentrated solution and causes foaming.

 The long non-polar end of soap gravitates towards and surrounds the dirt and

absorbs the dust in it.

 The short polar end with the carboxylate ion repels the water away from the dirt.

 A spherical aggregate of soap molecules is formed in the soap solution in water and

is called a micelle.

 Thus, the soap molecule dissolves the dirt and our clothes get clean.

Soaps do not form lather in hard water because

Hard water contains calcium and magnesium salts. Soap molecules react with calcium

and magnesium salts to form an insoluble precipitate called scum.

Two problems arise because of the use of detergents instead of soap:

i. Soaps are biodegradable, while detergents are non-biodegradable; hence,

detergents accumulate in the environment and cause problems.

ii. Certain phosphate additives are added to detergents. These phosphate additives

act as nutrients for algae which form a thick green scum over the river water and

upset the animal life in the river.