Chemistry that changes color in the cold. The chemical essence of color
Thermochromic paint is modern material, with which unusual coatings are created that can change color under the influence of different temperatures. Due to this effect, heat-sensitive compounds have found wide application in various industries, from the production of souvenirs to the painting of cars.
Properties of the active substance
The active ingredient in the composition is a thermochromic pigment. It is he who provides the reaction of the coating to heating or cooling, accompanied by a change in color. The amplitude of temperature fluctuations is 15–70 °C.
The value at which the reaction begins is individual for each specific composition.
KATO_Katosha - Chameleon hair (PRAVANA VIVIDS Mood Color)
PRAVANA VIVIDS Mood Color is the world's first pigment that changes the color of your hair depending on the temperature. THIS IS...
Thermochromic pigments are contained in the material in the form of liquid crystals, enclosed in microcapsules, which allows them to be mixed with various solutions, such as oil-based, rubber-based or acrylic-based paints. The active substance usually makes up from 5 to 30% of the total mass of the coloring agent. funds; this number depends on the desired result.
Types of thermal paints
Thermochromic compounds are divided into two groups:
- returnable,
- irrevocable.
The former include those coatings that give a reversible visual effect, that is, they are able to change the shade and return to their original state when the temperature returns to normal. This “trick” is repeated a large number of times.
In the second case, the paint changes color once and finally, the coating will no longer react to heat or cold.
Areas of use
Return-type thermochromic inks are used more widely than their “disposable” counterparts. These materials have gained high popularity among car owners who want to make their car original in terms of external design.
car cover
Heat-sensitive paint is a godsend for those who like to experiment and be creative in car care. Everyone can create a new interesting image for their iron horse with their own hands, because working with paint that changes color is not difficult. It can even be applied with an ordinary paint brush or roller, although the best option for painting a car body is, of course, a paint sprayer.
Thermochromic material can be not only a highlight of the decor, but also have an important practical function: if, when heated, the car coating becomes white or another light shade, then in hot weather the body will be able to reflect the sun's rays and surface the car will overheat less.
To create a complex visual effect, you can use the following technique: paint the car in several layers of thermal paint, using compositions with different temperature thresholds. How clear wall from old paint at home? To add “magic” will help drawings made using a stencil or applied by hand (if there are the makings of an artist).
Paints that change color with temperature The car, for the design of which heat-sensitive paints were masterfully used, simply cannot go unnoticed in the stream of other cars!
While rejoicing at the opportunity to decorate your vehicle, you should still know that thermochromic paint also has some disadvantages:
- low light resistance: in order to protect the coating of the car body from the damaging effects of ultraviolet radiation, you will have to apply a layer of special varnish and equip the parking lot with a canopy (the best option is a garage);
- in case of mechanical damage, a complete repainting of the machine will be required;
- difficulties in registering a car that does not have a permanent color;
- thermosensitive dye- an expensive material.
Dishes that change color
A tea or coffee mug, on the surface of which a funny inscription or drawing appears when a hot drink gets into it, is a good memorable gift. An appetizer platter with an emerging pattern is an interesting detail in a table setting. Paint that changes color with temperature for drawing?
And a variety of children's dishes that give a visual signal when porridge or milk is too hot is a useful thing in the everyday life of young mothers.
Important: thermochromic paints do not contain toxic substances, and dishes painted with these materials are safe for health.
clothing
The textile industry also uses compositions that change color depending on temperature. So, a plain T-shirt worn on the body may surprise you with a fashionable print, and a stylish pattern or label will appear on jeans.
Souvenirs and decor elements
In this industry, an unusually wide scope for the use of thermochromic materials is open: Christmas toys and garlands, other holiday paraphernalia, original lamps and candlesticks, key rings, gift stationery, and more. The great thing is that a lot of things can be made and painted with your own hands, for example, paint a picture or create a panel with a “secret”.
Printed products
Business cards that “come to life” from the touch of warm hands, advertising booklets or magazines promoting perfume (rub the page!), children's picture books, postcards - all this is often produced using heat-sensitive compounds, since their color palette is quite rich.
In general, everyone can find an original use of these unusual materials in everyday life on their own, showing imagination and making a little effort.
Additional Information:
Another advantage that thermochromic paint has is the price. It is quite low, taking into account the properties of this material (1500 rubles for a 25-gram jar, which is enough for a long time). Such solutions attract customers and are an excellent advertising move.
- Below + 20 degrees - for applying the substance to the dishes that will be used for soft drinks.
- + 29 ... + 31 degrees - suitable for surfaces that will change their color when exposed to body temperature (when touched). The use of this effect is widely used in advertising, on T-shirts, in magazines and on booklets.
- Above + 43 degrees - materials intended for products that will interact with hot temperatures (dishes for hot drinks). In this case, the color change effect performs not only a decorative, but also a warning function.
For application on mugs, thermochromic paint with a barrier below +20 degrees Celsius is used.
Usually thermochromic pigments are toxic and are used only to a limited extent, but the developers of The Unseen managed to get rid of this problem by finding and synthesizing similar, but harmless substances. How to change paint color at home? A change in temperature causes these molecules to take on one or another spatial conformation, changing the spectrum of absorbed radiation.
Depending on the specific paint in the kit, this can happen at different temperatures. For example, “cold” blue and white transition into each other around 15 ° C, and “hot” red and black - at 31 ° C.
Boker has developed several dyes that change their color in different temperature ranges. The transition points correspond to the transition between room and outdoor temperature, or correspond to the temperature of the human body. Among the compositions developed are black paint, which changes its color to red when exposed to hot air, there are paints that change from black to white, from silver to pale blue, from blue to white and from black to yellow.
To create a complex visual effect, you can use the following technique: paint the car in several layers of thermal paint, using compositions with different temperature thresholds. To add “magic” will help drawings made using a stencil or applied by hand (if there are the makings of an artist). The car, for the design of which heat-sensitive paints were masterfully used, simply cannot go unnoticed in the stream of other cars!
But even the first samples in promo videos allow us to imagine the effect of using such hair dye. When the curls - under the influence of temperature from the blowing hair dryer, change shades from dark, almost black with a slight reddish shimmer to bright red and even light red.
Looks interesting enough. In addition, the creators of the paint promise its maximum safety: that it will be no more harmful than conventional hair dyes that are sold today.
Thermochromic (thermosensitive) paints are very popular in the food industry. An image covered with such paint and placed on the product informs the consumer whether the product has reached the desired temperature, for example, in a refrigerator or oven. Thermochromic paint is also used by manufacturers of beer, alcoholic beverages (bottles, labels, stickers, etc.), where it signals that the drink is cooled, in the manufacture of ceramic dishes (cups, glasses, plates), and is also used in various kinds plastic PP, PVC, ABS, silicone rubber and other transparent or translucent plastic materials for injection molding, extrusion, offset, screen printing, screen printing, flexography.
We all have any tricks. Many of us know a few simple magic tricks that can surprise friends at a party or show to children and make them laugh. Today we will make a kind chemical experiment, which can also become a beautiful focal point.
Let's watch the video first:
So, in order to prepare our miracle liquid, you may have to go to the pharmacy, but we assure you - it's worth it.
We will need:
- Two glasses of the same size;
- Two small glasses (can be made of plastic);
- A container in which we will pour warm water;
- A spoon with which we will mix;
- Potato or corn starch;
- One gram of vitamin C;
- Tincture of iodine;
- Hydrogen peroxide (3%);
- Syringes for more accurate dosing of all components.
If vitamin C is in the form of tablets, then they must be crushed into powder. First of all, we need to add a gram of vitamin to a plastic glass and add 60 ml of warm water.
The next step is to prepare liquid starch by mixing one teaspoon of starch in 150 ml cold water. Next, add another 150 ml of hot water and stir well.
We take two identical glasses and pour 60 ml of warm water into them.
In the first glass, add 5 ml of tincture of iodine and 10-12 ml of liquid with vitamin C. After adding the liquid with vitamin, iodine will completely discolor.
In the second glass, add 15 ml of hydrogen peroxide and 7 ml of liquid starch.
The preparatory stage is over, which means that you can move on to the focus itself. We take glasses and pour the liquid from one to another.
After that, we just have to put one glass on the table and wait. The liquid will soon change its color to dark. In chemistry, this experiment is known as the iodine clock. If we state the essence of the experiment in the most accessible way, then we can say that this is a kind of confrontation between starch, which turns iodine into a dark liquid, and vitamin C, which prevents this. In the end, the vitamin is completely consumed and the liquid instantly changes its color. The magic worked. By the way, if you add a little more vitamin C powder to a dark liquid, the liquid will again become discolored for a while.
Real shot of hair dye color change under hot air
The Unseen / Vimeo
Thermochromic pigments are substances or mixtures of substances that change their color depending on temperature. Many substances have this ability, but as a rule, color change requires very high temperatures and is associated with phase changes or chemical reactions. There are several classes of substances for which thermochromic properties are well pronounced and appear at low temperatures. It is thanks to them that in stores you can find mugs, the pattern of which changes under the influence of hot water, thermometers and even fabrics.
Often, liquid crystals are used as thermochromes - substances whose molecules are ordered into columns or sheets, even despite the liquid state of aggregation. Temperature changes affect the dimensions of the structures, such as the width of the sheets. This is reflected in the optical properties of materials. The second class of thermochromes is organic dyes that can reversibly change their color due to chemical transformations. An example of such compounds are spiropyrans - in the structure of their molecules there are two rings of atoms connected in one place. When the temperature or acidity of the medium changes, the rings can open, greatly changing the properties and color of the substance. However, as a rule, such dyes are toxic to the skin, which limits their use.
The authors of the development were inspired stage from the film "Witchcraft", in which the heroines of the film change the hair color of one of them with the help of a spell. To reduce the toxicity of the paint, the developers used polymeric binders. "We can prevent the harmful effects of these chemicals through a process called 'polymer stabilization', in which chain-like molecules (polymers) wrap around the irritant," says Lauren Bocker, founder of the company.
Boker has developed several dyes that change their color in different temperature ranges. The transition points correspond to the transition between room and outdoor temperature, or correspond to the temperature of the human body. Among the compositions developed are black paint, which changes its color to red when exposed to hot air, there are paints that change from black to white, from silver to pale blue, from blue to white and from black to yellow.
There are other types of pigments that change color under the action of external influences. For example, photochromes change color under the action of light, mechanochromes - during deformation, electrochromics - under the influence of electric current. In addition to using these substances for decoration, scientists also use the transformations of compounds for fundamental purposes. So, a year ago, chemists from Germany and Japan created nanoscale "scissors" that can reversibly open and close under the influence of light. They were based on a DNA molecule modified with photochromic azobenzene.
Vladimir Korolev
Determination of color factors. What is color in terms of chemistry? It is impossible to consider the chemical essence of color without knowledge physical properties visible light. Great English physicist We owe I. Newton that he explained the phenomenon of decomposition white color on a set of rays of the color spectrum. Each wavelength corresponds to a certain energy that these waves carry. The color of any substance is determined by the wavelength, the energy of which prevails in this radiation. The color of the sky depends on how much sunlight reaches our eyes. Rays with a short wavelength (blue) are reflected from the molecules of air gases and scattered. Our eye perceives them and determines the color of the sky - blue, blue (table 3.).
The same happens in the case of colored substances. If a substance reflects rays of a certain wavelength, then it is colored. If the energy of light waves of the entire spectrum is equally absorbed or reflected, then the substance appears black or white. The human eye contains an optical system: the lens and the vitreous body. The retina contains light-sensitive elements: cones and rods. The cones allow us to distinguish colors.
Table 3. Color of substances having one absorption band in the visible part of the spectrum
Thus, what we call color is the result of two physical and chemical phenomena: the interaction of light with the molecules of a substance and the effect of waves coming from a substance on the retina of the eyes. So, first factor color formation - light.
Consider examples of the following, second factor- the structure of substances.
crystal structure have metals, they have an ordered structure of atoms and electrons. Color is related to the mobility of electrons. When illuminating metals, reflection predominates, their color depends on the wavelength they reflect. The white luster is due to the uniform reflection of almost the entire set of visible rays. This is the color of aluminum, zinc. Gold has a reddish-yellow color because it absorbs blue, indigo and violet rays. Copper also has a reddish color. Magnesium powder is black, which means that this substance absorbs the entire spectrum of rays.
Next, third the color appearance factor is the ionic state of substances. The color also depends on the environment around the colored particles. Cations and anions in solution are surrounded by a shell of a solvent that affects the ions.
Factors affecting color change chemical substances. When conducting a simple experiment with the addition of the following substances to a solution of beet juice (raspberry color): acetic acid; a solution of alkali or water, as a result, a change in the color of the beetroot solution can be observed. In the first case, the acidic medium changes the color of the beetroot solution to purple, in the second experiment, the alkaline medium changes the color of the solution to blue, and the addition of water ( neutral environment) does not cause color changes.
Chemists know the indicator for determining the alkaline environment - phenolphthalein. It changes the color of alkali solutions to crimson. With a change in the color of the iron ion when surrounded by potassium thiocyanate in a bloody color is associated historical fact. In 1720, political opponents of Peter I from the clergy organized in one of the St. Petersburg cathedrals a "miracle" icon of the Mother of God, which began to shed tears, which was commented on as a sign of her disapproval of Peter's reforms. Peter I carefully examined the icon and noticed something suspicious: he found small holes in the eyes of the icon. He also found the source of tears: it was a sponge soaked in a solution of iron thiocyanate, which has a blood-red color. The weight evenly pressed on the sponge, squeezing out drops through a hole in the icon. “Here is the source of miraculous tears,” said the emperor.
Chemicals are part of the nature that surrounds us from all sides. Animal blood and leaf greens contain similar structures, but blood contains iron ions - Fe, and plants - Mg. This ensures the color: red and green. Incidentally, the saying blue blood” is true for deep-sea animals that have vanadium in their blood instead of iron. Also, algae that grow in places where there is little oxygen have a blue color.
Plants with chlorophyll are able to form organomagnesium substances and use the energy of light. The color of photosynthetic plants is green.
Iron-containing hemoglobin is used to carry oxygen throughout the body. Hemoglobin with oxygen stains the blood bright red, and without oxygen gives the blood a dark color.
It is necessary to draw the following conclusions regarding the physicochemical nature of color:
The first factor in the formation of color is light;
The second factor is the chemical structure of substances;
The third factor in the appearance of color is the ionic state of chemicals, the color depends on the environment around the colored particles.
4.2. Chemistry of dyes .
Color harmony is one of constituent parts design art. The most ancient paints were charcoal, chalk, clay, cinnabar and some salts such as copper acetate (verdigris). Paints and dyes are used by artists, decorators and textile workers.
The use of the first dyes - inorganic pigments - began in the Stone Age. Primitive people used colored natural minerals to paint the body, various household items and clothing. Beautiful drawings in caves have survived to this day, outliving their creators for hundreds of centuries. It is colored minerals, together with noble metals, that have always been symbols of power and wealth of people. With the development of mankind, the need for dyes only grew.
Back in the tenth century BC, at the bottom mediterranean sea near the city of Thira (ancient Phenicia) they caught needle snails. Slaves dived into the sea for these snails day after day. Other slaves squeezed them out, ground them with salt and subjected them to further processing, which consisted of many operations. The extracted substance was at first white or pale yellow, but under the influence of air and sunlight gradually became lemon-yellow, then green, and finally acquired a magnificent violet-red color. Received purple for several centuries was the most valuable of all dyes. He was then a symbol of power - the right to wear purple-dyed robes was the privilege of the rulers and nobles closest to them. Dyeing only one square meter of fabric with a dye obtained in this way was very expensive. Indeed, to obtain one gram of purple, 10,000 snails had to be processed!
The backbreaking labor of Tyre's slaves is not the only example of its kind in history. A few hundred years later indigo- Violet-blue dye, extracted from the plant Indigofera tinctiria, has become one of the major sources of profit for the British East India Company. The ships of the East India Company annually delivered from 6 to 9 million kilograms of this valuable dye to all parts of the world. They used to paint sails, now jeans.
Nowadays, the manufacture of modern cheap and at the same time bright dyes of all colors and shades no longer requires the overwork of slaves or the population of the colonies. They, including purple and indigo, are produced in chemical plants. However, purple and indigo have lost their former glory. They were replaced by more lightfast synthetic dyes, a wide choice of which we have today.
The path to the current success was opened thanks to the work of many chemical scientists. In 1826, 1840 and 1841, Unferdorben, Fritzsche and Zinin independently obtained aniline from indigo. In 1834, Runge discovered aniline in coal tar, in the same year he discovered phenol, and a little later, the first dye from coal tar - rosolic acid giving purple color.
In 1856, the 18-year-old chemist Perkin, working during the holidays in his home laboratory, in an unsuccessful attempt to synthesize quinine, unexpectedly received a bright reddish-violet dye - movein. Together with his father and brother, Perkin founded a company and a year later organized the production of mauveine on a factory scale. Thus, Perkin laid the foundation for the creation of the aniline industry.
In 1868, Grebe and Liebermann revealed the secret alizarin- a red dye extracted from the roots of madder. Syntheses followed. eosin and other phthalein dyes by Bayer and Caro and deciphering the structure of anthracene dyes by E. Fischer and O. Fischer. By the end of the XIX century. these achievements culminated in the industrial introduction of the synthesis of indigo according to the method developed by Heimann and other chemists.
The merit of German chemists in the development of the paint and varnish industry is great. As early as 1911, German firms exported 22,000 tons of synthetic indigo. By simultaneously producing 1500 tons of cheap synthetic alizarin, they almost completely replaced natural alizarin, which led to a sharp reduction in madder breeding.
Why do substances illuminated by white light acquire a particular color? The fact is that passing through the dye, the light is absorbed by its molecules. The structure of dye molecules is such that light is absorbed selectively. The dye molecule “chooses” the rays that make up white light, the lines of the spectrum, which are characteristic only for it. Losing some of the colors, the incident beam is colored by the so-called complementary colors (green - red, yellow - violet, blue - orange). For example, the loss of red will result in green coloring.
What does the absorption spectrum of a substance depend on? Before us is the dye formula of a relatively simple structure: Its exact chemical name- n, n "-sodium. This substance is used as an indicator, it was called otherwise - methyl orange. However, this dye is not suitable for dyeing, since when acid is added, the yellow color turns red. It is no coincidence that organic dyes have a complex structure. Studies by many chemists have made it possible to establish a relationship between the color of the compound and its structure. The basis, or core, of the dye molecule, as a rule, forms a ring structure. Color carriers - chromophores - must be attached to it. These are always unsaturated groups:
CH=CH is the ethylene group;
С=О – carbonyl group (oxo group, keto group);
N=N - azo group;
N=O, nitroso group;
NO2 is a nitro group.
The nucleus and chromophore groups together form a colored system - a chromogen. In most cases, the presence of only one chromophore does not yet give color. For example, in the orange molecule b-carotene- carrot dye - contains 11 double bonds. In addition, the color depends on how exactly the chromophores are located and connected to each other. To enhance the color, deepen its hue and achieve greater color fastness, additional groups, auxochromes, must be attached to the core with the chromophore. These include primarily the hydroxyl group OH and the amino group NH2, which not only influence the color but also, due to their acidic or basic character, increase the affinity of the dye to the fiber. The modern electronic theory of color considers color as the result of interaction with the light of the electron cloud of the dye molecule. It is on its parameters, which are determined by the presence of chromophore and auxochromophore groups, that the absorption spectrum of the molecule depends.
Phosphors. Conventional dyes scatter the absorbed light in the form of infrared radiation invisible to the human eye. However, there are molecules capable, after being excited due to external energy, returning back to the unexcited state, to emit rays of visible color. These are phosphors. The energy required for their glow can be chemical (“phosphors”), mechanical (“triboluminophores”), electrical (“electroluminophores”) or light (“photoluminophores”), as well as under the influence of radiation.
Phosphorescent phosphors exist in nature. Glow can occur due to the slow oxidation of a substance in air (for example, white phosphorus, luciferin in some insects, microbes, fungi, fish). Such substances without access to an oxidizing agent (air oxygen) do not glow. Some substances can glow when rubbed or shaken (for example, crystalline chelidonin, some manganese-activated sulfides, etc.). This glow is called triboluminescence. Substances that glow in the presence of radiation or X-rays invisible to the eye are used to make permanently luminous compositions. As radioactive substance used, for example, paraffin, in the molecules of which some of the atoms of ordinary hydrogen (protium) are replaced by atoms of superheavy radioactive hydrogen (tritium). Due to the presence of radioactive elements in the composition, such visible light sources are hazardous to health. Electroluminophores are widely used in lighting engineering.
However, it is inorganic or organic photoluminophores that are used as phosphor dyes. Depending on the excitation time of their molecules, phosphors can glow in the dark with an excitation time of several hours (many such luminous toys are sold), or at short times, phosphors simply turn into a characteristic color. Of particular interest are such phosphors that actively absorb UV radiation. Clothing tinted with such phosphors brightly "burns" in the sun. The red clothes of the employees of the Ministry of Emergency Situations are visible for many kilometers even in the fog. Phosphor paints are used for road signs and advertisements, rescue boats. But there are also unexpected uses for such phosphors.
UV protection. There are many cosmetics on the market that protect a person from harmful UV radiation, for example, sunscreens. The main active components of these products are UV absorbers - the same phosphors that absorb harmful hard radiation.
But not only the human body needs to be protected from ultraviolet radiation. UV absorbers - light stabilizers - are widely used to protect polymers. An example is Tinuvin. In the unexcited state, a stable hydrogen bond is formed between the hydrogen of the hydroxyl group and the nearest nitrogen atom. Its stability is due to the formation of a stable hexagon. The absorption of a UV radiation quantum is sufficient to destroy this ring. When it is restored, energy is emitted, but this is no longer harmful ultraviolet, but safe infrared radiation. (The surface of all metal objects exposed to environment is destroyed. Their protection is most effective with colored pigments: aluminum powder, zinc dust, red lead, chromium oxide).
Optical brighteners. Each of you must have noticed that at the disco, when you turn on a special backlight, white shirts and blouses of people begin to glow brightly in blue. A sheet of white paper will shine even brighter. This means that special phosphors - optical brighteners - have been added to the fabric of your clothes and paper. Their action is similar to the action of ordinary "blue", which was previously added to the water during washing, to bleach clothes. Today, for the purpose of bleaching, substances are introduced into the composition of washing powders that give the fabric a bluish fluorescence.
The blue color complementary to yellow “kills” the yellowness of the fabric. The same thing is done by the phosphor that turns UV radiation into radiation. of blue color. At the same time, it protects the material from ultraviolet radiation.
Phosphor for greenhouse film. The usual greenhouse plastic film is already outdated (by the way, the “greenhouse effect” is due to the fact that UV and visible rays pass through the polyethylene layer almost without loss, and polyethylene is opaque for thermal infrared rays from the soil surface). There are new photoconverting films that glow red in the sun. It is emitted by a special phosphor synthesized on the basis of europium oxide, which converts green, blue and UV radiation into red. Of course, it's very beautiful, it's not about beauty.
A plant at the initial stage of development requires a large amount of red to grow green mass (leaves). This is the purpose of the phosphor. It has a complex structure that provides stepwise conversion of UV radiation to the required red color. Therefore, the amount of red color in the light falling on the leaves of plants increases several times, which leads to an increase in the yield of greenhouse crops. True, when the time comes for fruit ripening, such a film should be replaced with blue. On the contrary, it absorbs red rays. The leaves stop growing, all the energy of the plant is directed to the growth of fruits.
Lost river. Fluorescence is clearly visible even when 1 g of radomin 6G is dissolved in 100,000 liters of water. The ability of phosphors to be unusually easy to detect in negligible concentrations is used to determine the direction of underground water currents. An example is the resolution of the question of the "disappearance" of the Danube. In the upper reaches of this river, near the railway station Immedingen, most of Danube water is lost in loose limestone rocks. In order to establish the direction of water movement in 1877, 10 kg of fluorescein was poured into the Danube near this station. After 60 hours, one of the exposed posts found a distinct fluorescence in a small river. In modern times, this property of the phosphor has proven to be very useful in environmental inspections of leaks and wastewater production. Let's not forget about the system of protection by phosphor printing of documents and, finally, banknotes.
quantum dots. Phosphor nanoparticles (quantum dots) absorbed by microorganisms with nutrient media, allow you to trace their movement and development in a living organism. The selective absorption of such particles by malignant cells is already being used to diagnose cancer and other diseases at an early stage.
In addition to those described above, there are many interesting dyes. For example, photochromic dyes have been developed that change color with an increase in the dose of UV radiation, an increase in temperature, and the action of an electric field. There are dyes that color films differently in reflected and transmitted light. A large article can be written about interference coloring with multilayer pearlescent pigments, about holographic coloring, about the use of liquid crystal structures, about digital printing and much more.
Despite the fact that the basic rules for creating chromophore molecules are known, the discovery of a new dye even today is sometimes caused by a happy accident. The technology of dyes is both chemistry, and physiology, and art.
5. Basic patterns of color perception:
Panteleev Pavel Alexandrovich
The paper gives explanations for the appearance of color in various compounds, and also investigates the properties of chameleon substances.
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Color chemistry. Substances-chameleons
Section: natural science
Completed by: Panteleev Pavel Nikolaevich,
Student 11 "A" class
Middle secondary school №1148
them. F. M. Dostoevsky
Lecturer: Karmatskaya Lyubov Aleksandrovna
1. Introduction. Page 2
2. Nature of color:
2.1. organic substances; Page 3
2.2. inorganic substances. Page 4
3. The influence of the environment on the color. Page 5
4. Substances-chameleons. Page 7
5. Experimental part:
5.1. The transition of chromate to dichromate and vice versa; Page 8
5.2. Oxidizing properties of chromium (VI) salts; Page 9
5.3. Oxidation of ethanol with a chromium mixture. Page 10
6. Photochromism. Page 10
7. Conclusions. Page 13
8. List of used sources. Page 14
1. Introduction.
At first glance, it may seem difficult to explain the nature of color. Why do substances have different colors? How does color even come about?
It is interesting that creatures live in the depths of the ocean, in the body of which blood flows blue color. One of these representatives is holothurians. At the same time, the blood of fish caught in the sea is red, like the blood of many other large creatures.
What determines the color of various substances?
First of all, the color depends not only on how the substance is colored, but also on how it is illuminated. After all, in the dark everything seems black. The color is also determined by the chemical structures that prevail in the substance: for example, the color of the leaves of plants is not only green, but also blue, purple, etc. This is due to the fact that in such plants, in addition to chlorophyll, which gives the green color, other compounds predominate.
Blue blood in holothurians is explained by the fact that they have vanadium instead of iron in the pigment that provides the color of the blood. It is its compounds that give the blue color to the liquid contained in holothurians. In the depths where they live, the oxygen content in the water is very low and they have to adapt to these conditions, so compounds have arisen in organisms that are completely different from those of the inhabitants of the air environment.
But we have not yet answered the above questions. In this work, we will try to give complete, detailed answers to them. To do this, a number of studies should be carried out.
The purpose of this work will be to give an explanation of the appearance of color in various compounds, as well as to investigate the properties of chameleon substances.
In accordance with the goal, tasks were set
In general, color is the result of the interaction of light with the molecules of matter. This result is explained by several processes:
* the interaction of magnetic vibrations of the light beam with the molecules of matter;
* selective absorption of certain light waves by molecules with different structures;
* exposure to rays reflected or passed through a substance on the retina or on an optical device.
The basis for explaining color is the state of electrons in a molecule: their mobility, the ability to move from one energy level to another, to move from one atom to another.
Color is associated with the mobility of electrons in a molecule of a substance and with the possibility of electrons moving to still free levels when absorbing the energy of a light quantum (elementary particle of light radiation).
Color arises as a result of the interaction of light quanta with electrons in the molecules of matter. However, due to the fact that the state of electrons in the atoms of metals and non-metals, organic and inorganic compounds is different, the mechanism for the appearance of color in substances is also different.
2.1 Color of organic compounds.
For organic matter, which have color (and not all of them have this property), molecules are similar in structure: they are usually large, consisting of tens of atoms. For the appearance of color in this case, it is not the electrons of individual atoms that matter, but the state of the system of electrons of the entire molecule.
Ordinary sunlight is a stream of electromagnetic waves. A light wave is characterized by its length - the distance between adjacent maxima or two adjacent troughs. It is measured in nanometers (nm). The shorter the wave, the greater its energy, and vice versa.
The color of a substance depends on which waves (rays) of visible light it absorbs. If sunlight is not absorbed by the substance at all, but reflected and scattered, then the substance will appear white (colorless). If the substance absorbs all the rays, then it appears black.
The process of absorption or reflection of certain rays of light is associated with the structural features of the substance molecule. The absorption of a light flux is always associated with the transfer of energy to the electrons of a substance molecule. If the molecule contains s-electrons (forming a spherical cloud), then a lot of energy is required to excite them and transfer them to another energy level. Therefore, compounds with s-electrons always appear colorless. At the same time, p-electrons (forming a cloud in the shape of a figure eight) are easily excited, since the connection they make is less strong. Such electrons are contained in molecules that have conjugated double bonds. The longer the conjugation chain, the more p-electrons and the less energy is required to excite them. If the energy of the visible light waves (wavelengths from 400 to 760 nm) is sufficient to excite electrons, then the color that we see appears. The rays expended on the excitation of the molecule will be absorbed by it, and the unabsorbed rays will be perceived by us as the color of the substance.
2.2 Color of inorganic substances.
For inorganic substancescolor is due to electronic transitions and charge transfer from an atom of one element to an atom of another. The decisive role here is played by the outer electron shell of the element.
As in organic substances, the appearance of color here is associated with the absorption and reflection of light.
In general, the color of a substance is the sum of the reflected waves (or those that have passed through the substance without delay). At the same time, the color of a substance means that certain quanta are absorbed by it from the entire range of wavelengths of visible light. In molecules of colored substances, the energy levels of electrons are located close to each other. For example, substances: hydrogen, fluorine, nitrogen - seem to us colorless. This is due to the fact that visible light quanta are not absorbed by them, since they cannot transfer electrons to more high level. That is, ultraviolet rays pass through these substances, which are not perceived by the human eye, and therefore the substances themselves have no color for us. In colored substances, for example, chlorine, bromine, iodine, the electronic levels are closer to each other, so the light quanta in them are able to transfer electrons from one state to another.
An experience. Influence of a metal ion on the color of compounds.
Instruments and reagents: four test tubes, water, iron(II), cobalt(II), nickel(II), copper(II) salts.
Execution of experience. Pour 20-30 ml of water into test tubes, add 0.2 g of iron, cobalt, nickel and copper salts each and mix until dissolved. The color of the iron solution became yellow, cobalt - pink, nickel - green, and copper - blue.
Conclusion: As is known from chemistry, the structure of these compounds is the same, but they have a different number of d-electrons: for iron - 6, for cobalt - 7, for nickel - 8, for copper - 9. This number affects the color of the compounds. Therefore, you can see the difference in color.
3. The influence of the environment on the color.
Ions in solution are surrounded by a solvent shell. The layer of such molecules directly adjacent to the ion is calledsolvation shell.
In solutions, ions can act not only on each other, but also on the solvent molecules surrounding them, and those, in turn, on the ions. Upon dissolution and as a result of solvation, a color appears in a previously colorless ion. Replacing water with ammonia deepens the color. Ammonia molecules are more easily deformed and the color intensity is enhanced.
Now Let us compare the color intensity of copper compounds.
Experience No. 3.1. Comparison of color intensity of copper compounds.
Instruments and reagents: four tubes, 1% CuSO solution 4, water, HCl, ammonia solution NH 3, 10% solution of potassium hexacyanoferrate(II).
Execution of experience. Place 4 ml of CuSO into one test tube 4 and 30 ml H 2 O, in the other two - 3 ml CuSO 4 and 40 ml H 2 O. Add 15 ml of concentrated HCl to the first tube - a yellow-green color appears, to the second - 5 ml of a 25% ammonia solution - a blue color appears, in the third - 2 ml of a 10% solution of potassium hexacyanoferrate(II) - we observe a red- brown sediment. Add CuSO solution to the last test tube 4 and leave to control.
2+ + 4Cl - ⇌ 2- + 6H 2 O
2+ + 4NH 3 ⇌ 2+ + 6H 2 O
2 2 + 4- ⇌ Cu 2 + 12 H 2 O
Conclusion: With a decrease in the amount of reagent (substances involved in chemical reaction ) required for the formation of the compound, the color intensity increases. When new copper compounds are formed, charge transfer and color change occur.
4. Substances-chameleons.
The concept of "chameleon" is known primarily as a biological, zoological term denotinga reptile that has the ability to change the color of its skin when irritated, change the color of the environment, etc.
However, "chameleons" can also be found in chemistry. So what's the connection?
Let's go back to chemistry:
Chameleon substances are substances that change their color in chemical reactions and indicate changes in the environment under study. We highlight the general - a change in color (coloration). This is what connects these concepts. Chameleon substances have been known since ancient times. In the old manuals chemical analysis it is recommended to use the "chameleon solution" to determine the content of sodium sulfite Na in samples of unknown composition 2 SO 3 , hydrogen peroxide H 2O2 or oxalic acid H 2 C 2 O 4 . "Chameleon solution" is a solution of potassium permanganate KMnO 4
, which during chemical reactions, depending on the medium, changes its color in different ways. For example, in an acidic environment, a bright purple solution of potassium permanganate becomes colorless due to the fact that from the MnO permanganate ion 4
-
a cation is formed, i.e.positively charged ion Mn 2+ ; in a strongly alkaline medium from bright violet MnO 4
- it turns out the green manganate ion MnO 4
2-
. And in a neutral, slightly acidic or slightly alkaline environment, the final reaction product will be an insoluble black-brown precipitate of manganese dioxide MnO 2
.
We add that due to its oxidizing properties,those. the ability to donate or take electrons from atoms of other elements,and visual color change in chemical reactions, potassium permanganate has found wide application in chemical analysis.
So, in this case, the "chameleon solution" (potassium permanganate) is used as an indicator, i.e.a substance that indicates the presence of a chemical reaction or changes that have occurred in the medium under study.
There are other substances called "chameleons". We will consider substances containing the element chromium Cr.
Potassium chromate - inorganic compound, metal saltpotassium and chromic acid with the formula K 2 CrO 4 , yellow crystals, soluble in water.
Potassium bichromate (potassium bichromate, potassium chromium peak) - K 2Cr2O7 . inorganic compound, orange crystals, soluble in water. Highly toxic.
5. Experimental part.
Experience No. 5.1. The transition of chromate to dichromate and vice versa.
Instruments and reagents: potassium chromate solution K 2 CrO 4 , potassium bichromate solution K 2Cr2O7 , sulfuric acid, sodium hydroxide.
Execution of experience. Sulfuric acid is added to a solution of potassium chromate, as a result, the color of the solution changes from yellow to orange.
2K 2 CrO 4 + H 2 SO 4 \u003d K 2 Cr 2 O 7 + K 2 SO 4 + H 2 O
I add alkali to a solution of potassium bichromate, as a result, the color of the solution changes from orange to yellow.
K 2 Cr 2 O 7 + 4NaOH \u003d 2Na 2 CrO 4 + 2KOH + H 2 O
Conclusion: In an acidic environment, chromates are unstable, the ion yellow color turns into Cr ion 2 O 7 2- orange, and in an alkaline medium, the reaction proceeds in the opposite direction:
2Cr 2 O 4 2- + 2H + acidic medium - alkaline medium Cr 2 O 7 2- + H 2 O.
Oxidizing properties of chromium (VI) salts.
Instruments and reagents: potassium bichromate solution K 2Cr2O7 , sodium sulfite solution Na 2 SO 3 , sulfuric acid H 2 SO 4 .
Execution of experience. To solution K 2Cr2O7 , acidified with sulfuric acid, add a solution of Na 2 SO 3. We observe a color change: the orange solution turned green-blue.
Conclusion: In an acidic environment, chromium is reduced by sodium sulfite from chromium (VI) to chromium (III): K 2 Cr 2 O 7 + 3Na 2 SO 3 + 4H 2 SO 4 \u003d K 2 SO 4 + Cr 2 (SO 4) 3 + 3Na 2 SO 4 + 4H 2 O.
Experience No. 5.4. Oxidation of ethanol with a chromium mixture.
Instruments and reagents: 5% potassium bichromate solution K 2Cr2O7 , 20% sulfuric acid H 2 SO 4 , ethanol(ethanol).
Performing the experiment: To 2 ml of a 5% solution of potassium bichromate, add 1 ml of a 20% solution of sulfuric acid and 0.5 ml of ethanol. We observe a strong darkening of the solution. We dilute the solution with water to better see its shade. We get a yellow-green solution.
To 2 Cr 2 O 7 + 3C 2 H 5 OH + H 2 SO 4 → 3CH 3 -COH + Cr 2 O 3 + K 2 SO 4 + 4H 2 O
Conclusion: In an acidic environment, ethyl alcohol is oxidized with potassium bichromate. This produces an aldehyde. This experience shows the interaction of chemical chameleons with organic substances.
Experience 5.4. clearly illustrates the principle by which indicators operate to detect alcohol in the body. The principle is based on the specific enzymatic oxidation of ethanol, accompanied by the formation of hydrogen peroxide (H 2 O 2 ), causing the formation of a colored chromogen,those. organic matter containing a chromophore group (chemical group consisting of carbon, oxygen, nitrogen atoms).
Thus, these indicators visually (on a color scale) show the alcohol content in human saliva. They are used in medical institutions, when establishing the facts of alcohol consumption and intoxication. The scope of indicators is any situation when it is necessary to establish the fact of alcohol consumption: conducting pre-trip inspections of vehicle drivers, identifying drunk drivers on roads by traffic police, using them in emergency diagnostics as a means of self-control, etc.
6. Photochromism.
Let's get acquainted with interesting phenomenon, where there is also a change in the color of substances, photochromism.
Today, glasses with chameleon glasses are unlikely to surprise anyone. But the history of the discovery of unusual substances that change their color depending on the light is very interesting. In 1881, the English chemist Phipson received a letter from his friend Thomas Griffith describing his unusual observations. Griffith wrote that the front door of the post office, located opposite his windows, changes color during the day - darkens when the sun is at its zenith, and brightens at dusk. Intrigued by the message, Phipson examined lithopon, the paint that had been used to paint the post office door. His friend's observation was confirmed. Phipson was unable to explain the cause of the phenomenon. However, many researchers are seriously interested in the reversible color reaction. And at the beginning of the 20th century, they managed to synthesize several organic substances called "photochromes", that is, "light-sensitive paints". Since the time of Phipson, scientists have learned a lot about photochromes -Substances that change color when exposed to light.
Photochromism, or tenebescence, is the phenomenon of a reversible change in the color of a substance under the action of visible light, ultraviolet.
Exposure to light causes in a photochromic substance, atomic rearrangements, change in the population of electronic levels. In parallel with a change in color, a substance can change its refractive index, solubility, reactivity, electrical conductivity, and other chemical and physical characteristics. Photochromism is inherent in a limited number of organic and inorganic, natural and synthetic compounds.
There are chemical and physical photochromism:
- chemical photochromism: intramolecular and intermolecular reversible photochemical reactions (tautomerization (reversible isomerism), dissociation (cleavage), cis-trans-isomerization, etc.);
- physical photochromism: the result of the transition of atoms or molecules into different states. The change in color in this case is due to a change in the population of the electronic levels. Such photochromism is observed when only powerful light fluxes are exposed to the substance.
Photochromes in nature:
- Mineral tugtupit able to change color from white or pale pink to bright pink.
Photochromic materials
There are the following types of photochromic materials: liquid solutions and polymer films (macromolecular compounds) containing photochromic organic compounds, glasses with silver halide microcrystals evenly distributed in their volume (silver compounds with halogens), photolysis ( decay by light) which causes photochromism; Alkaline and alkaline earth metal halide crystals activated with various additives (e.g. CaF 2 /La,Ce; SrTiO 3 /Ni,Mo).
These materials are used as light filters with variable optical density (that is, they regulate the flow of light) in eye protection and devices from light radiation, in laser technology, etc.
Photochromic lenses
Photochromic lens exposed to light, partially covered with paper. A second level of color is visible between the light and dark parts, since photochromic molecules are located on both surfaces of the lens polycarbonate and others plastics . Photochromic lenses typically darken in the presence of UV and brighten in its absence in less than a minute, but the full transition from one state to another occurs from 5 to 15 minutes.
Conclusions.
So, the color of various compounds depends on:
* from the interaction of light with the molecules of matter;
* in organic substances, color occurs as a result of the excitation of the electrons of the element and their transition to other levels. The state of the system of electrons of the entire large molecule is important;
* in inorganic substances, color is due to electronic transitions and charge transfer from an atom of one element to an atom of another. An important role is played by the outer electron shell of the element;
* the color of the compound is affected external environment;
*important role plays the number of electrons in the compound.
List of sources used
1. Artemenko A. I. "Organic chemistry and man" ( theoretical basis, advanced course). Moscow, "Enlightenment", 2000.
2. Fadeev G. N. "Chemistry and color" (book for extracurricular reading). Moscow, "Enlightenment", 1977.