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


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

Apparatus and Chemical

  • Three test tube (20 mL)
  • Dropper
  • Test tube stand three beakers (100 mL) :three glass rod
  • A measuring cylinder (50 mL)
  • A measuring scale
  • Underground water
  • Hard water
  • Distilled water
  • Soap solution


Hardness of water is generally due to the presence of salts of calcium and magnesium (hydrogencarbontes, Chloride and sulphates) in water. These salts are soluble in water. When soap is added in 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(HCO3)2 + 2Na+ stearate → Ca(stearate)2 + 2NaHCO3
MgCl2 + 2Na+ stearate → Mg(stearate)2 + 2NaCl

As shown above the salts of calcium and magnesium show similar reaction. Therefore, the presence of calcium and magnesium salts in water precipitates the soap thereby reducing its cleansing power and foaming capacity.


  • Take three 100 mL beakers and label them as A, B and C
  • Take 20 mL of distilled water in beaker A. In beaker B take 20 mL of underground water. In beaker C take 20 mL of hard water.
  • Put 10 drops of soap solution in each beaker with a dropper and stir the content of each beaker with separate glass rods.
  • Place three test tube in test tube stand and label them as A, B and C
  • Pour 3 mL of the above solution in the corresponding test tubes.
  • Take test tube A and shake it ten times by placing thumb on its mouth.
  • Foam or lather will be formed by shaking the test tube. Measure he length of the foam produced immediately with the help of a measuring scale
  • Similarly, repeat steps 6 and 7 with the remaining two samples.

Observations and calculations

  • Volume of the distilled water, underground water or hard water = ___________ mL
  • Drops of soap solution added to each beaker = ___________ mL
  • Volume of solution taken in each test tube = ___________ mL
  • Number of times each test tube is shaken = ___________ mL
S.No. Mixture
( water + soap)
Test tube reading
Initial length Final length
(cm) (cm)
Length of the foam produced
1.Distilled water
(soft water)
2.Well water or
underground water
3.Hard water

Results and Discussion

Find out from the observation that which sample of water produces the maximum length of foam (lather)

For cleansing purpose, the foam needs to be produced which depends on free availability of soap. In hard water it is trapped due to scum, this makes the hard water unsuitable for washing.


  • Stir the mixture carefully so as to avoid spilling of soap solution.
  • Shake every tube for equal numbers of times and in similar manner.
  • Measure the length of the foam produced immediately after its production.


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