Explain why adding soap to water will help remove dirt and oil?

Question

Explain why adding soap to water will help remove dirt and oil. Give a scientific reason why to earn points.
Keep In Mind that a soap molecule is long with one end attracted to oil molecules
and that one end of a soap molecule is polar, and the other end is nonpolar. Soap will dssolve , and the soap molecules will float freely in water.

Answer 1

In as plain English as possible:

The non-polar end adsorbs the oil or other hydrophobic dirt. The ionic end is highly soluble in water. This allows for an emulsion to be formed. The alkali metal (sodium or potassium ion) does not play a role in the action of the soap.

Soap use is not a chemical reaction, but a physical one. Under normal conditions, the soap does not react with the dirt chemically. If "hard water" minerals are present (magnesium or calcium) these can chemically react with the soap and lessen its effectiveness by removing the soap from solution.

The structure of the emulsion is such that the oil or oily dirt is surrounded by soap molecules with the ionic part of the molecules toward the outside where water will react with the ionic end (by hydrogen bonding) and keep the oil in "solution." Hot water helps in the formation and suspension of the emulsion.

This interface of oil and water is based upon the the old adage that "like dissolves like." The long hydrocarbon part of the soap adsorbs the oil, the ionic end is dissolved in the water.

Answer 2

Soap is made up of what is called a surfactant; surfactant is simply a molecule that has a charged head, with a very long hydrophobic tail; surfactant has traditionally been derivatives of fat. When soap is dropped into water, the hydrophobic tails will zoom together and the surfactant form what's called a micelle. Things like dirt and oil, which usually sits in a separate layer than water, will start to break up and sit into these spherical micelles, and the layer of what you might recognize as "fat" is simply your surfactant holding the oil and dirt. A needle can float on top of water by surface tension. This also explains why some insects can crawl across water. But to accomplish this, the needle must be placed carefully on top of the water - any disturbance will make the needle lose its grip and it will sink. When you add soap to water, the soap disturbs the normal hydrogen bonds that makes the water's surface tension so strong. Instead of hydrogen bonding with each other, the water begins to hydrogen bond with the polar head of the surfactant; also, the water must deal with the long hydrophobic tail. This effectively ruins the surface tension, and the needle sinks.

Answer 3

It can do these things because one part of the soap molecule is hydrophilic (water-binding) and the other is hydrophobic (water-repellent). The hydrophilic part allows the hydrophobic fatty acids to come into contact with other hydrophobic substances, such as the dirt on the surface that is being cleaned. When the grime adheres to the soap's fatty acids, it becomes encapsulated in droplets of water. Dirt, oil and bacteria are easily scrubbed off and washed away in this suspended state.

Answer 4

Fats & oils tend not to be polar. Their molecules have no particular charge and so are not attracted to polar substances such as salt. Instead, they prefer to bond with other non-polar substances. Fats & oils tend to be electrical insulators.

Soap molecule is a half-way house. It consists of a long strand with an ionic hydrophilic (water loving), grease repelling (lipophobic) group on one end and a non-polar grease loving (lipophilic), water-loving(hydrophobic) group on the other. If you drop soap into clean water, all the molecules gather on the surface with their hydrophilic ionic ends stuck in the water and the hydrophobic ends waving in the air. Slide a dirty dish in, and the lipophilic end of each molecule sticks to the grease as it slips past. As the dish sinks, it take the soap molecules with it, attached by their "heads" to the grease but still waving their hydrophilic tails in the water like microscopic tadpoles.

All you have to do now is bash at the dirt with a sponge or cloth, and it can be persuaded to leave the plate, for as it lifts off the surface, it becomes insulated from the water as new soap molecules rush in and try to bury their heads in it. The end result is a small blob of grease completely surrounded by a layer of soap molecules, all with their lipophilic heads pointing inwards and their hydrophilic tails pointing outwards. As far as the grease is concerned, all it can see are lipophilic molecules, and as far as the water is concerned, all it can see is a rather large hydrophilic lump. Eventually, all the soap molecules are used up and you to dump your water and start again. Pass the tea towel!!

Answer 5

Soaps are sodium or potassium fatty acids salts, produced from the hydrolysis of fats in a chemical reaction called saponification. Each soap molecule has a long hydrocarbon chain, sometimes called its 'tail', with a carboxylate 'head'. In water, the sodium or potassium ions float free, leaving a negatively-charged head.

Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.

The organic part of a natural soap is a negatively-charged, polar molecule. Its hydrophilic (water-loving) carboxylate group (-CO2) interacts with water molecules via ion-dipole interactions and hydrogen bonding. The hydrophobic (water-fearing) part of a soap molecule, its long, nonpolar hydrocarbon chain, does not interact with water molecules. The hydrocarbon chains are attracted to each other by dispersion forces and cluster together, forming structures called micelles. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

Grease and oil are nonpolar and insoluble in water. When soap and soiling oils are mixed, the nonpolar hydrocarbon portion of the micelles break up the nonpolar oil molecules. A different type of micelle then forms, with nonpolar soiling molecules in the center. Thus, grease and oil and the 'dirt' attached to them are caught inside the micelle and can be rinsed away.

Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:


CH3(CH2)16CO2-Na+ + HCl ----> CH3(CH2)16CO2H + Na+ + Cl-
These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.


2 CH3(CH2)16CO2-Na+ + Mg2+ ----> [CH3(CH2)16CO2-]2Mg2+ + 2 Na+
The insoluble salts form bathtub rings, leave films that reduce hair luster, and gray/roughen textiles after repeated washings. Synthetic detergents, however, may be soluble in both acidic and alkaline solutions and don't form insoluble precipitates in hard water. But that is a different story...

Answer 6

The process of cleaning a soiled surface consists of the following physical-chemical steps:

1. Wetting of the surface and, in the case of textiles, penetration of the fibre structure by wash liquor containing the detergent. Detergents (and other surface-active agents) increase the spreading and wetting ability of water by reducing its surface tension—that is, the affinity its molecules have for each other in preference to the molecules of the material to be washed.

2. Absorption of a layer of the soap or detergent at the interfaces between the water and the surface to be washed and between the water and the soil. In the case of ionic surface-active agents (explained below), the layer formed is ionic (electrically polar) in nature.

3. Dispersion of soil from the fibre or other material into the wash water. This step is facilitated by mechanical agitation and high temperature; in the case of toilet soap, soil is dispersed in the foam formed by mechanical action of the hands.

4. Preventing the soil from being deposited again onto the surface cleaned. The soap or detergent accomplishes this by suspending the dirt in a protective colloid, sometimes with the aid of special additives. In a great many soiled surfaces the dirt is bound to the surface by a thin film of oil or grease. The cleaning of such surfaces involves the displacement of this film by the detergent solution, which is in turn washed away by rinse waters. The oil film breaks up and separates into individual droplets under the influence of the detergent solution. Proteinic stains, such as egg, milk, and blood, are difficult to remove by detergent action alone. The proteinic stain is nonsoluble in water, adheres strongly to the fibre, and prevents the penetration of the detergent. By using proteolytic enzymes (enzymes able to break down proteins) together with detergents, the proteinic substance can be made water-soluble or at least water-permeable, permitting the detergent to act and the proteinic stain to be dispersed together with the oily dirt. The enzymes may present a toxic hazard to some persons habitually exposed.

In order to perform as detergents (surface-active agents), soaps and detergents must have certain chemical structures: their molecules must contain a hydrophobic (water-insoluble) part, such as a fatty acid or a rather long chain carbon group, such as fatty alcohols or alkylbenzene. The molecule must also contain a hydrophilic (water-soluble) group, such as -COONa, or a sulfo group, such as -OSO3Na or -SO3Na (such as in fatty alcohol sulfate or alkylbenzene sulfonate), or a long ethylene oxide chain in nonionic synthetic detergents. This hydrophilic part makes the molecule soluble in water. In general, the hydrophobic part of the molecule attaches itself to the solid or fibre and onto the soil, and the hydrophilic part attaches itself to the water.

Answer 7

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