Cleaning Capacity of Soap

The aim of this experiment is to study the comparative cleansing capacity of a sample of soap in soft and hard water. Alcohol undergoes oxidation upon burning in the presence of excess oxygen. This process is known as combustion.

Hardness in water is caused due to the presence of the water soluble salts of calcium and magnesium (bicarbonates, chlorides and sulphates) in water.

When a piece of soap is dissolved in hard water, it reacts with the salts of calcium and magnesium and forms insoluble calcium and magnesium salts of the fatty acid. This precipitate is known as scum which floats on top of the water surface. The cleansing capacity of soap in solution then becomes ineffective and reduces its foaming capacity.

Ca(HCO₃)₂(aq) + 2C₁₇H₃₅COONa(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2NaHCO₃(aq)

Sodium stearate (soap) Scum

CaSO₄(aq) + 2C₁₇H₃₅COONa(aq) → (C₁₇H₃₅COO)₂Ca(s) + Na₂SO₄(aq)

Apparatus and materials are required

Apparatus:

Beaker– (50 mL)
Spatula
Magnetic stirrer

Materials:

Scale
Digital balance
Soap
Well water/Underground water
Distilled water

Chemical:

Calcium carbonate

To perform this experiment, steps to be followed in the following manner:

Procedure

The three beakers are labelled A, B and C. Take 20 mL of distilled water in beaker A, 20 mL of underground water in beaker B, and in beaker C, add 2 g of calcium hydrogen carbonate (or calcium sulphate) to 20 mL of distilled water.
Stir the contents of beaker C so that calcium hydrogen carbonate (or calcium sulphate) dissolves in water.
Drop 1 g of soap in each beaker A, B, and C. Stir the contents of these beakers, an equal number of times, to form the respective solutions.
Measure the length of the foam produced immediately with the help of a measuring scale in all three samples and record your observations.

In this experiment, we learnt that:

The hardness of water is caused by the presence of calcium or magnesium salts dissolved in water, which react with soap to form calcium or magnesium stearate, also called scum, which floats on top of the solution.

The amount of lather formed from soap depends on the hardness of water, and hence more lather is produced in soft water, which has no calcium or magnesium ions.

FAQs on Cleaning Capacity of Soap

Q1: What is the cleansing action of soap?

Answer: The dirt present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap will form a micelle and trap the dirt at the centre of the crystal. These micelles remain suspended in the water. Hence, the dust particles are easily rinsed away by water.

Q2: Define hard and soft water?

Answer: Hard water is the water containing a high amount of minerals of calcium and magnesium. Soft water is the surface water or chemically softened water.

Q3: What is soap?

Answer: Soap is a sodium salt or potassium salt of long chain fatty acids having cleansing action in water. They are used as cleansing agents to remove dirt, oil from the skin and clothes.