Colored precipitates with chromium. Color Chemistry of Qualitative Reactions All Gases and Precipitates in Chemistry

The Pambak River in the Lori region in northern Armenia has acquired a reddish hue, water samples have been taken for examination.

April 1999 after the NATO bombing of Yugoslavia and the destruction of petrochemical enterprises, a poisonous "black rain" passed over the town of Pancevo, containing a huge amount of heavy metals and organic compounds harmful to human life. Soil and groundwater were seriously polluted, which turned out to be contaminated with ethylene and chlorine. A huge amount of oil, oil products, ammonia and amino acids got into the Danube.

June-July 2000 in some regions of Dagestan and North Ossetia, in particular, in the city of Vladikavkaz, there were "colored rains". As a result of analyzes of water samples, an increased content of chemical elements was found. They exceeded the maximum allowable concentrations of cobalt (more than four times) and zinc (more than 434 times). Laboratory studies confirmed that the composition of the polluted rain was identical to the chemical composition of the samples taken on the territory of JSC "Electrozinc", which violated the standards for maximum allowable emissions into the atmosphere, approved by the Ministry of Environmental Protection.

In 2000 and 2002"rusty" precipitation fell in the Altai Territory and the Altai Republic. The weather anomaly was caused by strong emissions of combustion products at the Ust-Kamenogorsk metallurgical plant.

July-September 2001"red rains" repeatedly fell out in the Indian state of Kerala. Several hypotheses of the origin of red particles were put forward at once: someone considered them to be red dust carried by the wind from the Arabian Desert, someone recognized them as fungal spores or ocean algae. A version of their extraterrestrial origin was put forward. According to scientists, a total of about 50 tons of this strange substance fell on the ground along with precipitation.

In October 2001 residents of the southwestern regions of Sweden fell under abnormal rain. After the rain, gray-yellow stains remained on the surface of the earth. Swedish experts, and in particular a researcher from the Gothenburg Geoscience Center Lars Fransen, said that strong winds "dragged" red sandy dust from the Sahara, raised it to a height of up to 5 thousand meters and then poured it along with the rain in Sweden.

Summer 2002 green rain poured over the Indian village of Sangranpur near the city of Kolkata. Local authorities announced that there was no chemical attack. The examination of scientists who arrived at the site determined that the green cloud is nothing more than pollen from flowers and mangoes contained in bee excrement, and does not pose a danger to humans.

In 2003 in Dagestan, precipitation in the form of salt deposits fell. Cars standing in the open air were covered with a layer of salt. According to meteorologists, the reason for this was a cyclone that came from the regions of Turkey and Iran. Fine particles of sand and dust raised by a strong wind from the developed quarries in the territory of Dagestan mixed with water dust raised from the surface of the Caspian Sea. The mixture was concentrated in clouds that moved to the coastal regions of Dagestan, where unusual rain fell.

Winter 2004 orange-colored snow fell in eastern Poland. At the same time, residents of Transcarpathia observed him in the villages of Quiet and Gusinoe. According to one version, sandstorms in Saudi Arabia became the reason for the orange color of the snow: grains of sand, picked up by a strong wind, accumulated in the upper atmosphere and fell along with snow in Transcarpathia.

April 19, 2005 red rain fell in Kantemirovskiy and Kalacheevskiy districts of the Voronezh region. Precipitation left an unusual trace on the roofs of houses, fields, agricultural machinery. In a soil sample, traces of ocher, a natural pigment for paint production, were found. It contained hydroxides of iron and clay. Further investigation showed that there had been a release at the ocher factory in the village of Zhuravka, which led to the color of rain clouds in red. According to experts, the precipitation did not pose a danger to the health of people and animals.

April 19, 2005 over several regions of the Stavropol Territory, the sky acquired a yellowish tint, and then it began to rain, the drops of which were colorless. After drying, the drops were left on cars and on dark beige clothes, which were not washed off afterwards. The same rain fell on April 22 in Orel. The analyzes carried out showed that the sediments contained alkali, namely, nitrogenous compounds. The precipitation was very concentrated.

April 2005 for several days, orange rains were falling in Ukraine - in the Nikolaev region and in the Crimea. Colored precipitation also covered Donetsk, Dnepropetrovsk, Zaporozhye, Kherson regions these days. Ukrainian weather forecasters said that the orange color of the rain acquired due to a dust hurricane. The wind brought dust particles from North Africa.

February 2006 gray-yellow snow fell on the territory of the village of Sabo, located 80 km south of the city of Okha in the north of Sakhalin. According to eyewitnesses, oily spots of a gray-yellow color and with an unusual strange smell formed on the surface of the water obtained by melting suspicious snow. Experts believe that unusual precipitation could be the consequences of the activity of one of the Far Eastern volcanoes. Possibly, the pollution of the environment by products of the oil and gas industry is to blame. The reason for the yellowing of the snow was not exactly established.

February 24-26, 2006 in some areas of Colorado (USA) it was brown snow, almost like chocolate in color. "Chocolate" snow in Colorado - a consequence of a long drought in neighboring Arizona: there are giant clouds of dust that mixes with snow. Sometimes volcanic eruptions give the same result.

March 2006 creamy-pink snow fell in the north of Primorsky Krai. Experts explained the unusual phenomenon by the fact that the cyclone had previously passed through the territory of Mongolia, where strong dust storms raged at that time, covering large expanses of desert territories. Dust particles were drawn into the vortex of the cyclone and colored the precipitation.

March 13, 2006 in South Korea, including Seoul, yellow snow fell. The snow was yellow because it contained yellow sand brought from the deserts of China. The country's meteorological service has warned that snow containing fine sand can be dangerous for the respiratory system.

November 7, 2006 in Krasnoyarsk, light snow fell with green rain. He walked for about half an hour and, having melted, turned into a thin layer of greenish clay. People exposed to green rain experienced tearing and headaches.

January 31, 2007 in the Omsk region, on an area of ​​​​about 1.5 thousand square kilometers, yellow-orange snow fell with a pungent odor, covered with oily spots. Having passed through the entire Irtysh region, a plume of yellow-orange precipitation touched the Tomsk region along the edge. But the main part of the "acidic" snow fell in the Tarsky, Kolosovsky, Znamensky, Sedelnikovsky and Tyukalinsky districts of the Omsk region. In the colored snow, the iron content was exceeded (according to preliminary laboratory data, the concentration of iron in the snow was 1.2 mg per cubic centimeter, while the maximum allowable rate was 0.3 mg). According to Rospotrebnadzor, such a concentration of iron is not dangerous for human life and health. Abnormal precipitation was studied by laboratories in Omsk, Tomsk and Novosibirsk. At first it was assumed that the snow contained the poisonous substance heptyl, which is a component of rocket fuel. The second version of the appearance of yellow precipitation was the emissions of the metallurgical enterprises of the Urals. However, Tomsk and Novosibirsk experts came to the same conclusion as Omsk - the unusual color of the snow is due to the presence of clay-sand dust, which could get into the Omsk region from Kazakhstan. No toxic substances were found in the snow.

March 2008 yellow snow fell in the Arkhangelsk region. Experts suggested that the yellow color of the snow is due to natural factors. This is due to the high content of sand that got into the clouds as a result of dust storms and tornadoes that occurred elsewhere on the planet.

Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent. Compounds of hexavalent chromium are most often colored yellow or red, while trivalent chromium is characterized by greenish tones. But chromium is also prone to the formation of complex compounds, and they are painted in a variety of colors. Remember: all chromium compounds are poisonous.

Potassium dichromate K 2 Cr 2 O 7 is perhaps the most famous of the chromium compounds and is the easiest to obtain. A beautiful red-yellow color indicates the presence of hexavalent chromium. Let us carry out several experiments with it or with sodium dichromate very similar to it.

We strongly heat in the flame of a Bunsen burner on a porcelain shard (a piece of crucible) such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 ° C with the formation of a dark liquid. Let's warm it up for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. We will dissolve part of it in water (it will turn yellow), and leave the other part on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4, green chromium oxide (III) and oxygen:

2K 2 Cr 2 O 7 → 2K 2 CrO 4 + Cr 2 O 3 + 3/2O 2
Due to its tendency to release oxygen, potassium dichromate is a strong oxidizing agent. Its mixtures with coal, sugar or sulfur ignite vigorously on contact with the flame of a burner, but do not give an explosion; after combustion, a voluminous layer of green is formed - due to the presence of chromium oxide (III)-ash.

Carefully! Burn no more than 3-5 g on a porcelain shard, otherwise the hot melt may start to splatter. Keep your distance and wear safety goggles!

We scrape off the ash, wash it with water from potassium chromate and dry the remaining chromium oxide. Let's prepare a mixture consisting of equal parts of potassium nitrate (potassium nitrate) and soda ash, add it to chromium oxide in a ratio of 1:3 and melt the resulting composition on a shard or magnesia stick. Dissolving the cooled melt in water, we get a yellow solution containing sodium chromate. Thus, molten saltpeter oxidized trivalent chromium to hexavalent. By fusion with soda and saltpeter, all chromium compounds can be converted into chromates.

For the next experiment, let's dissolve 3 g of powdered potassium bichromate in 50 ml of water. To one part of the solution, add a little potassium carbonate (potash). It will dissolve with the release of CO2, and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions (Caution!), Then the red-yellow color of the bichromate will appear again.

Pour 5 ml of potassium dichromate solution into a test tube, boil with 3 ml of concentrated hydrochloric acid under draft or in the open air. Yellow-green poisonous chlorine gas is released from the solution, because chromate will oxidize HCl to chlorine and water. Chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.

In another test tube, carefully add 1-2 ml of concentrated sulfuric acid to potassium dichromate (in an amount that fits on the tip of a knife). (Caution! The mixture may splatter! Wear safety goggles!) We heat the mixture strongly, as a result, brownish-yellow hexavalent chromium oxide CrOz is released, which is poorly soluble in acids and well in water. It is anhydride of chromic acid, but sometimes it is called chromic acid. It is the strongest oxidizing agent. Its mixture with sulfuric acid (chromium mixture) is used for degreasing, since fats and other hard-to-remove contaminants are converted into soluble compounds.

Attention! Extreme care must be taken when working with the chromium mixture! If splashed, it can cause severe burns! Therefore, in our experiments, we will refuse to use it as a cleaning agent.

Finally, consider the reactions of detection of hexavalent chromium. Place a few drops of potassium dichromate solution in a test tube, dilute it with water and carry out the following reactions.

When a solution of lead nitrate is added (Caution! Poison!) Yellow lead chromate (chrome yellow) precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.

Add hydrogen peroxide (properly stored) and acidify the solution with sulfuric acid. The solution will take on a deep blue color due to the formation of chromium peroxide. The peroxide, when shaken with some ether (Caution! Fire hazard!) will turn into an organic solvent and turn it blue.

The latter reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. But, for example, nitric acid does not destroy chromium, as we can easily verify by using pieces of damaged chromium plating. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can be added), chromium and many chromium-containing steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide will precipitate, which will dissolve in excess NaOH and form green sodium chromite.

Filter the solution and add 30% hydrogen peroxide (Caution! Poison!). When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether. Instead of the method described above, thin filings of a metal sample can be alloyed with soda and nitrate, washed, and the filtered solution tested with hydrogen peroxide and sulfuric acid.

Finally, let's test with a pearl. Traces of chromium compounds give a bright green color with brown.

Lesson objectives:

• determination of the factors that cause the coloring of chemicals;
• expansion and systematization of knowledge on the chemical foundations of the theory of the origin of color;
• development of cognitive interest in the study of qualitative reactions.

Formed competencies of students:

• the ability to analyze the phenomena of the surrounding world in chemical terms;
• the ability to explain chemical phenomena associated with the appearance of color solutions;
• willingness to work independently with information;
• willingness to interact with colleagues and speak in front of an audience.

"All living things strive for color." W. Goethe

Knowledge update

In previous lessons, we have studied the properties of inorganic and organic substances, often using qualitative reactions that indicate the presence of a particular substance by color, smell, or sediment. The crossword puzzle offered to you consists of the names of chemical elements that have color differences.

Crossword solution:

Vertically:

1) Substance that turns flames purple (potassium).

2) The lightest silvery metal (lithium).

Horizontally:

3) The name of this element is "green branch" (thallium)

4) The metal that stains glass blue (niobium)

5) The name of the metal means sky blue (cesium)

6) Violet vapors of this substance were first obtained by Courtois thanks to his cat (iodine).

Motivation of educational activity.

Please note that the solution to the crossword puzzle was related to the color of the substances. But not only chemicals, but the world around us is colorful.

"All living things strive for color." These words of the great genius of poetry truly reflect the peculiarity of the emotions that this or that color evokes in us. We perceive it associatively, i.e. recall something familiar and familiar. The perception of color is accompanied by certain emotions. (Demonstration of paintings by artists).

Students answer questions about emotions on the perception of color.

• The blue color evokes calmness, it is pleasant, it increases the assessment of self-affirmation.
• Green - the color of green plants, the mood of peace, tranquility.
• Yellow is the spirit of happiness, fun, associated with the sun.
• Red is the color of activity, action, you want to achieve results.
• Black - causes sadness, irritation.

Why is the world around us so colorful?

Today we are trying to find the answer to the question "What is a color?" in terms of chemistry.

The topic of the lesson is "Color Chemistry of Qualitative Reactions".

Determination of color factors

It is impossible to consider the chemical essence of color without knowledge of the physical properties of visible light. Without light there is no coloring of objects, everything seems dark. Light is electromagnetic waves. How much joy a rainbow in the sky brings to both children and adults, however, it appears only if the sun's rays are reflected in water droplets and return to the human eye with a multi-colored spectrum. We owe the great English physicist Isaac Newton the fact that he explained this phenomenon: white is a combination of rays of different colors. 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 1.)

Table 1 - Color of substances having one absorption band in the visible part of the spectrum.

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. From biology lessons, you know that 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.

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.

1 color formation factor is light.

Consider examples of the next factor - the structure of substances.

Metals have a crystalline structure, 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. (Demonstration of the collection of metals). 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.

Let's see how the color of a substance changes from the state of the structure using sulfur as an example.

Demonstration of the video film "Chemical elements".

We conclude: sulfur in the crystalline state is yellow, and in the amorphous state it is black, i.e. in this case, the color factor is the structure of the substance.

What happens to the color of substances when the structure is destroyed, for example, during the dissociation of salt molecules, if these solutions are colored.

CuS0 4 (blue) Cu 2+ + SO 4 2-

NiS0 4 (green) Ni 2+ + SO 4 2-

CuCI 2 (blue) Cu 2+ + 2CI -

FeCI 3 (yellow) Fe 3+ +3CI -

In these solutions, the same anions, different cations give color.

The following solutions have the same cation, but different anions, so the anions are responsible for the color:

K 2 Cr 2 O 7 (orange) 2K + +Cz 2 O 4 2-

K 2 Cr0 4 (yellow) 2K + + Cz0 4 2-

KMnO 4 (violet) K + + Mn04 -

The 3rd factor in the appearance of color 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.

We carry out the following experiment. There is a solution of beet juice (crimson color). Add the following to this solution:

1. an experience. Beet juice solution and acetic acid
2. an experience. Beet juice solution and NH 4 0H solution
3. an experience. A solution of beet juice and water.

In experiment 1, an acidic medium causes a color change to purple, in experiment 2, an alkaline medium changes the color of the beets to blue, and the addition of water (neutral medium) does not cause color changes.

A well-known indicator for the determination of an alkaline environment is phenolphthalein, which changes the color of alkali solutions to crimson.

Experience is being made:

NaOH + phenolphthalein -> crimson color

We conclude: the 4th color change factor is the environment.

Let us consider the case of the environment of an atom of one element by various complexes.

An experiment is being carried out: a qualitative reaction to the Fe 3+ ion:

FeCl 3 + KCNS -> red color

FeCl 3 + K 4 (Fe(CN) 6) -> p-p dark blue

A historical fact is associated with a change in the color of the iron ion when surrounded by potassium thiocyanate in a bloody color.

Student messages.

In 1720, political opponents of Peter I from the clergy organized a "miracle" in one of the St. Petersburg cathedrals - the icon of the Mother of God 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 the 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 sovereign.

We are experimenting.

We write words on paper with solutions of CuS0 4 (blue) and FeСI 3 (yellow), then we process the sheet with yellow blood salt K 4 (Fe (CN) 6). The word CuSO 4 (cyan) turns red and the word FeCI 3 (yellow) turns blue-green. There is no change in the oxidation state of the metal, only the environment changed:

2CuS0 4 + K 4 (Fe(CN) 6) Cu 2 (Fe(CN) 6) + 2K 2 SO 4

4FeCl 3 + 3 K 4 (Fe(CN) 6) Fe 4 (Fe(CN) 6) 3 +12 KCI

5th color factor - environment of ions by complexes.

Conclusion.

We have identified the main factors influencing the manifestation of the color of substances.

We realized that color is the result of the absorption of a certain part of the visible spectrum of sunlight by a substance.

A qualitative reaction is a special reaction that detects ions or molecules by color.

Messages of students on the topic "Color serves people".

Animal blood and leaf greens contain similar structures, but blood contains iron ions - Fe, and plants - Mg. This ensures the color: red and green. By the way, the saying "blue blood" is true for deep-sea animals, whose blood contains vanadium 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.

Paints and dyes are used by artists, decorators and textile workers. Harmony of color is an integral part of the art of "design". The most ancient paints were charcoal, chalk, clay, cinnabar and some salts such as copper acetate (verdigris).

Phosphor paints are used for road signs and advertisements, rescue boats.

For the purpose of bleaching, substances are introduced into the composition of washing powders that give the fabric a bluish fluorescence.

The surface of all metal objects under the influence of the environment is destroyed. Their protection is most effective with colored pigments: aluminum powder, zinc dust, red lead, chromium oxide.

Reflection.

1. What factors cause the color of chemicals?

2. What substances can be determined by qualitative reactions by color change?

3. What factors determine the color of potassium and copper salts?

Nature, of which chemicals are a part, surrounds us with mysteries, and trying to solve them is one of life's greatest joys.

Today we tried to approach the truth "Chemistry of color" from one side, and maybe you will discover another. The most important thing is that the world of color is cognizable.

Man is born
To create, dare - and nothing else,
To leave a good mark in life
And solve all the difficult problems.
For what? Look for your answer!

Homework.

Give examples of qualitative reactions to iron ions by color change.