CBSE Class 10 Science Practical Skills – Cleaning Capacity of Soap in Hard and Soft Water
BASIC BUILDING CONCEPTS
Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is, it dissolves in water, while the other end is hydrophobic, that is, it dissolves in hydrocarbons. When soap is at the surface of water, the hydrophobic tail of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and hydrocarbon tail protruding out of water. Inside the water, these molecules have a unique orientation that keeps the hydrocarbon portion out of the water. This is accomplished by forming clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface. This formation is called micelle. Soap in the form of a micelle is able to clean, since the oily dirt will be collected in the centre of micelle.
To study the comparative cleaning capacity of a sample of soap in soft and hard water.
Tap water, distilled water,
calcium hydrogen carbonate (0.5 g),
calcium chloride (0.5 g),
calcium sulphate (0.5 g),
soap sample (1 g),
five test tubes,
three beakers (100 ml),
four glass rods,
a measuring scale and three droppers.
Water that does not contain a significant amount of dissolved minerals such as compounds of calcium and magnesium is soft water. It produces lather with a little amount of soap. Hard water is water that has high mineral content, it has high concentrations of Ca2+ and Mg2+ ions. Calcium and magnesium hydrogen carbonates, chlorides and sulphates in water render it hard. When soap is added to hard water, it does not form lather. The calcium and magnesium salts in hard water react with soap and form scum, which is insoluble and floats on top of water. Due to the formation of scum, which is calcium or magnesium salt of soap (soap is sodium or potassium salt of fatty acids), soap becomes ineffective in solution and no cleansing occurs. The equations involved in the formation of scum are as follows:
As the calcium and magnesium salts in water react with soap, cleansing power of soap is reduced and its foaming capacity also decreases.
1. Take three beakers (100 ml) and label them as ‘A’, ‘B’ and ‘C
2. Prepare 30 ml soap solution by adding 1 g soap to water in beaker ‘C’.
3. Pour 20 ml tap water in beaker ‘A’ and 50 ml distilled water in beaker ‘B’.
4. Place five test tubes, labelled A, B, C, D and E, in a test tube stand.
5. Using a dropper pour 5 ml of tap water in test tube A, and 5 ml distilled water in the other test tubes labelled as B, C, D and E.
6. To test tube C, D and E add a pinch of calcium hydrogencarbonate, calcium chloride and calcium sulphate respectively using a spatula.
7. Mix the solution well using a glass rod.
8. Using a dropper, add 3 ml of soap solution to the test tube A. Measure the length of the solution present in the test tube A and record this in table.
9. Shake test tube A, ten times by placing thumb on its mouth.
10. Observe whether foam is formed by shaking the test tube and record your observations.
11. Measure the length of content (foam and solution) present in the test tube immediately with the help of a measuring scale.
12. Repeat steps 8 to 11 with the remaining water samples (B, C, D and E) and record your observations.
- Volume of water samples taken in test tube = 5 ml.
- Volume of soap solution added to each test tube = 3 ml.
- Maximum length of foam is produced in water sample(s) …………………………………
- Scum is formed in water sample(s) ……………………………
- The water sample most suitable for washing purpose is …………………………..
For cleaning, foam needs to be produced. Production of foam depends on the free availability of hydrophobic portion of the soap. In hard water due to formation of scum hydrophobic portion of the soap gets trapped. Therefore, hard water is not suitable for cleaning purpose.
- Use same amount of soap for all the samples.
- Take equal volumes of water samples.
- Shake each test tube equal number of times and in a similar manner.
- Length of foam should be measured immediately after shaking.
What is hard water?
Hard water is the water that has high mineral content and produces scum on addition of soap.
What is soft water?
The water which has low mineral content and produces lather on mixing soap to it is called soft water.
Why does hard water produce scum when soap is added to it?
Hard water produces scum when soap is added due to the presence of Ca2+ and Mg2+ salts in it, which react with soap forming precipitates which are calcium and magnesium salts of soap.
How does presence of Ca2+ and Mg2+ ions in water lead to the formation of scum?
Ca2+ and Mg2+ ions present in hard water react with soap to form calcium and magnesium salts of soap which are insoluble in water.
Give the equation involved in the formation of scum.
Does the presence of all salts of calcium and magnesium cause hardness in water?
Which salts cause hardness in water?
The hydrogencarbonates, chlorides and sulphates of calcium and magnesium cause hardness in water.
Which ions are present in soft water?
Only sodium ion is present in soft water.
Which out of hard water or soft water is better for cleansing?
Can hard water be softened?
By boiling or by chemical treatment.
Can hard water be used for washing purposes?
No, hard water cannot be used for washing purposes.
Because a large amount of soap gets washed away due to the formation of scum. This also sticks to fibres of garments.
Out of soaps and detergents which form lather in hard water?
Would sodium chloride present in water render it hard?
No, sodium chloride does not render water hard.
Can you explain why sodium salts in water do not make it hard water?
Soaps are sodium salts of fatty acids. On adding sodium salts to water containing soap, no scum is formed as can be understood by the following equation.
NCERT LAB MANUAL QUESTIONS
Do both hard water and soft water produce foam with soap?
No, only soft water produces foam with soap.
Why is scum formed when hard water is treated with soap?
Scum is formed when hard water is treated with soap because Ca2+, Mg2+ ions present in hard water react with soap and form an insoluble precipitate scum.
With their prolong use, white scales get deposited in the interior of boilers and electric kettles. What is the reason for this observation?
On prolong use, scales are deposited in the interior of boilers and electric kettles due to the precipitation of CaCO,, and CaS04. The precipitation of CaC03 is also caused due to the thermal decomposition of bicarbonate ions.
What do you understand by temporary and permanent hardness of water?
Temporary hardness is caused due to the presence of calcium and magnesium bicarbonates in water and can be removed by boiling or by adding Na2C03 to hard water.
Permanent hardness is caused due to the presence of chlorides and sulphates of calcium and magnesium and can be removed by using an ion exchanger.
What is the reaction between soap molecules and ions present in hard water?
PRACTICAL BASED QUESTIONS
Multiple Choice Questions /VSA (1 Mark)
Soaps work well in
(a) hard water
(b) soft water
(c) acidified water
(d) very cold water
Presence of which salt makes the water hard?
(a) salts of sodium
(b) salts of potassium
(c) salts of calcium
(d) all of these
Scum is the precipitate formed when soap in water reacts with
(a) calcium and magnesium salts
(b) calcium and sodium salts
(c) magnesium and sodium salts
(d) calcium and potassium salts
Which of the following statement is correct about the formation of lather by soap?
(a) In hard water soap forms more lather.
(b) In soft water soap forms more lather.
(c) In both hard and soft water soap forms same amount of lather.
(d) In both hard and soft water soap does not form lather.
Temporary hardness in water is caused by
(a) chlorides of sodium and magnesium
(b) sulphates of calcium and potassium
(c) hydrogencarbonates of calcium and magnesium
(d) hydrogencarbonates of sodium and potassium
Temporary hardness of water can be removed ! by
(b) reacting with NaCl
(c) reacting with KC1
Permanent hardness in water is caused by
(а) chlorides and sulphates of sodium and potassium
(б) chlorides and sulphates of sodium and calcium
(c) chlorides and sulphates of calcium and magnesium
(d) chlorides and sulphates of magnesium and potassium.
The soap solution required to compare the cleaning capacity of soap is made by dissolving soap in
(a) well water
(b) tap water
(c) filtered water
(d) distilled water
Which of the following is best suited for washing clothes with soap?
(a) Distilled water containing calcium hydrogen- carbonate
(b) Distilled water containing calcium chloride
(c) Distilled water containing calcium sulphate
(d) Only distilled water.
Why can’t hard water be used for washing clothes?
(a) It forms scum with soap.
(b) It hardens the texture of clothes.
(c) It does not react with soap.
(d) This statement is wrong. Hard water is suited well for washing clothes.
When hard water is used in boilers for a long time, which phenomenon is observed?
(а) The amount of steam formed in boilers ; increases.
(b) The boilers work more efficiently.
(c) White scales get deposited on the interiors of boilers.
(d) Boilers do not work with hard water.
Clothes do not get cleaned when hard water is used for washing them. This is because
(a) hard water reacts with fibres of clothes.
(b) the scum formed by reaction of soap with hard water sticks to the fibres of clothes.
(c) hard water itself is dirty.
(d) fibres of clothes do not get wetted by hard water so they do not get cleaned.
Which of the following statements is wrong?
(а) Soaps and detergents both form lather with soft water.
(б) Soaps form lather with hard water, detergents do not form lather with hard water.
(c) Soaps do not form lather with hard water, detergents form lather with hard water.
(d) Detergents are better cleaning agents than soaps.
Soaps act as cleansing agents by
(а) releasing sodium/potassium ions in solution
(b) undergoing a reaction with fat
(c) forming micelles and trapping dust within the micelle
(d) all of the above.
In the presence of calcium and magnesium salts in water, the cleansing capacity of soap
(c) no change
(d) depends upon the quality of soap.
Identify the statement which is not correct.
(a) Soap forms micelles in soft water.
(b) Soap forms scum in hard water.
(c) Soap forms lather in soft water.
(d) Both ends of a soap molecule are hydrophobic.
On the addition of Na2C03 to hard water
(a) the foaming capacity increases
(b) the foaming capacity decreases
(c) the foaming capacity increases and then decreases
(d) the foaming capacity decreases and then increases.
The correct equation involved in removal of hardness from water is
Short Answer Type Questions
1.Differentiate between soft water and hard water.
2. Are soap molecules hydrophobic or hydrophilic in nature?
3. How is soap able to clean oily dirt?
4. Soap cannot be used in hard water. Why?
5. Give one advantage of
- detergent over soap.
- soap over detergent
Multiple Choice Questions /VSA
Short Answer Questions
2. A soap molecule is made up of two parts: a long hydrocarbon part and a short ionic part. The long hydrocarbon chain is hydrophobic in nature but the ionic part is hydrophilic in nature.
3. Inside water, soap molecules form a unique orientation around oil or dirt molecule, called micelles. Soap in the form of micelles is able to clean because oily dirt accumulates in the centre
of micelle. Besides this, soap micelles can be easily rinsed away as they are present in the form of colloid.
4. Hard water contains Ca2+ and Mg2+ ions. When soap is added to hard water, Ca2+ and Mg2+ ions react with soap to form calcium and magnesium salts of soap. There salts of soap are insoluble in water and form scum. As a result, a lot of soap is wasted. Because of this reason soap cannot be used in hard water.
- Detergents can be used in hard water but soap cannot be used in hard water.
- Soaps are biodegradable and hence they do not cause pollution. Detergents are non-biodegradable and hence cause water pollution.