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प्रश्न
Explain the formation of scum when hard water is treated with soap.
उत्तर १
Hard water often contains salts of calcium and magnesium. Soap molecules react with calcium and magnesium salts and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.
\[\ce{\underset{Hard water}{Ca^{2+}} + \underset{Soap}{2RCOONa} -> \underset{Cal. salt (ppt{.})}{(RCOO)2Ca} + 2Na+}\]
\[\ce{\underset{Hard water}{Mg^{2+}} + \underset{Soap}{2RCOONa} -> \underset{Mag. salt (ppt{.})}{(RCOO)2Mg} + 2Na+}\]
उत्तर २
Scum is formed when a soap is used in hard water. This happens because a large amount of soap is utilised in reacting with the magnesium and calcium ions of hard water. An insoluble curdy precipitate, scum, sticks to the clothes making them difficult to be cleaned.
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संबंधित प्रश्न
Both soap and detergent are some type of salts. What is the difference between them?
Describe in brief the cleansing action of soap.
While preparing soap a small quantity of common salt is generally added to the reaction mixture of vegetable oil and sodium hydroxide. Which one of the following may be the purpose of adding common salt?
(A) To reduce the basic nature of the soap
(B) To make the soap neutral
(C) To enhance the cleansing power of the soap
(D) To favour the precipitation of the soap
In the neighbourhood of your school, hard water required for an experiment is not available. Select from the following group of salts available in your school, a group each member of which, if dissolved in distilled water, will make it hard:
(a) Sodium chloride, calcium chloride
(b) Potassium chloride, sodium chloride
(c) Sodium chloride, magnesium chloride
(d) Calcium chloride, magnesium chloride
Which of the following sets of materials can be used for conducting a saponification reaction for the preparation of soap?
(a) Ca(OH)2 and neem oil
(b) NaOH and neem oil
(c) NaOH and mineral oil
(d) Ca(OH)2 and mineral oil
Hard water is not available for an experiment. Some salts are given below :
(I) Sodium chloride
(II) Sodium sulphate
(III) Calcium chloride
(IV) Calcium sulphate
(V) Potassium chloride
(VI) Magnesium sulphate
Select from the following group of these salts, each member of which may be dissolved in water to make it hard.
(A) I, II, V
(B) I, III, V
(C) III, IV, VI
(D) II, IV, VI
A student took four test tubes P, Q, R and S and filled about 8 mL of distilled water in each. After that he dissolved an equal amount of Na2SO4 in P, K2SO4 in Q, CaSO4 in R and MgSO4 in S. On adding an equal amount of soap solution and shaking each test tube well, a good amount of lather will be obtained in the test tubes:
(a) P and Q
(b) P and R
(c) P, Q and S
(d) Q, R and S
What change will you observe if you test soap solution with a litmus paper (red and blue)? Give reason for your observation.
Why is soap not suitable for washing clothes when the water is hard?
Which of the two is better for washing clothes when the water is hard: soap or detergent? Give reason for your answer.
Explain the cleansing action of soap. Draw diagrams to illustrate your answer.
What are detergents chemically? List two merits and two demerits of using detergents for cleansing. State the reason for the suitability of detergents for washing, event in the case of water having calcium and magnesium ions.
Give a scientific explanation.
Soap forms a precipitate in hard water.
Soap is a salt of ______ and sodium hydroxide.
Write an answer to the following question.
Name three chemicals responsible for the surface activity of various detergents.
Match the following
| 1. | Soap | C6H5 OH |
| 2. | Cement | CaSO4.2H2O |
| 3. | Fertilizers | NaOH |
| 4. | Gypsum | RCC |
| 5. | Phenol | NPK |
Analogy:
Used to join components: Adhesives:: Clean the body ______.
______ to remove strong strains on the clothes.
Why do soaps not work effectively in hard water?
