Question

While studying the comparative cleansing action of soap with hard water and soft water,a student observed white scum when he used tap water and foam when he used distilled water.Explain the reason behind the observation.

Answer 1

Our Objective

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

The Theory

Ordinary water does not remove dirt from clothes or skin because the dirt present is oily or greasy in nature. Soaps are one of the most commonly used cleansing agents and are capable of reacting with water to remove  dirt. They are either animal origin or plant in origin.
Chemically, they are water soluble sodium or potassium salts of higher fatty acids such as stearic acid, palmitic acid or oleic acid. Fatty acids are found in fats and oils. When fat or oil is treated with alkali such as caustic soda or caustic potash soap and glycerin are produced.
                             

The soap molecule is generally represented as RCOONa. In solution, it ionizes to form RCOO- and Na+. Each soap molecule has a polar head group (carboxylate ion, COO- group) and a long non-polar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the non-polar tail attracts the water insoluble oily or greasy dirt particles.

When a dirty cloth is placed in soap solution, the long non-polar hydrocarbon tail of soap molecules points towards the oily dirt particles and the polar heads point towards the water. This forms a spherical structure with polar parts of the molecule on the surface and non-polar parts in the center. This spherical structure is called micelle. This micelle is attracted towards water and carries the oily dirt particles along with it. This causes the dirt particles to detach from the fibres of the cloth. In this manner, clothes become free from dirt or dust.



Other important cleansing agents are synthetic detergents. These are sodium salts of long chain sulphonic acids and are generally represented as RSO3Na. The cleansing action of soaps and detergents are same.

Rubbing of clothes with brush or agitation in a washing machine loosens the bond between the dirt particles and the fibres of clothes. This supports the cleansing action of soaps and detergents.

Classification of Water

Water, along with soap, is used for washing purposes. On the basis of effective washing with soap, water has been classified as soft water and hard water.

Soft water: Water which produces good lather with soap is called soft water. When water falls as rain, it is naturally soft. Washing with soap is easy in soft water.

Hard water: Water which does not produce good lather with soap is called hard water. It is difficult to wash with soap in hard water. Water seeping through the ground becomes hard water. It is not useful for laundry and laboratory purposes.

Cleaning Capacity of Soap with Hard and Soft Water

Although soap is a good cleaning agent, its cleaning capacity is reduced when used in hard water. Hardness of water is due to the presence of sulphates, chlorides or bicarbonate salts of Ca2+ or Mg2+ ions. Soaps are sodium or potassium salts of long chain fatty acids. When soap is added to hard water, the Ca2+ and Mg2+ ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding calcium and magnesium salts which are precipitated as scum. The insoluble scum sticks on the clothes and so the cleaning capacity of soap is reduced.



The cleaning action of soap is very effective in soft water because it contains negligible calcium and magnesium ions.

Synthetic detergents are used in the case of hard water also because the calcium and magnesium salts of detergents are soluble in water. Detergents are more soluble than soaps and hence form more lather than soaps.

Answer 2

Tap water is hard water which contains calcium and magnesium salts. Soap reacts with Ca and Mg salts to form insoluble substance called scum.

While distilled water is soft water which doesn't have Ca and Mg salts.Distilled water is pure and doesn't have any impurities therefore foam is formed.