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PHYSICO-CHEMICAL FUNDAMENTALS OF WASHING

 

From cleaning it is quite natural to go to the wash, which is what we will do.

Wash - a physical-chemical process, its main characters - surfactants. The molecules of these substances consist of two parts - hydrophilic, i.e., having an affinity for water, and hydrophobicthat water does not interact, but willingly comes into contact with pollutants, for example, it is difficult washed fats and oils. These groups are hydrophilic and hydrophobic are at different ends of the long molecules. Such molecules are attached with their hydrophobic ends to oily surfaces and hydrophilic sticking out, like the needles of the hedgehog:

Surfactant molecules surround the molecules of fat

Water these "needles" wets well, it surrounds such a "hedgehog", lifts it from the surface and drift away. Something like this operates and soap and washing powder. And to quickly remove dirt with a cloth or with our hands, we three sponge, brush, together...

As soon as the soap is the oldest surfactant, with him and start.

Dissolve in a small amount of water, a little soap, add in a test tube solution of phenolphthalein. The color will become crimson red. Hence, the alkaline environment. In fact, the usual soap - sodium salt of fatty acids - oleic, stearic, for example, C17H35COONa (and liquid soap potassium salt of the same acid). When dissolved in water, these salts are hydrolyzed fall into the acid and alkali. But fatty acids are weak, and the alkali in this case is strong, so the solution has an alkaline reaction.

Used to think that soap is good wipes and washes because it forms the alkali. It turned out that this is not at all. In contrast, the alkali (for example, washing soda) washes because it connects with fats and forms in the solution of surface-active substances such as soap.

By the way, the soap is not too difficult to get by. There are several ways. Here is one of them. Prepare a hot concentrated solution of washing soda, pour it into a test tube and gradually, drop by drop, add the vegetable oil until it will no longer dissolve. Instead of butter you can get beeswax. In the resulting solution sprinkle a pinch of salt. So do on a soap plants - this process is known as salting out. After adding solid salt soap floats to the surface and it is easy to separate from solution.

Now soap for Laundry use less, and washing powders increasingly. The composition of these powders are surface-active substances obtained synthetically. So they called a synthetic detergents.

We will put this experience. Cut the flap dirty tissue in three pieces and dip each piece in the glasses. In the first glass just pour the heated water in the second - soap solution and a third solution of any washing powder, which will be home. Lightly RUB the cloth, rinse them in clean water, dry and carefully make out. That piece of fabric that had been in the water became slightly clearer. Shred of soap noticeably brightened up. But the net will be the piece of tissue that was extracted from a glass with a solution of washing powder. Therefore, synthetic detergents act more vigorously than ordinary soap.

Many washing powders have another important property: they wash in any water - soft, hard, even in the sea. And soap?

Take ordinary water and dissolve in it some salt of calcium or magnesium. You can buy in the drugstore bitter salt, you can take a dry sea salt (it is also sold in pharmacies) or a solution of calcium chloride. So you make the water hard, because hard water differs from soft, which contains a lot of salts of calcium and magnesium - the so-called hardness salts.

Again, take a piece of dirty cloth and try to wash it with soap in such hard water. Nothing will come of it - even the foam is not formed. Hardness salts come with soap in the reaction, the formation of calcium and magnesium Soaps, and they are insoluble in water. Our soap has lost all its useful properties.

But if hard water to dissolve the detergent, such as "Lotus", it will wash the dirt almost the same as before - hard water do not harm them. Surfactant included in the composition of the powder does not react with the salts of rigidity, and therefore, do not lose their properties.

Solutions washing powders, as solutions of soap, can be alkaline; in this case, they recommend washed cotton and linen, not wool, not silk. However, there is a neutral tool, they are often made not in the form of powders and liquids; they are good for wool, silk and synthetic fabrics. If you have any doubts whether to erase whatever powder wool sweater, then perform the test with phenolphthalein. The solution became red - this means that there are free alkali, and she's wool is contraindicated because it can destroy the fibers. But if the solution remained colorless or colored quite a bit, feel free to immerse in it, and wool and silk clothes.

In the old days when soap was a luxury item, for washing often used other, more affordable substances, which, although to a lesser extent, but still wash the dirt. Try it and you, how are these substances. Can take to experience the mustard powder or decoction beans, but even better - the roots of some plants such as primrose, crow's eye, cyclamen, hooded. These roots contain saponins - substances with detergent action (possibly in old books you heard this expression - soap root). All of these natural substances are washed, of course, worse than soap, but you can easily make sure that they are washed.

Razgovor about detergents finish experience, in which, by adding a surfactant and thereby altering the surface tension of the water, we will make the object move through the water.

From thin copper wire to make a flat spiral in a few turns, lightly grease it with oil or vaseline and very carefully lower the water surface. The surface tension of water does not give a spiral sink, and the water is not wet. Now the pipette gently drip in the middle of a spiral one drop of soap solution. Spiral now will start spinning. Spreading out on the surface, a soap solution reaches the end of the spiral goes out and develops small jet thrust. When the coil stops, add a drop of soap solution again - rotation resumes.

This helix can serve as a device for determining the surface activity of various liquids. Replace the soap solution with another substance, the spiral will move with different speed. If you drop the salt solution, the circular motion will not disappear completely. And in detergent spiral will quickly drown. He washes away the layer of oil that holds the wire on the water.

O. Holguín. "Experiments without explosions"
M., "Chemistry", 1986

 

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