Tuesday 13 December 2011

CBSE - Class 10 - Ch4 - Soaps and Detergents

Soaps and Detergents

Soap & Detergents (Concept Map)
Q1: Why soap does not work well with hard water containing Ca2+ or Mg2+ ions?

Answer: Because it reacts with Ca2+ or Mg2+ to forma white precipitate which is called scum and soap goes waste. Therefore soap forms no or little lather with hard water.

Q2 Differences between Organic Compounds and Inorganic Compounds.


Organic CompoundsInOrganic Compounds
Mostly covalent bonding.Mostly Ionic bonding.
Most of the compounds are gases and liquids. If solids have low melting points.Most of these are solids with high melting points
Generally insoluble in water. Many are soluble in water.
Soluble in organic solvents like alcohol, toulene etc. Generally insoluble in organic solvents.
Aq. solution do not conduct electricity. Since Aq. soln. forms ions, conducts electricity.
Burn and decompose easily. Does not burn easily.
Slow reactions Fast reactions generally.
Q3 What is soap? Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?

Answer: Soap are sodium or potassium salts of long-chain carboxylic acids. In general a a soap is the dual-nature molecule, sodium dodecyl sulphate , with its long, snake-like structure. Its head is an ionized sulphonic acid group –SO−4 Na+, ion paired with a sodium cation. The remainder of the molecule (tail) is a wholly non-ionic straight-chain alkyl group.

The acid-head of the soap is capable of ionic reactions and thus dissolves in water, that's why it is hydorphilic (water loving) in nature. While the carbon chain dissolves in oil and is hydrophobic (water hating) by nature. When soap is added to the water, the hydrophilic end aligns along the surface of water and the hydrophobic tail remains out of water. Such cluster of molecules in which hydrophobic tail are in interior of the cluster and ionic ends are on the surface of the cluster is called micelle.

The soap micelles thus helps in dissolving dirt/grease in water and help in cleaning clothes.
No, micelle will not be formed when added in solvents like ethanol.

Q4: What is Glycerol?

Answer: Glycerol is a sweet viscous liquid soluble in water. It is a byproduct of saponification process. It goes by a common name glycerine. Its formula is CH2OH.CHOH.CH2OH. It is used in the manufacture of explosives, plastics, and pharmaceuticals. It is also used to prevent freezing.

Q5(NCERT): Would you be able to check if water is hard by using a detergent?

Answer: Detergent works both in hard water and soft water, it cannot be used to detect if water is hard or not. Detergents are ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds are effective in hard water because they do not form insoluble precipitates with the calcium and magnesium ions in hard water.

Q6(NCERT): People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes? 

Answer: The soap micelles formed by soap entrap dirt and grease particles and lie on the surface of the clothes. When the clothes are agitated, these micelles get detached from the surface of the clothes and go into water.


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