Question

Have you ever observed while cleaning cloth that foam is formed with difficulty and an insoluble substance (scum) remains after washing with water? This is caused by the reaction of soap with the calcium and magnesium salts, which cause the hardness of water. Hence, you need to use a larger amount of soap. This problem is overcome by using another class of compounds called detergents as cleansing agents. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, they remain effective in hard water. Detergents are usually used to make shampoos and products for cleaning clothes. 

(i) How will you test the hardness of water? 

(ii) Why soap forms scum (insoluble substance) with hard water? 

(iii) How one can overcome the problem of scum formation? What values do you identify in this context?

Answer 1

(i) The hardness of water can be determined using soap. 

If the given amount of soap has difficulty in producing foam (lather) and produces an insoluble precipitate (scum) instead, in the water then the water is hard. 

If the soap produces foam easily in the water, then the water is soft. 

(ii) Hard water contains calcium and magnesium ions. Scum is formed as a result of reaction between calcium or magnesium ions with soap. 

(iii) The problem of scum formation can be avoided by using detergents. The charged ends of these compounds do not form insoluble precipitates with calcium or magnesium ions in hard water which makes detergents effective in hard water. 

From this, one can learn that soaps are ineffective in hard water (water containing calcium and magnesium ions) as they do not produce foam in hard water. This is rectified by the usage of another class of compounds called detergents. Thus one must not use soaps in hard water, as it will be a waste of time and money. At the same time one must also realize that detergents are non-biodegradable. These compounds cannot be degraded by bacteria and therefore can potentially harm the environment.