Hard-water scum

Many areas have household water containing calcium, magnesium, and iron ions. Although these mineral-rich waters can be healthful for drinking, the ions react with soaps to form insoluble salts called hard-water scum. The following equation shows the reaction of a soap with calcium, common in areas where water comes in contact with limestone rocks. 

The hard water scum that forms a ring around the bathtub is... 

Detergents are made by, for example, treating petroleum hydrocarbons with sulphuric acid, yielding sulphonated products which are water soluble. These can also solubilise fats and oils since, like the stearate ion, they have an oil-miscible hydrocarbon chain and a water-soluble ionic end. The calcium salts of these substances, however, are soiu u-ic in water and, therefore, remove hardness without scum formation. 

Detergents are designed to be effective in hard water meaning water containing calcium salts that form insoluble calcium carboxylates with soaps These precipitates rob the soap of Its cleansing power and form an unpleasant scum The calcium salts of synthetic deter gents such as sodium lauryl sulfate however are soluble and retain their micelle forming ability even m hard water... 

Anionic surfactants are the most commonly used class of surfactant. Anionic surfactants include sulfates such as sodium alkylsulfate and the homologous ethoxylated versions and sulfonates, eg, sodium alkylglycerol ether sulfonate and sodium cocoyl isethionate. Nonionic surfactants are commonly used at low levels ( 1 2%) to reduce soap scum formation of the product, especially in hard water. These nonionic surfactants are usually ethoxylated fatty materials, such as H0CH2CH20(CH2CH20) R. These are commonly based on triglycerides or fatty alcohols. Amphoteric surfactants, such as cocamidopropyl betaine and cocoamphoacetate, are more recent surfactants in the bar soap area and are typically used at low levels (<2%) as secondary surfactants. These materials can have a dramatic impact on both the lathering and mildness of products (26). 

The soaps produced by saponification may give excessive foam during spray cleaning or react with hard-water salts to form scum and scale. 

EDTA sequesters calcium and magnesium from hard water, preventing them from forming insoluble soap films (scum) with soaps and detergents. Chelators are sometimes used to sequester metal ions that interfere with dyes and perfumes. 

Soaps are made by heating sodium hydroxide with a fat such as coconut oil, olive oil, or beef fat, which contain esters formed between glycerol and fatty acids (see Section 19.7). The sodium hydroxide attacks the esters and forms the soluble soap. In the case of beef fat, stearic acid forms the soap sodium stearate, seen in (3). Soaps, however, form a scum in hard water. The scum is an impure precipitate of calcium stearate. 

Figure 5.3 As seen on the side of the tub and drain stopper right), a white residue known as soap scum forms when soap reacts with calcium salts present in hard water.

Water softener manufacturers remind us of the many undesirable effects produced by hard water. Besides enhancing the likelihood of harmful scale deposits in plumbing, water heaters, and dishwashers, hard water also has been associated with bathtub scum, deposits on laundry, scale on glasses and dishes, scratchy skin, and unmanageable hair. A simple chemical process explains the origin of hard water. Additional chemical reactions provide an explanation for the scum and deposits that readily form when soap and hard water combine. 

A second problem with hard water is that these metals react with soap molecules and form a scum to which bathtub rings, etc., are attributed. Hard water is therefore not the best water to use for efficient soapy water cleaning processes, since the metal-soap precipitation reaction competes with the cleaning action. Water softeners assist with solving this problem too. 

Choose acidic formulations for products designed to dissolve soap scum, hard water spots, stains, rust, or encrustations that are mainly calcium or magnesium salts [10]. 

Borax is used as a cleaning agent and water softener that removes ions of elements such as magnesium and calcium that cause hard water. When these hard water elements are mixed with soap, they prevent soap from sudsing and form a scum or residue that is deposited on hard surfaces. Borax can ehminate this residue ring by replacing the Mg and Ca ions with the more soluable Na and ions. Borax is the third most important boron compound. 

These Insoluble soaps separate as scum in water and are useless as cleansing agent. In fact these are hlnderance to good washing, because the precipitate adheres onto the fibre of the cloth as gummy mass. Hair washed with hard water looks duU because of this sticky precipitate. Dye does not absorb evenly on cloth washed with soap using hard water, because of this gummy mass. 

Unpleasant feeling. Hard water gives the symptom of soap curd and scum in bathtubs, and is caused by calcium and magnesium salts. 

Hard" water contains impurities dissolved in it, such as chalk, and when ordinary soap is put into hard water, a precipitate, or scum, forms. This is not only unattractive, it is also wasteful, since it prevents the soap from doing its job until all the chemicals causing the scum have been pushed out of the water. One of the main chemical ingredients of hard water is calcium sulfate. 

A scum forms on the surface of boiling hard water. What is this scum Why does it form ... 

Sometimes people refer to water as being "hard" or "soft." If someone says water is hard, he means that there are a lot of calcium (Ca2+) or magnesium (Mg2+) ions dissolved in it. Hard water causes several problems. First, it can cause scales to form on the inside of pipes, water heaters, or teakettles. These scales occur when the calcium or magnesium precipitates out of solution and sticks to the insides of pipes. The scales build up and eventually the pipes are completely clogged. Hard water also prevents soap from lathering, and it reacts with soap to leave behind a sticky film commonly called soap scum. 

In hard water areas all over the world, it is difficult to make the soap lather. Instead, the water becomes cloudy. This cloudiness is caused by the presence of a solid material (a precipitate) formed by the reaction of the dissolved substances in the water with soap (basically sodium stearate) and it is a real problem. This white precipitate is known as scum (Figure 11.35). 

Figure 11.35 Soap and hard water form scum.

To overcome the problem of scum formation, soapless detergents have been developed. They do not produce a scum because they do not react with the substances in hard water. For further discussion of soapless detergents see Chapter 15, p. 239. 

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