To study the comparative cleaning capacity of a sample of soap in soft and hard water - Science Practicals

To study the comparative cleaning capacity of a sample of soap in soft and hard water.


Hardness of water is caused by the presence of the salts of calcium and magnesium (hydrogencarbonates, chlorides and sulphates) in water. These salts are soluble in water. When soap is added to hard water, it reacts with the salts to form a scum, which is insoluble and floats on top of the water surface. The scum is formed due to the formation of insoluble calcium or magnesium salts of the fatty acid used in the soap formation. The soap in solution then becomes ineffective.
Ca(HCO 3)2 (aq) + 2Na+ -stearate (aq) ⎯→ Ca(stearate)2 (s) + 2NaHCO3 (aq)
CaSO4 (aq) + 2Na+ -stearate (aq) ⎯→ Ca(stearate)2 (s) + Na2SO4 (aq)
The salts of calcium and magnesium show similar reactions. Therefore, the presence of calcium and magnesium salts in water precipitates the soap thereby reducing its cleansing power and foaming capacity.

Materials Required
Underground water (well water), distilled water, calcium hydrogencarbonate or calcium sulphate, soap sample, a physical balance and weight box, three test tubes and a test tube stand, three beakers (100 mL), three glass rods, a measuring cylinder (50 mL), and a measuring scale,


Step 1: Three beakers are taken and labelled as A, B and C.
Step 2: 20 mL of distilled water is taken in beaker A. In beaker B, 20 mL of underground water is put, and 2 g of calcium hydrogencarbonate (or calcium sulphate) is added in beaker C to 20 mL of distilled water.
Step 3: The contents of beaker C is stirred so that calcium hydrogen carbonate (or calcium sulphate) dissolves in water.
Step 4: 1 g of soap is put in each beaker A, B, and C (after weighing it using a physical balance).
Step 5: The contents of these beakers are stirred with separate glass rods.
Step 6: Three test tubes are placed in a test tube stand and labelled as tube A, B and C.
Step 7: 3 mL of the above prepared soap solution is poured in the corresponding test tubes.
Step 8: Test tube A is taken and shaken ten times by placing thumb on its mouth.
Step 9: Foam or lather is formed by shaking the test tube. The length of the foam produced is measured immediately with the help of a measuring scale.
Step 10: Similarly, steps 8 and 9 are repeated with the remaining two samples.

(i) Mass of the soap sample taken in each beaker = _____ g
(ii) Volume of the distilled water and underground water = _____ mL
added in each beaker
(iii) Volume of soap sample taken in each test tube =  _____  mL
(iv) Number of times each test tube taken =  _____

Results and Discussions
Infer from the observations that which solution of the soap sample produces the maximum length of foam (lather). For cleansing purpose, the foam needs to be produced which depends on free availability of hydrophobic portion of soaps (or alkyl groups). In hard water it is trapped due to scum or precipitation, this makes the hard water unsuitable for washing.

1. Use same sample of soap for soft water and hard water.
2. Stir the mixture carefully while dissolving soap in water so as to avoid spilling of soap solution.
3. The quantity of soap sample in all solutions must be same. The amount of distilled water added in every soap sample must be same. That is the concentration of all test solutions must be same.
4. The mass of the soap samples must be determined very carefully using a physical balance. In case of any need, take help from your teacher.
5. Shake every tube for equal number of times and in a similar manner.
6. Measure the length of the foam produced immediately after its production.

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