>> Physical and chemical phenomena (chemical reactions). Let's experiment at home. External effects in chemical reactions
Physical and chemical phenomena (chemical reactions)
The material in this paragraph will help you figure out:
>what is the difference between physical and chemical phenomena.(chemical reactions);
> what external effects accompany chemical reactions.
In natural history lessons, you learned that various physical and chemical phenomena occur in nature.
Physical phenomena.
Each of you has repeatedly observed how ice melts, water boils or freezes. Ice, water and water vapor consist of the same molecules, so they are one substance (in different states of aggregation).
Phenomena in which a substance does not transform into another are called physical.
Physical phenomena include not only changes in substances, but also the glow of hot bodies, the passage of electric current in metals, the spread of the smell of substances in the air, the dissolution of fat in gasoline, and the attraction of iron to a magnet. Such phenomena are studied by the science of physics.
Chemical phenomena (chemical reactions).
One of the chemical phenomena is combustion. Let's consider the process of burning alcohol (Fig. 46). It occurs with the participation of oxygen contained in the air. When burned, alcohol seemingly turns into a gaseous state, just as water turns into steam when heated. But that's not true. If the gas obtained as a result of the combustion of alcohol is cooled, then part of it will condense into liquid, but not into alcohol, but into water. The rest of the gas will remain. With the help of additional experiment it can be proven that this residue is carbon dioxide.
Rice. 46. Burning alcohol
So the alcohol that burns and oxygen, which participates in the combustion process, are converted into water and carbon dioxide.
Phenomena in which some substances are transformed into others are called chemical phenomena or chemical reactions.
Substances that enter into a chemical reaction are called starting substances, or reagents, and those that are formed are called final substances, or reaction products.
The essence of the chemical reaction considered is conveyed by the following entry:
alcohol + oxygen -> water + carbon dioxide
starting materials final substances
(reagents) (reaction products)
The reactants and products of this reaction are made up of molecules. During combustion, a high temperature is created. Under these conditions, the molecules of the reagents disintegrate into atoms, which, when combined, form molecules of new substances - products. Therefore, all atoms are conserved during the reaction.
If the reactants are two ionic substances, then they exchange their ions. Other variants of interaction of substances are also known.
External effects accompanying chemical reactions.
By observing chemical reactions, you can record the following effects:
Change in color (Fig. 47, a);
gas release (Fig. 47, b);
formation or disappearance of sediment (Fig. 47, c);
appearance, disappearance or change in odor;
release or absorption of heat;
the appearance of a flame (Fig. 46), sometimes a glow.
Rice. 47. Some external effects during chemical reactions: a - appearance
coloring; b - gas release; c - appearance of sediment
Laboratory experiment No. 3
The appearance of color as a result of the reaction
Are solutions of soda ash and phenolphthalein colored?
Add 2 drops of phenolphthalein solution to a portion of soda solution I-2. What color appeared?
Laboratory experiment No. 4
Release of gas as a result of the reaction
Add a little chloride acid to the soda ash solution. What are you observing?
Laboratory experiment No. 5
The appearance of a precipitate as a result of the reaction
Add 1 ml of copper sulfate solution to the soda ash solution. What's happening?
The appearance of a flame is a sign of a chemical reaction, that is, it indicates a chemical phenomenon. Other external effects can be observed during physical phenomena. Let's give a few examples.
Example 1. Silver powder obtained in a test tube as a result of a chemical reaction is gray in color. If you melt it and then cool the melt, you will get a piece of metal, but not gray, but white, with a characteristic shine.
Example 2. If you heat natural water, gas bubbles will begin to emerge from it long before boiling. This is dissolved air; its solubility in water decreases when heated.
Example 3. An unpleasant odor in the refrigerator disappears if granules of silica gel, one of the silicon compounds, are placed in it. Silica gel absorbs molecules of various substances without destroying them. Activated carbon works in a similar way in a gas mask.
Example 4 . When water turns into steam, heat is absorbed, and when water freezes, heat is released.
To determine what kind of transformation has occurred - physical or chemical, you should carefully observe it, as well as comprehensively examine the substances before and after the experiment.
Chemical reactions in nature, everyday life and their significance.
Chemical reactions occur constantly in nature. Substances dissolved in rivers, seas, and oceans interact with each other, some react with oxygen. Plants absorb carbon dioxide from the atmosphere, water and dissolved substances from the soil and process them into proteins, fats, glucose, starch, vitamins, other compounds, as well as oxygen.
This is interesting
As a result of photosynthesis, about 300 billion tons of carbon dioxide are absorbed from the atmosphere each year, 200 billion tons of oxygen are released, and 150 billion tons of organic substances are formed.
Reactions involving oxygen, which enters living organisms during respiration, are very important.
Many chemical reactions accompany us in everyday life. They occur during frying meat, vegetables, baking bread, souring milk, fermenting grape juice, bleaching fabrics, burning various types of fuel, hardening cement and alabaster, blackening silver jewelry over time, etc.
Chemical reactions form the basis of such technological processes as the extraction of metals from ores, the production of fertilizers, plastics, synthetic fibers, medicines, and other important substances. By burning fuel, people provide themselves with heat and electricity. Using chemical reactions, they neutralize toxic substances and process industrial and household waste.
The occurrence of some reactions leads to negative consequences. Rusting of iron shortens the life of various mechanisms, equipment, vehicles, and leads to large losses of this metal. Fires destroy housing, industrial and cultural facilities, and historical values. Most foods spoil due to their interaction with oxygen in the air; in this case, substances are formed that have an unpleasant odor, taste and are harmful to humans.
conclusions
Physical phenomena are the phenomena in which each substance is conserved.
Chemical phenomena, or chemical reactions, are the transformation of one substance into another. They can be accompanied by various external effects.
Many chemical reactions occur in the environment, in plants, animals and humans, and accompany us in everyday life.
?
100. Match:
1) dynamite explosion; a) physical phenomenon;
2) solidification of molten paraffin; b) chemical phenomenon.
3) food burning in a frying pan;
4) the formation of salt during the evaporation of sea water;
5) separation of a strongly shaken mixture of water and vegetable oil;
6) fading of dyed fabric in the sun;
7) passage of electric current in the metal;
101. What external effects are accompanied by such chemical transformations: a) burning of a match; b) rust formation; c) fermentation of grape juice.
102. Why do you think some food products (sugar, starch, vinegar, salt) can be stored indefinitely, while others (cheese, butter, milk) quickly spoil?
Experimenting at home
External effects in chemical reactions
1. Prepare small amounts of aqueous solutions of citric acid and baking soda. Pour portions of both solutions together into a separate glass. What's happening?
Add a few soda crystals to the remainder of the citric acid solution, and a few citric acid crystals to the remainder of the soda solution. What effects do you observe - the same or different?
2. Pour some water into three small glasses and add 1-2 drops of brilliant green alcohol solution, known as “zelenka,” to each. Add a few drops of ammonia to the first glass, and citric acid solution to the second. Has the color of the dye (green) in these glasses changed? If so, how exactly?
Write down the results of the experiments in a notebook and draw conclusions.
Popel P. P., Kryklya L. S., Chemistry: Pidruch. for 7th grade. zagalnosvit. navch. closing - K.: VC "Academy", 2008. - 136 p.: ill.
Lesson content lesson notes and supporting frame lesson presentation interactive technologies accelerator teaching methods Practice tests, testing online tasks and exercises homework workshops and trainings questions for class discussions Illustrations video and audio materials photographs, pictures, graphs, tables, diagrams, comics, parables, sayings, crosswords, anecdotes, jokes, quotes Add-ons abstracts cheat sheets tips for the curious articles (MAN) literature basic and additional dictionary of terms Improving textbooks and lessons correcting errors in the textbook, replacing outdated knowledge with new ones Only for teachers calendar plans training programs methodological recommendationsGoals: know
1) features of physical and chemical phenomena;
2) signs of chemical reactions.
1) distinguish between physical and chemical phenomena;
2) recognize chemical reactions based on their signs.
During the classes
I. Organizational moment
Hello children, sit down. Let's start the chemistry lesson.
II. Communicating the topic and purpose of the lesson
The topic of our lesson is “Physical and chemical phenomena. Signs of chemical reactions” (write in notebooks).
Today we will have to learn about physical and chemical phenomena and the signs of chemical reactions. This is what we need to find out.
What will you and I need to be able to do? And we will have to be able to distinguish chemical phenomena from physical ones and recognize chemical reactions by their signs.
III. Learning new material
So let's begin.
Teacher: In the world, everything is in motion, everything changes. Changes that occur in substances are called phenomena. For example: evaporation of water, melting of iron, rusting of metals, etc. There are physical and chemical phenomena.
Physical phenomena are accompanied by changes in shape, state of aggregation, volume, temperature, degree of grinding of a substance, etc., but there is no transformation of some substances into others. The composition of the substance remains unchanged.
For example, the melting of ice or the boiling of water are physical phenomena and here a change in the state of aggregation of a substance occurs, while the substance itself - water - remains unchanged. In this case, a change in the physical properties of the substance occurs.
In addition to physical properties, each substance has certain chemical properties.
The chemical properties of a substance are the ability of a given substance to transform into other substances. The chemical properties of substances are manifested in chemical phenomena.
Chemical phenomena, which are called chemical reactions, are accompanied by the transformation of some substances into others.
As a result of a chemical reaction, new substances are always formed, which differ from the original ones in composition and properties.
Thus, during physical phenomena, the qualitative and quantitative composition of substances is preserved, but during chemical phenomena, the composition of the original substances is not preserved, they are transformed into other substances.
You were given a creative task to take home - write a story about an encounter with physical and chemical phenomena or draw what you saw. So guys, who's ready?
In the meantime, the stories will be heard, everyone else will have to think about what phenomenon we are talking about - physical or chemical.
Guys, please.
Student 1: Using information from biology, you know that potato tubers contain starch, which is formed in the leaves in the light and then deposited in the tubers. If you cut this tuber and drop tincture of iodine onto the cut, the brown color of iodine will turn into blue. This happened because a reaction occurred between starch and iodine, and a new blue substance was formed (demonstrates experiment).
Okay, well done, sit down.
Guys, what phenomenon do you think the student spoke about?
Students: It was about a chemical phenomenon.
What do you think?
That's right, it's a chemical phenomenon.
Who else is ready?
Well done, okay, sit down.
Guys, what phenomenon were we talking about here?
Students: It was again about a chemical phenomenon.
Who else thinks?
Well done guys, you answered correctly. Let's listen to the student again.
Student 3: While studying literature for young chemists, I decided to do such an experiment at home. I took a glass of soapy water, added a few drops of table vinegar and found that instead of foam, the liquid contained light gray flakes of sediment, let's see what happens (demonstrates experience).
Well done, okay, sit down. What phenomenon did the Student tell us about?
Students: Here we talked about a chemical phenomenon. (Who else will say?)
That's right, children, this is a chemical phenomenon.
Who else prepared the performance?
Please, let's listen.
Student 4: And I decided to draw my phenomena. Look what I did and listen to my story.
Clear hot day (shows the picture). Water evaporates from the surface of the earth in the form of steam, which is always in the air. The further from the surface of the earth, the lower the temperature and therefore the smallest droplets of water are formed from the steam. Fog is made up of these droplets. Clouds are the same fog in the air high above the ground (changes the pattern). Small droplets, merging with each other in the clouds, gradually increase in size. The cloud darkens and becomes a cloud. Heavy drops of water cannot stay in the air and fall to the ground in the form of rain (changes the picture). In winter, snowflakes are formed from steam. Reservoirs freeze in winter, becoming covered with ice. This is where the fun kids are. So guys, what phenomena have I presented to you?
Students: Physical phenomena are presented here, namely changes in the state of aggregation of water.
Well done, sit down, the work is well prepared.
Student 5: I depicted the process of washing hands. When we wash our hands with soap, the soap in the water breaks down into two substances: an alkali and a fatty acid. The alkali binds the fat that covers our skin, and the fatty acid forms a rich foam. The foam captures and, together with water, carries away all the smallest particles of dirt that were on our skin.
Student 6: This picture shows the process of preparing the dough. After kneading the flour, add sugar, butter, and yeast. Some of the starch contained in flour is converted into sugar. Here, yeast “attacks” this sugar and decomposes the sugar into alcohol and carbon dioxide. In the dough, gas tends to escape and at the same time it rises and loosens the dough. This is why the dough becomes porous and the bread or cake is riddled with holes.
And here there is a series of chemical reactions.
Okay, well done, sit down. You probably understood the material. I give all participants in creative work a “5” (six ratings).
So, guys, what phenomena are called physical?
Students: Physical phenomena are phenomena that are accompanied by changes in shape, state of aggregation, volume, temperature, degree of grinding, without the transformation of some substances into others. The composition of the substance remains unchanged.
Okay, well done, sit down. What phenomena are called chemical? Who's to say?
Students: Chemical phenomena are phenomena that are accompanied by the transformation of some substances into others. In this case, the composition of the original substances is not preserved, but during the chemical reaction they are converted into other substances.
Well done, good, right, sit down.
So, let's comment once again on the performance of your classmates. (On the desk) -
Reactants Sign of a chemical reaction
When clicked, reacting substances appear immediately, then signs of these reactions:
Let's open our notebooks and write down the signs of a chemical reaction. We wrote down the title “Signs of chemical reactions.”
- Color change.
- Gas release.
- Precipitation.
And the topic of our lesson is “Signs of chemical reactions.”
So by what signs are chemical reactions recognized? (We list).
But these are not all the signs, there are a number of other signs that I will tell you about now.
Attention, children, we continue to study a new topic. Now I will show you experiments that are accompanied by signs that we have already talked about and those that you do not yet know about
(I demonstrate experiments with signs of chemical reactions from the disk “School chemical experiment”, grade 8, part 1):
- precipitation;
- dissolution of sediment;
- color change;
- sound effect;
- gas release.
So, by what signs are chemical reactions recognized?
Student: Lists the signs of chemical reactions and writes them down in a notebook.
Who will repeat? Another student repeats.
Okay, well done, now kids, let's have a rest! Otherwise I see that you are tired.
(Physical minute)
So, let's move on to consolidation.
IV. Fixing the material
Guys, open your textbooks, paragraph 28, page 97. Look carefully and find:
Read an example of a reaction that occurs with a color change. Who found it? Please... What example did we look at?
A chemical reaction that occurs with the formation of a precipitate... What was the example in our lesson?
With the release of gas? What kind of experience did we have with this sign in class?
With the release of heat.
With color change.
With the appearance of a smell.
So what are the characteristics of chemical reactions?
Attention, let's move on to the next job. As a reinforcement, you will complete test tasks (Appendix 1) and do a self-assessment. There are test tasks on your tables. Each question has answer options. You will need to select the correct answer (one or more) and circle it. But first, don’t forget to write your first and last name on the piece of paper. Do it (5-7 minutes).
Now let’s check how you understand the topic of the lesson. Exchange sheets with each other and check the test using the key (I display the key with the correct answers on the board). Look at the board and check your answer. If the option is incorrect, cross it out and circle the correct one. If there are no errors, put “5”. If 1-2 errors are made “4”. If there are more than 2 errors - “3”.
Have you checked? Tell me honestly who got “5”, who got “4”, who got “3”.
Well done, you have mastered the topic well!
So guys, what did you learn in class?
Student: We learned what the signs of chemical reactions are.
What work have we done?
Student: Completed creative tasks, demonstrated experiments and drawings, and also watched video clips “Signs of chemical reactions”
What have we learned?
Student: We learned to recognize chemical reactions by their signs.
What else?
Student: We consolidated the methodology for performing tests.
Well done guys, good.
So, what signs of chemical reactions do you know? (students' answers)
Well done guys, you learned the new topic well.
VI. Homework assignment
And now the homework: study paragraph 28, No. 1, 2 - mandatory for everyone, and No. 3 - for students with “4” and “5” in the subject.
Homework written down, lesson over. Goodbye.
Literature:
- textbook “Chemistry. 8th grade". I.I.Novoshinsky, N.S.Novoshinskaya;
- A. Khrapovsky. Entertaining essays on chemistry;
chemical reactions that proceed in only one direction are called from left to right"), as a result of which the starting substances are transformed into reaction products. Such chemical processes are said to proceed “to the end.” These include combustion reactions, and reactions accompanied by the formation of poorly soluble or gaseous substances Reversible are called chemical reactions that occur simultaneously in two opposite directions (“from left to right” and “from right to left”). In the equations of such reactions, the equal sign is replaced by two oppositely directed arrows. Among two simultaneously occurring reactions, they are distinguished straight( flows from left to right) and reverse(proceeds “from right to left”). Since during a reversible reaction the starting substances are simultaneously consumed and formed, they are not completely converted into reaction products. Therefore, reversible reactions are said to proceed “not completely.” As a result, a mixture of starting substances and reaction products is always formed. 6. Based on the participation of catalysts, chemical reactions are divided into homogeneous catalytic non-catalytic Phenomena in which some substances are transformed into others that differ from the original ones in composition and properties are called chemical. Examples: oxidation of substances in air, combustion, rusting of iron, etc. Chemical phenomena - chemical transformations, chemical reactions, chemical interactions. Phenomena in which the form or physical state of substances changes are called physical. Examples: boiling, melting, evaporation, freezing, release of heat, light, etc. Chemical phenomena are always accompanied by physical ones. Examples: when magnesium burns, heat and light are released; In a galvanic cell, an electric current arises as a result of chemical reactions. A chemical compound is a chemically individual substance consisting of atoms of one or different elements. Examples: oxygen, Berthollet salt, zinc, sulfur, etc. Mixture - several chemical compounds mixed with each other. Examples: air, metal alloys, sea water, etc. A mechanical mixture and a chemical compound have the following main differences. When preparing a mechanical mixture, the components can be taken in any quantitative ratios. When producing chemical compounds, the starting substances are taken in strictly defined quantities. During mechanical mixing of substances, as a rule, no release or absorption of energy is observed. Chemical reactions are accompanied by thermal effects. The individual properties of the substances that make up the mechanical mixture are preserved, since the components in the mixture are chemically unchanged. During chemical reactions, the properties of the original substances are not preserved, since as a result of their interaction, new substances with new properties are formed. A mixture, unlike a chemical compound, can be separated into its component parts using differences in their physical properties. Example 1 When a mixture of iron and iron sulfide weighing 6.4 g was dissolved in hydrochloric acid, 1.79 l (n.e.) of a mixture of gases was released. Determine the mass fraction (%) of iron in the mixture. Given: t(mixture) - 6.4 g K(gases) = 1.79 l Find: tt"(Fe) in the mixture Solution: 1) Write the reaction equations: Fe + 2HCl = FeCl2 + H2t, FeS + 2HC1 = FeCl2 + H2Sf. 2) Let us express the mass of Fe in the mixture as x g, and the volume of released H2 as y l. Then the mass of FeS will be equal to (6.4 - x) g, and the volume of H2S will be (1.79 - y) l. 5) Calculate the mass fraction (%) of Fe in the mixture: w(Fe) = -T-7-" 100% =17.34%. 6.4 Answer; u>(Fe) = 17.34%. Example 2 When under normal conditions, 20 liters of a gas mixture consisting of nitrogen and oxygen have a mass of 28 g. Determine the volumes of gases in the mixture Given: /n(mixture) = 28 g K(mixture) = 20 l Find: K(N2) and V( 02). Solution: 1) Let us denote F(N2) in the mixture by x l, then Y(02) will be (20 - x) l 2) Find the amounts of substances N2 and 02, and then their masses: x 20 V. (N2) = ~2M M°L; V(2) ~ 22.4 M°L; 28 l: l h 32 (20 - x) t(*2> 3 lijg; t(°2)-- 3) According to the condition, the mass of the mixture of gases is 28 g, i.e.: 28 x 32(20-*) +-~gm-:- = 28, x = 3.2 liters. Thus, in the mixture contains 3.2 liters of N2 and 16.8 liters of 02. Answer: K(K2) = 3.2 liters; G(02) = 16.8 liters. Example 3 Determine the mass fraction of each gas in a mixture consisting of hydrogen and methane, if its hydrogen density is 5. Given: Dn2=5 Find: w(H2) and q>(CH4) Solution: 1) Let us denote w(tl2) by x%, and w(CH4) - (100 - x) %. 2) Find the molecular weight of the mixture: M = M(H2) £>n2, M = 2-5 = 10. 3) Express the mass fractions of gases in the molecular weight of the mixture: „, 2 x _ 16 (100 - x ) ^> = -100-" "W"-ioo-" 100 100 Therefore, the gas mixture contains 42.85% H2 and 57.15% CH4. Answer: u>(H2) - 42.85%; w(CH4) = 57.15%. Questions and tasks for independent solution 1. Give examples of five physical phenomena. Indicate the signs by which you classify phenomena as physical. 2. Give examples of five chemical phenomena. Indicate the signs by which you classify the phenomena as chemical. 3. Which phenomena (physical or chemical) should include the formation of smoke: a) during the combustion of coal; b) concentrated hydrochloric acid in air; c) when vapors of hydrochloric acid and ammonia come into contact? 4. What phenomena (physical or chemical) include the transformation of some allotropic modifications of elements into others: oxygen into ozone, white phosphorus into red, orthorhombic sulfur into monoclinic, etc.? 5. Do physical and chemical phenomena always appear separately? Give examples of phenomena occurring simultaneously. 6. What phenomena (physical or chemical) include: a) preparation of powder from a piece of chalk; b) ignition of a match; c) gas release when baking soda is treated with acid; d) sublimation of solid iodine. 7. What is a mixture and a chemical compound? 8. What distinctive features of mixtures and chemical compounds do you know? 9. Write down separately the names of chemical compounds and mixtures from the following: salt, soil, nitrogen, air, river water, milk, zinc, granite, marble, argon. 10. Pure substances (as opposed to mixtures) include: a) copper sulfate; b) sea water; to the air; d) Berthollet salt. 11. Mixtures (as opposed to pure substances) include: a) hydrochloric acid; b) slaked lime; c) chlorine water; d) drinking water. Determine the mass fraction of sulfur in the initial mixture. Answer: 57.1%. 28. When 5.1 g of an aluminum-magnesium alloy was treated with hydrochloric acid, 5.6 liters of hydrogen were released. Determine the mass fraction (%) of Mg in the alloy. Answer: 47.05%. 29. Under normal conditions, a gas mixture with a volume of 12 liters, consisting of ammonia and carbon monoxide (IV), has a mass of 18 g. How many liters of each of these gases are contained in the mixture? Answer: 4.62 l NH3 and 7.38 l C02. 30. Determine the volume fractions (%) of a gas mixture consisting of carbon oxides (II, IV), if its hydrogen density is 19.6. Answer 30% SD; 70% CO2. 31. Determine the mass fraction (%) of dolomite CaC03 MgCO^ in the ore if the decomposition of 10 g of it releases 0.96 l of carbon monoxide (IV) (n.a.). Answer: 39.4%. 32. What mass of dolomite containing 8% impurities must be taken to obtain 56 m3 of carbon monoxide (IV) (n.a.)? Answer: 250 kg. 33. When 1.6 g of a mixture of ZnO and ZnC03 was calcined, 1.248 g of zinc oxide was obtained. What is the composition of the starting mixture? Answer: 37.5% ZnO. 34. What mass fraction (%) of chlorine is contained in a mixture consisting of 2 g of sodium chloride and 2 g of ammonium chloride? Answer: 63.5%. 35. There is a mixture of calcium and calcium oxide weighing 10 g. What is the mass of each substance in the mixture if the reaction of 2 g of it with water produces 224 ml of hydrogen? Answer: 2g Ca, 8g CaO. 36. When 5.85 g of a mixture of aluminum and its oxide was treated with an excess solution of potassium hydroxide, 1.26 liters of gas (n.e.) were released. Determine the mass fraction of aluminum in the mixture. Answer: 17.3%. 37. To convert 1 g of a mixture of potassium carbonate and potassium hydroxide into potassium chloride, 0.626 g of hydrogen chloride was consumed. How many grams of potassium hydroxide were in the mixture? Answer: 0.8 g. 38. When 6.9 g of zinc alloy with aluminum is dissolved in sulfuric acid, 2.688 liters of hydrogen are released. Determine the zinc content in the alloy. Answer: 94.8%. 39. The volume of a mixture of carbon (II) monoxide and oxygen is 100 ml. After burning carbon monoxide, the volume decreased by 30 ml due to the oxygen in the mixture. Find the volumetric composition of the original mixture. Answer: 60% CO; 40% 02. 40. An excess of barium chloride solution was added to a solution of 2.404 g of a mixture of salts CuS04 5H20 and FeS04 7H20. The mass of the resulting sediment was 2.172 g. Determine the composition of the initial mixture. Answer: 67.4% CuS04-5H20 and 32.6% FeS04-7H20. 41. What volume of air is required for complete combustion of 1 m3 of a gas mixture having a volume composition of 20% H2, 30% CH4 and 50% CO? The volume fraction of oxygen in air is 20.95%. Answer: 4.53 m3. 42. Determine the mass fraction (%) of sodium chloride in a mixture of sodium and potassium chlorides if 0.325 g of this mixture reacted with silver nitrate to form 0.7175 g of precipitate. Answer: 53.5%.
I guarantee that you have noticed more than once something like how your mother’s silver ring darkens over time. Or how a nail rusts. Or how wooden logs burn to ash. Well, okay, if your mother doesn’t like silver, and you’ve never gone hiking, you’ve definitely seen how a tea bag is brewed in a cup.
What do all these examples have in common? And the fact that they all relate to chemical phenomena.
A chemical phenomenon occurs when some substances are transformed into others: new substances have a different composition and new properties. If you also recall physics, then remember that chemical phenomena occur at the molecular and atomic level, but do not affect the composition of atomic nuclei.
From the point of view of chemistry, this is nothing more than a chemical reaction. And for each chemical reaction it is certainly possible to identify characteristic features:
- During the reaction, a precipitate may form;
- the color of the substance may change;
- the reaction may result in the release of gas;
- heat can be released or absorbed;
- the reaction may also be accompanied by the release of light.
Also, a list of conditions necessary for a chemical reaction to occur has long been determined:
- contact: To react, substances must touch.
- grinding: for the reaction to proceed successfully, the substances entering into it must be crushed as finely as possible, ideally dissolved;
- temperature: many reactions directly depend on the temperature of substances (most often they need to be heated, but some, on the contrary, need to be cooled to a certain temperature).
By writing the equation of a chemical reaction in letters and numbers, you thereby describe the essence of a chemical phenomenon. And the law of conservation of mass is one of the most important rules when drawing up such descriptions.
Chemical phenomena in nature
You, of course, understand that chemistry does not only happen in test tubes in a school laboratory. You can observe the most impressive chemical phenomena in nature. And their significance is so great that there would be no life on earth if not for some of the natural chemical phenomena.
So, first of all, let's talk about photosynthesis. This is the process by which plants absorb carbon dioxide from the atmosphere and produce oxygen when exposed to sunlight. We breathe this oxygen.
In general, photosynthesis occurs in two phases, and only one requires lighting. Scientists conducted various experiments and found that photosynthesis occurs even in low light. But as the amount of light increases, the process accelerates significantly. It was also noticed that if the plant's light and temperature are simultaneously increased, the rate of photosynthesis increases even more. This happens up to a certain limit, after which a further increase in illumination ceases to accelerate photosynthesis.
The process of photosynthesis involves photons emitted by the sun and special plant pigment molecules - chlorophyll. In plant cells it is contained in chloroplasts, which is what makes the leaves green.
From a chemical point of view, during photosynthesis a chain of transformations occurs, the result of which is oxygen, water and carbohydrates as an energy reserve.
It was originally thought that oxygen was formed as a result of the breakdown of carbon dioxide. However, Cornelius Van Niel later found out that oxygen is formed as a result of the photolysis of water. Later studies confirmed this hypothesis.
The essence of photosynthesis can be described using the following equation: 6CO 2 + 12H 2 O + light = C 6 H 12 O 6 + 6O 2 + 6H 2 O.
Breath, ours including you, – this is also a chemical phenomenon. We inhale the oxygen produced by plants and exhale carbon dioxide.
But not only carbon dioxide is formed as a result of respiration. The main thing in this process is that through breathing a large amount of energy is released, and this method of obtaining it is very effective.
In addition, the intermediate result of the different stages of respiration is a large number of different compounds. And those, in turn, serve as the basis for the synthesis of amino acids, proteins, vitamins, fats and fatty acids.
The breathing process is complex and divided into several stages. Each of them uses a large number of enzymes that act as catalysts. The scheme of chemical reactions of respiration is almost the same in animals, plants and even bacteria.
From a chemical point of view, respiration is the process of oxidation of carbohydrates (optionally: proteins, fats) with the help of oxygen; the reaction produces water, carbon dioxide and energy, which cells store in ATP: C 6 H 12 O 6 + 6 O 2 = CO 2 + 6H 2 O + 2.87 * 10 6 J.
By the way, we said above that chemical reactions can be accompanied by the emission of light. This is also true in the case of breathing and its accompanying chemical reactions. Some microorganisms can glow (luminesce). Although this reduces the energy efficiency of breathing.
Combustion also occurs with the participation of oxygen. As a result, wood (and other solid fuels) turns into ash, and this is a substance with a completely different composition and properties. In addition, the combustion process releases a large amount of heat and light, as well as gas.
Of course, not only solid substances burn; it was simply more convenient to use them to give an example in this case.
From a chemical point of view, combustion is an oxidation reaction that occurs at a very high speed. And at a very, very high reaction rate, an explosion can occur.
Schematically, the reaction can be written as follows: substance + O 2 → oxides + energy.
We consider it as a natural chemical phenomenon rotting.
Essentially, this is the same process as combustion, only it proceeds much more slowly. Rotting is the interaction of complex nitrogen-containing substances with oxygen with the participation of microorganisms. The presence of moisture is one of the factors contributing to the occurrence of rotting.
As a result of chemical reactions, ammonia, volatile fatty acids, carbon dioxide, hydroxy acids, alcohols, amines, skatole, indole, hydrogen sulfide, and mercaptans are formed from protein. Some of the nitrogen-containing compounds formed as a result of decay are poisonous.
If we turn again to our list of signs of a chemical reaction, we will find many of them in this case. In particular, there is a starting material, a reagent, and reaction products. Among the characteristic signs, we note the release of heat, gases (strong-smelling), and color change.
For the cycle of substances in nature, decay is very important: it allows the proteins of dead organisms to be processed into compounds suitable for assimilation by plants. And the circle begins again.
I'm sure you've noticed how easy it is to breathe in summer after a thunderstorm. And the air also becomes especially fresh and acquires a characteristic smell. Every time after a summer thunderstorm, you can observe another chemical phenomenon common in nature - ozone formation.
Ozone (O3) in its pure form is a blue gas. In nature, the highest concentration of ozone is in the upper layers of the atmosphere. There it acts as a shield for our planet. Which protects it from solar radiation from space and prevents the Earth from cooling, since it also absorbs its infrared radiation.
In nature, ozone is mostly formed due to air irradiation with ultraviolet rays from the Sun (3O 2 + UV light → 2O 3). And also during electrical discharges of lightning during a thunderstorm.
During a thunderstorm, under the influence of lightning, some oxygen molecules break up into atoms, molecular and atomic oxygen combine, and O 3 is formed.
That is why we feel especially fresh after a thunderstorm, we breathe easier, the air seems more transparent. The fact is that ozone is a much stronger oxidizing agent than oxygen. And in small concentrations (like after a thunderstorm) it is safe. And it’s even useful because it decomposes harmful substances in the air. Essentially disinfects it.
However, in large doses, ozone is very dangerous for people, animals and even plants; it is toxic to them.
By the way, the disinfecting properties of laboratory-obtained ozone are widely used for ozonizing water, protecting products from spoilage, in medicine and cosmetology.
Of course, this is not a complete list of amazing chemical phenomena in nature that make life on the planet so diverse and beautiful. You can learn more about them if you look around carefully and keep your ears open. There are a lot of amazing phenomena around that are just waiting for you to become interested in them.
Chemical phenomena in everyday life
These include those that can be observed in the everyday life of a modern person. Some of them are very simple and obvious, anyone can observe them in their kitchen: for example, making tea. Tea leaves heated with boiling water change their properties, and as a result the composition of the water changes: it acquires a different color, taste and properties. That is, a new substance is obtained.
If you add sugar to the same tea, the chemical reaction will result in a solution that will again have a set of new characteristics. First of all, a new, sweet taste.
Using strong (concentrated) tea leaves as an example, you can conduct another experiment yourself: clarify the tea with a slice of lemon. Due to the acids contained in lemon juice, the liquid will once again change its composition.
What other phenomena can you observe in everyday life? For example, chemical phenomena include the process combustion of fuel in the engine.
To simplify, the combustion reaction of fuel in an engine can be described as follows: oxygen + fuel = water + carbon dioxide.
In general, several reactions occur in the chamber of an internal combustion engine, which involve fuel (hydrocarbons), air and an ignition spark. More precisely, not just fuel - a fuel-air mixture of hydrocarbons, oxygen, nitrogen. Before ignition, the mixture is compressed and heated.
The combustion of the mixture occurs in a split second, eventually breaking the bond between the hydrogen and carbon atoms. This releases a large amount of energy, which drives the piston, which then moves the crankshaft.
Subsequently, hydrogen and carbon atoms combine with oxygen atoms to form water and carbon dioxide.
Ideally, the reaction of complete combustion of fuel should look like this: C n H 2n+2 + (1.5n+0,5) O 2 = nCO 2 + (n+1) H 2 O. In reality, internal combustion engines are not that efficient. Suppose that if there is a slight lack of oxygen during a reaction, CO is formed as a result of the reaction. And with a greater lack of oxygen, soot (C) is formed.
Plaque formation on metals as a result of oxidation (rust on iron, patina on copper, darkening of silver) - also from the category of household chemical phenomena.
Let's take iron as an example. Rust (oxidation) occurs under the influence of moisture (air humidity, direct contact with water). The result of this process is iron hydroxide Fe 2 O 3 (more precisely, Fe 2 O 3 * H 2 O). You may see it as a loose, rough, orange or red-brown coating on the surface of metal products.
Another example is a green coating (patina) on the surface of copper and bronze products. It is formed over time under the influence of atmospheric oxygen and humidity: 2Cu + O 2 + H 2 O + CO 2 = Cu 2 CO 5 H 2 (or CuCO 3 * Cu(OH) 2). The resulting basic copper carbonate is also found in nature - in the form of the mineral malachite.
And another example of a slow oxidation reaction of a metal in everyday conditions is the formation of a dark coating of silver sulfide Ag 2 S on the surface of silver products: jewelry, cutlery, etc.
“Responsibility” for its occurrence lies with particles of sulfur, which are present in the form of hydrogen sulfide in the air that we breathe. Silver can also darken upon contact with sulfur-containing food products (eggs, for example). The reaction looks like this: 4Ag + 2H 2 S + O 2 = 2Ag 2 S + 2H 2 O.
Let's go back to the kitchen. Here are a few more interesting chemical phenomena to consider: scale formation in the kettle one of them.
In domestic conditions there is no chemically pure water; metal salts and other substances are always dissolved in it in varying concentrations. If the water is saturated with calcium and magnesium salts (bicarbonates), it is called hard. The higher the salt concentration, the harder the water.
When such water is heated, these salts undergo decomposition into carbon dioxide and insoluble sediment (CaCO 3 andMgCO 3). You can observe these solid deposits by looking into the kettle (and also by looking at the heating elements of washing machines, dishwashers, and irons).
In addition to calcium and magnesium (which form carbonate scale), iron is also often present in water. During chemical reactions of hydrolysis and oxidation, hydroxides are formed from it.
By the way, when you are about to get rid of scale in a kettle, you can observe another example of entertaining chemistry in everyday life: ordinary table vinegar and citric acid do a good job of removing deposits. A kettle with a solution of vinegar/citric acid and water is boiled, after which the scale disappears.
And without another chemical phenomenon there would be no delicious mother’s pies and buns: we’re talking about extinguishing soda with vinegar.
When mom extinguishes baking soda in a spoon with vinegar, the following reaction occurs: NaHCO 3 + CH 3 COOH =CH 3 COONa + H 2 O + CO 2 . The resulting carbon dioxide tends to leave the dough - and thereby changes its structure, making it porous and loose.
By the way, you can tell your mom that it is not at all necessary to extinguish the soda - she will react anyway when the dough gets into the oven. The reaction, however, will be a little worse than when extinguishing soda. But at a temperature of 60 degrees (or better than 200), soda decomposes into sodium carbonate, water and the same carbon dioxide. True, the taste of ready-made pies and buns may be worse.
The list of household chemical phenomena is no less impressive than the list of such phenomena in nature. Thanks to them, we have roads (making asphalt is a chemical phenomenon), houses (brick firing), beautiful fabrics for clothing (dying). If you think about it, it becomes clearly clear how multifaceted and interesting the science of chemistry is. And how much benefit can be derived from understanding its laws.
Among the many, many phenomena invented by nature and man, there are special ones that are difficult to describe and explain. These include burning water. How is this possible, you might ask, since water doesn’t burn, it’s used to extinguish fire? How can it burn? Here's the thing.
Burning water is a chemical phenomenon, in which oxygen-hydrogen bonds are broken in water mixed with salts under the influence of radio waves. As a result, oxygen and hydrogen are formed. And, of course, it is not the water itself that burns, but hydrogen.
At the same time, it reaches a very high combustion temperature (more than one and a half thousand degrees), plus water is formed again during the reaction.
This phenomenon has long been of interest to scientists who dream of learning how to use water as fuel. For example, for cars. For now, this is something from the realm of science fiction, but who knows what scientists will be able to invent very soon. One of the main snags is that when water burns, more energy is released than is spent on the reaction.
By the way, something similar can be observed in nature. According to one theory, large single waves that seem to appear out of nowhere are actually the result of a hydrogen explosion. Electrolysis of water, which leads to it, is carried out due to the impact of electrical discharges (lightning) on the surface of salt water of the seas and oceans.
But not only in water, but also on land you can observe amazing chemical phenomena. If you had a chance to visit a natural cave, you would probably be able to see bizarre, beautiful natural “icicles” hanging from the ceiling - stalactites. How and why they appear is explained by another interesting chemical phenomenon.
A chemist, looking at a stalactite, sees, of course, not an icicle, but calcium carbonate CaCO 3. The basis for its formation is wastewater, natural limestone, and the stalactite itself is built due to the precipitation of calcium carbonate (downward growth) and the force of adhesion of atoms in the crystal lattice (broader growth).
By the way, similar formations can rise from the floor to the ceiling - they are called stalagmites. And if stalactites and stalagmites meet and grow together into solid columns, they get the name stalagnates.
Conclusion
There are many amazing, beautiful, as well as dangerous and frightening chemical phenomena happening in the world every day. People have learned to benefit from many: creating building materials, preparing food, making transport travel great distances, and much more.
Without many chemical phenomena, the existence of life on earth would not be possible: without the ozone layer, people, animals, plants would not survive due to ultraviolet rays. Without plant photosynthesis, animals and people would have nothing to breathe, and without the chemical reactions of respiration, this issue would not be relevant at all.
Fermentation allows you to cook food, and the similar chemical phenomenon of rotting decomposes proteins into simpler compounds and returns them to the cycle of substances in nature.
The formation of an oxide when copper is heated, accompanied by a bright glow, the burning of magnesium, the melting of sugar, etc. are also considered chemical phenomena. And they find useful uses.
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