How to distinguish between physical and chemical phenomena. Substances and their properties
I bet you've noticed more than once something like Mom's silver ring darkens with time. Or how a nail rusts. Or how wooden logs burn to ash. Well, okay, if mom doesn’t like silver, and you never went hiking, you saw exactly how a tea bag is brewed in a cup.
What do all these examples have in common? And the fact that they are all 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 remember physics, then remember that chemical phenomena occur at the molecular and atomic level, but do not affect the composition of the nuclei of atoms.
From the point of view of chemistry, this is nothing more than a chemical reaction. And for each chemical reaction, it is necessarily possible to identify characteristic features:
- a precipitate may form during the reaction;
- the color of the substance may change;
- the consequence of the reaction may be the evolution 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 defined:
- contact: To react, the substances must come into contact.
- grinding: for the successful course of the reaction, the substances entering into it must be crushed as finely as possible, ideally - dissolved;
- temperature: very many reactions directly depend on the temperature of substances (most often they need to be heated, but some vice versa - cooled to a certain temperature).
By writing down 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 in compiling such descriptions.
Chemical phenomena in nature
Of course, you understand that chemistry does not only take place in test tubes in the school laboratory. The most impressive chemical phenomena you can observe in nature. And their significance is so great that there would be no life on earth if it were not for some of the natural chemical phenomena.
So, first of all, let's talk about photosynthesis. This is the process during which plants absorb carbon dioxide from the atmosphere and under the influence of sunlight produce oxygen. We breathe this oxygen.
In general, photosynthesis proceeds in two phases, and lighting is needed for only one. Scientists conducted various experiments and found that photosynthesis proceeds even in low light. But with an increase in the amount of light, the process is greatly accelerated. It has also been observed that if the plant's light and temperature are both increased at the same time, 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 pigment molecules of plants - chlorophyll. In plant cells, it is found in chloroplasts, which is what makes the leaves green.
From a chemical point of view, photosynthesis is a chain of transformations that results in oxygen, water, and carbohydrates as an energy store.
Initially, it was believed that oxygen was formed as a result of the splitting carbon dioxide. Later, however, Cornelius Van Niel found out that oxygen is formed as a result of the photolysis of water. Recent studies have confirmed this hypothesis.
The essence of photosynthesis can be described using the following equation: 6CO 2 + 12H 2 O + light \u003d C 6 H 12 O 6 + 6O 2 + 6H 2 O.
Breath, our with you in including, – it 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 due to 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 breathing is big number various 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 which uses a large number of enzymes that act as catalysts. Scheme chemical reactions respiration is almost the same in animals, plants and even bacteria.
From the point of view of chemistry, respiration is the process of oxidation of carbohydrates (as an option: proteins, fats) with the help of oxygen, as a result of the reaction, water, carbon dioxide and energy are obtained that cells store in ATP: C 6 H 12 O 6 + 6O 2 \u003d 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. In the case of breathing and the chemical reactions that go with it, this is also true. Glow (luminesce) can some microorganisms. Although the energy efficiency of breathing decreases.
Combustion also occurs with the participation of oxygen. As a result, wood (and other solid fuels) turns into ash, a substance with a completely different composition and properties. In addition, during the combustion process, a large amount of heat and light, as well as gas, is released.
Of course, not only solid substances burn, but with their help it was more convenient to give an example in this case.
From a chemical point of view, combustion is an oxidative reaction that proceeds 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.
As a natural chemical phenomenon, we consider and decay.
In fact, this is the same process as combustion, only it proceeds much more slowly. Decay 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 decay.
As a result of chemical reactions, ammonia, volatile fatty acids, carbon dioxide, hydroxy acids, alcohols, amines, skatole, indole, hydrogen sulfide, 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 as well. In particular, there is an initial substance, a reagent, reaction products. From characteristic features note the release of heat, gases (strong-smelling), color change.
For the circulation of substances in nature, decay has a very great importance: allows you to process the proteins of dead organisms into compounds suitable for absorption by plants. And the circle starts over.
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 (O 3) in its pure form is a gas of blue color. In nature, the highest concentration of ozone is in the upper atmosphere. There he acts as a shield for our planet. Which protects it from solar radiation from space and does not allow the Earth to cool down, since it also absorbs its infrared radiation.
In nature, ozone is mostly formed due to the irradiation of air with the ultraviolet rays of the Sun (3O 2 + UV light → 2O 3). And also with electrical discharges of lightning during a thunderstorm.
In a thunderstorm, under the influence of lightning, part of the oxygen molecules breaks up into atoms, molecular and atomic oxygen combine, and O 3 is formed.
That is why we feel a special freshness 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 a small concentration (as after a thunderstorm) is safe. And even useful, because it decomposes harmful substances in the air. In fact, it disinfects it.
However, in large doses, ozone is very dangerous for people, animals and even plants, for them it is poisonous.
By the way, the disinfecting properties of ozone obtained in the laboratory are widely used for ozonizing water, protecting products from spoilage, in medicine and cosmetology.
Of course, this is far from full list 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 seen in Everyday life modern man. Some of them are quite simple and obvious, anyone can observe them in their kitchen: for example, brewing tea. The tea leaves heated with boiling water change their properties, as a result, the composition of water also changes: it acquires a different color, taste and properties. That is, a new substance is obtained.
If sugar is poured into the same tea, as a result of a chemical reaction, a solution will be obtained, which will again have a set of new characteristics. First of all, new, sweet, taste.
Using the example of strong (concentrated) tea brewing, you can independently conduct another experiment: lighten 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 fuel combustion in the engine.
To simplify, the reaction of fuel combustion in the engine can be described as follows: oxygen + fuel = water + carbon dioxide.
In general, several reactions take place in the chamber of an internal combustion engine, in which fuel (hydrocarbons), air and an ignition spark are involved. Or rather, 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 fraction of a second, as a result, the bond between the hydrogen and carbon atoms is destroyed. Due to this, a large amount of energy is released, which sets the piston in motion, and that - the crankshaft.
Subsequently, hydrogen and carbon atoms combine with oxygen atoms, water and carbon dioxide are formed.
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 oxygen is not enough during the reaction, CO is formed as a result of the reaction. And with a greater lack of oxygen, soot is formed (C).
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. Rusting (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 reddish-brown coating on the surface of metal products.
Another example is the green coating (patina) on the surface of copper and bronze items. It is formed over time under the influence of atmospheric oxygen and humidity: 2Cu + O 2 + H 2 O + CO 2 \u003d 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 oxidative reaction of a metal in domestic conditions is the formation of a dark coating of silver sulfide Ag 2 S on the surface of silver items: jewelry, cutlery, etc.
The “responsibility” for its occurrence is borne by sulfur particles, which are present in the form of hydrogen sulfide in the air that we breathe. Silver can also darken upon contact with sulfur-containing foods (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 you can consider a few more interesting chemical phenomena: 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 various concentrations. If the water is saturated with calcium and magnesium salts (hydrocarbonates), 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 an insoluble precipitate (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 (from which carbonate scale is formed), iron is also often present in water. During the chemical reactions of hydrolysis and oxidation, hydroxides are formed from it.
By the way, when you are about to get rid of scale in the kettle, you can observe another example of entertaining chemistry in everyday life: ordinary table vinegar and citric acid do well with 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 are talking about extinguishing soda with vinegar.
When mom extinguishes 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, makes it porous and loose.
By the way, you can tell your mother 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 go a little worse than when soda is quenched. But at a temperature of 60 degrees (and preferably 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 (asphalt making is a chemical phenomenon), houses (brick firing), beautiful fabrics for clothes (dyeing). 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. They also include burning water. How can this be, you ask, because water does not burn, it extinguishes fire? How can she burn? And here's the thing.
The burning of water is a chemical phenomenon, at which oxygen-hydrogen bonds are broken in water with an admixture of salts under the influence of radio waves. The result is oxygen and hydrogen. 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 again formed during the reaction.
This phenomenon has long been of interest to scientists who dream of learning how to use water as a fuel. For example, for cars. So far, this is something from the realm of fantasy, 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, appearing as if from nowhere, are actually the result of a hydrogen explosion. The electrolysis of water, which leads to it, is carried out due to the ingress of electrical discharges (lightning) on the surface of the salt water of the seas and oceans.
But not only in water, but also on land, one can observe amazing chemical phenomena. If you had a chance to visit a natural cave, you would surely 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 are wastewater, natural limestone, and the stalactite itself is built due to the precipitation of calcium carbonate (growth down) and the adhesion force of atoms in the crystal lattice (growth in breadth).
By the way, similar formations can rise from the floor to the ceiling - they are called stalagmites. And if stalactites and stalagmites meet and coalesce into solid columns, they get a name stalagnates.
Conclusion
Many amazing, beautiful, as well as dangerous and frightening chemical phenomena occur in the world every day. From many, people have learned to benefit: they create building materials, cook food, make vehicles travel long 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 makes it possible to cook food, and the similar chemical phenomenon of putrefaction decomposes proteins into more simple connections and returns those to the cycle of substances in nature.
The formation of 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 find them a useful use.
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Physical and chemical phenomena
By conducting experiments and observations, we are convinced that substances can change.
Changes in substances that do not lead to the formation of new substances (with different properties) are called physical phenomena.
1. Water when heated, it can turn into steam, and when cooled - into the ice .
2.Length of copper wires changes in summer and winter: increases with heating and decreases with cooling.
3.Volume the air in the balloon increases in a warm room.
Changes occurred with substances, but at the same time water remained water, copper - copper, air - air.
New substances, despite their changes, were not formed.
An experience
1. We close the test tube with a stopper with a tube inserted into it
2. Dip the end of the tube into a glass of water. Heat the test tube by hand. The volume of air in it increases, and part of the air from the test tube goes into a glass of water (air bubbles are released).
3. As the tube cools, the volume of air decreases and water enters the tube.
Conclusion. Air volume changes are a physical phenomenon.
Tasks
Give 1-2 examples of changes occurring with substances that can be called a physical phenomenon. Write examples in your notebook.
Chemical phenomenon (reaction) - a phenomenon in which new substances are formed.
How can you tell what happened chemical reaction ? Precipitation occurs during some chemical reactions. Other signs are a change in the color of the original substance, a change in its taste, the release of gas, the release or absorption of heat and light.
See the table for examples of such reactions.
Signs of chemical reactions |
||||
Changing the color of the original substance |
Changing the taste of the original substance |
Precipitation |
Gas evolution |
The appearance of an odor |
Reaction |
sign |
|
Color change |
Taste change |
|
Gas evolution |
Various chemical reactions are constantly taking place in animate and inanimate nature. Our organism is also a real factory of chemical transformations of some substances into others.
Let's observe some chemical reactions.
Experiments with fire cannot be carried out on your own !!!
Experience 1
Hold over the fire a piece of white bread containing organic matter.
Watching:
1. charring, that is, a change in color;
2. the appearance of an odor.
Conclusion . A chemical phenomenon occurred (a new substance was formed - coal)
Experience 2
Prepare a glass of starch. Add some water, mix. Then a drop of iodine solution.
We observe a sign of the reaction: a change in color (blue starch)
Conclusion. A chemical reaction has taken place. The starch has been transformed into another substance.
Experience 3
1. Dilute a small amount of baking soda in a glass.
2. Add a few drops of vinegar there (you can take lemon juice or a solution of citric acid).
Observe the release of gas bubbles.
Conclusion. Gas evolution is one of the signs of a chemical reaction.
Some chemical reactions are accompanied by the release of heat.
Tasks
Place a few pieces of raw potatoes in a glass jar (or glass). Add hydrogen peroxide from your first aid kit to them. Explain how you can tell that a chemical reaction has taken place.
Often, from many people who discuss a particular process, you can hear the words: "This is physics!" or Indeed, almost all phenomena in nature, in everyday life and in space, which a person encounters during his life, can be attributed to one of these sciences. It is interesting to understand how physical phenomena differ from chemical ones.
science physics
Before answering the question of how physical phenomena differ from chemical ones, it is necessary to understand what objects and processes each of these sciences investigates. Let's start with physics.
From the ancient Greek language the word "fisis" is translated as "nature". That is, physics is the science of nature, which studies the properties of objects, their behavior under various conditions, transformations between their states. The purpose of physics is to determine the laws that govern what happens. natural processes. For this science, it does not matter what the object under study consists of, and what its chemical composition is, for it it is only important how the object will behave if it is affected by heat, mechanical force, pressure, and so on.
Physics is divided into a number of sections that study a certain narrower range of phenomena, for example, optics, mechanics, thermodynamics, atomic physics, and so on. In addition, many independent sciences depend entirely on physics, such as astronomy or geology.
Unlike physics, chemistry is a science that studies the structure, composition and properties of matter, as well as its change as a result of chemical reactions. That is, the object of study of chemistry is the chemical composition and its change during a certain process.
Chemistry, like physics, has many sections, each of which studies a certain class chemical substances, for example, organic and inorganic, bio- and electrochemistry. Research in medicine, biology, geology and even astronomy is based on the achievements of this science.
It is interesting to note that chemistry, as a science, was not recognized by ancient Greek philosophers because of its focus on experiment, as well as because of the pseudoscientific knowledge that surrounded it (recall that modern chemistry was "born" from alchemy). Only since the Renaissance, and largely thanks to the work of the English chemist, physicist and philosopher Robert Boyle, chemistry began to be perceived as a full-fledged science.
Examples of physical phenomena
Numerous examples can be cited that are subject to physical laws. For example, every student knows already in the 5th grade a physical phenomenon - the movement of a car along the road. At the same time, it does not matter what this car consists of, where it takes energy from to move, the only important thing is that it moves in space (along the road) along a certain trajectory at a certain speed. Moreover, the processes of acceleration and deceleration of the car are also physical. Vehicle traffic and other solids deals with the section of physics "Mechanics".
Another well-known is the melting of ice. Ice, being a solid state of water, at atmospheric pressure can exist for an arbitrarily long time at temperatures below 0 o C, but if the ambient temperature is increased by at least a fraction of a degree, or if heat is directly transferred to ice, for example, by taking it in hand, then it will start to melt. This process, which goes with the absorption of heat and a change in the aggregate state of matter, is an exclusively physical phenomenon.
Other examples of physical phenomena are the floating of bodies in liquids, the rotation of planets in their orbits, electromagnetic radiation bodies, the refraction of light when passing through the boundary of two different transparent media, the flight of a projectile, the dissolution of sugar in water, and others.
Examples of chemical phenomena
As mentioned above, any processes that occur with a change in the chemical composition of the bodies participating in them are studied by chemistry. If we return to the example of a car, we can say that the process of burning fuel in its engine is a vivid example of a chemical phenomenon, since as a result of it, hydrocarbons, interacting with oxygen, lead to the formation of completely different main ones of which are water and carbon dioxide.
To another one of clear examples considered class of phenomena is the process of photosynthesis in green plants. Initially, they have water, carbon dioxide and sunlight, but after the completion of photosynthesis, the initial reagents are no longer there, and glucose and oxygen are formed in their place.
In the general case, we can say that any living organism is a real chemical reactor, since it undergoes great amount transformational processes, for example, the breakdown of amino acids and the formation of new proteins from them, the conversion of hydrocarbons into energy for muscle fibers, the process of human respiration, in which hemoglobin binds oxygen, and many others.
One of the amazing examples of chemical phenomena in nature is the cold glow of fireflies, which is the result of the oxidation of a special substance - luciferin.
In the technical field, an example is the manufacture of dyes for clothing and food.
Differences
How are physical phenomena different from chemical ones? The answer to this question can be understood if we analyze the above information about the objects of study of physics and chemistry. The main difference between them is the change in the chemical composition of the object under consideration, the presence of which indicates transformations in it, while in the case of unchanged chemical properties bodies speak of a physical phenomenon. It is important not to confuse the change in chemical composition and a change in structure, which refers to the spatial arrangement of the atoms and molecules that form bodies.
Reversibility of physical and irreversibility of chemical phenomena
In some sources, when answering the question of how physical phenomena differ from chemical ones, one can find information that physical phenomena are reversible, while chemical ones are not, however, this is not entirely true.
The direction of any process can be determined using the laws of thermodynamics. These laws say that any process can go on spontaneously only in the case of a decrease in its Gibbs energy (a decrease in internal energy and an increase in entropy). However, this process can always be reversed if an external energy source is used. For example, let's say that scientists recently discovered the reverse process of photosynthesis, which is a chemical phenomenon.
This question was specifically placed in a separate paragraph, since many people consider combustion to be a chemical phenomenon, but this is not true. However, it would also be wrong to consider the combustion process as a physical phenomenon.
A common combustion phenomenon (bonfire, combustion of fuel in an engine, gas burner or burner, etc.) is a complex physical and chemical process. On the one hand, it is described by a chain of chemical oxidation reactions, but on the other hand, as a result of this process, strong thermal and light electromagnetic radiation occurs, and this is already the field of physics.
Where is the boundary between physics and chemistry?
Physics and chemistry are two different sciences that have various methods research, while physics can be both theoretical and practical, while chemistry is mainly a practical science. However, in some areas, these sciences are so close that the boundary between them blurs. The following are examples of scientific fields in which it is difficult to determine "where is physics and where is chemistry":
- quantum mechanics;
- nuclear physics;
- crystallography;
- Materials Science;
- nanotechnology.
As can be seen from the list, physics and chemistry closely intersect when the phenomena under consideration are on an atomic scale. Such processes are usually called physicochemical. It is curious to note that the only person who received Nobel Prize in chemistry and physics at the same time, is Maria Sklodowska-Curie.
>> Physical and chemical phenomena (chemical reactions). Experimenting at home. External effects in chemical reactions
Physical and chemical phenomena (chemical reactions)
The material of the paragraph will help you find out:
> what is the difference between physical and chemical phenomena.(chemical reactions);
> what external effects accompany chemical reactions.
In the lessons of natural history, 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, therefore they are one substance (in different states of aggregation).
Phenomena in which a substance does not change into another is called physical.
Physical phenomena include not only the change of substances, but also the glow of incandescent bodies, the passage electric current in metals, the spread of the smell of substances in the air, the dissolution of fat in gasoline, 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. Consider the process of burning alcohol (Fig. 46). It occurs with the participation of oxygen, which is contained in the air. Burning, alcohol, it would seem, passes into a gaseous state, just as water turns into steam when heated. Ho it's not. If the gas obtained as a result of the combustion of alcohol is cooled, then part of it will condense into a liquid, but not into alcohol, but into water. The rest of the gas will remain. With the help of additional experience, it can be proved that this residue is carbon dioxide.
Rice. 46. Burning alcohol
Thus the alcohol that burns, and oxygen, which is involved in the combustion process, are converted into water and carbon dioxide.
Phenomena in which one substance is converted into another, called chemical phenomena, or chemical reactions.
Substances that enter into a chemical reaction are called initial substances, or reagents, and those that are formed are called final substances, or reaction products.
The essence of the considered chemical reaction is conveyed by the following record:
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, it creates heat. Under these conditions, the molecules of the reagents break down into atoms, which, when combined, form the 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 the interaction of substances are also known.
External effects accompanying chemical reactions.
By observing chemical reactions, you can fix the following effects:
Color change (Fig. 47, a);
gas release (Fig. 47, b);
the formation or disappearance of sediment (Fig. 47, c);
the appearance, disappearance or change of smell;
release or absorption of heat;
the appearance of a flame (Fig. 46), sometimes a glow.
Rice. 47. Some external effects in chemical reactions: a - the appearance
coloring; b - gas evolution; c - the appearance of sediment
Lab experience #3
The appearance of color as a result of the reaction
Are soda ash and phenolphthalein solutions colored?
Add 2 drops of phenolphthalein solution to a portion of the I-2 soda solution. What color appeared?
Laboratory experiment No. 4
Emission of gas as a result of the reaction
Add a little hydrochloric acid to the soda ash solution. What are you watching?
Laboratory experiment No. 5
The appearance of a precipitate as a result of the reaction
To a solution of soda ash, add 1 ml of solution blue vitriol. What's happening?
The appearance of a flame is a sign of a chemical reaction, that is, it indicates precisely a chemical phenomenon. Other external effects can also be observed during physical phenomena. Let's give some examples.
Example 1 Silver powder obtained in a test tube as a result of a chemical reaction has a gray color. If it is melted and then the melt is cooled, we get a piece of metal, but not gray, but white, with a characteristic luster.
Example 2 If heated natural water, then gas bubbles will begin to be released from it long before boiling. It is dissolved air; its solubility in water decreases when heated.
Example 3. An unpleasant smell in the refrigerator disappears if silica gel granules, one of the Silicium compounds, are placed in it. Silica gel absorbs molecules of various substances without destroying them. Activated charcoal in a gas mask works similarly.
Example 4 . When water turns into steam, heat is absorbed, and when water freezes, heat is released.
To determine whether a transformation has taken place - physical or chemical, one 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 are constantly taking place in nature. Substances dissolved in rivers, seas, oceans interact with each other, some react with oxygen. Plants absorb carbon dioxide from the atmosphere, from the soil - water, substances dissolved in it and process them into proteins, fats, glucose, starch, vitamins, other compounds, as well as oxygen.
It is interesting
As a result of photosynthesis, about 300 billion tons of carbon dioxide are absorbed from the atmosphere annually, 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 the roasting of meat, vegetables, baking bread, souring of milk, fermentation of grape juice, bleaching of fabrics, burning various kinds fuel, hardening of cement and alabaster, blackening of silver jewelry over time, etc.
Chemical reactions form the basis of such technological processes as the production of metals from ores, the production of fertilizers, plastics, synthetic fibers, drugs, and other important substances. By burning fuel, people provide themselves with heat and electricity. With the help of chemical reactions, toxic substances are neutralized, industrial and domestic waste is processed.
Certain reactions lead to negative consequences. The rusting of iron reduces the life of various mechanisms, equipment, vehicles, and leads to large losses of this metal. Fires destroy housing, industrial and cultural facilities, 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 phenomena in which each substance is preserved.
Chemical phenomena, or chemical reactions, are the transformations of one substance into another. They can be accompanied by various external effects.
Many chemical reactions take place in environment, in plants, animal and human organisms, accompany us in everyday life.
?
100. Find a match:
1) dynamite explosion; a) a physical phenomenon;
2) solidification of molten paraffin; b) a chemical phenomenon.
3) burning food in a pan;
4) the formation of salt during the evaporation of sea water;
5) separation of a highly agitated mixture of water and vegetable oil;
6) fading of dyed fabric in the sun;
7) the passage of electric current in the metal;
101. What external effects are accompanied by such chemical transformations: a) burning 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) spoil quickly?
Experimenting at home
External effects in chemical reactions
1. Prepare small amounts of aqueous solutions of citric acid and baking soda. Pour together portions of both solutions into a separate beaker. What's happening?
Add some soda crystals to the rest of the citric acid solution, and a few citric acid crystals to the rest 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 an alcohol solution of brilliant green, known as greenery, to each. Add a few drops of ammonia to the first glass, and a solution of citric acid to the second. Has the color of the dye (brilliant green) changed in these glasses? If yes, how exactly?
Record the results of the experiments in a notebook and draw conclusions.
Popel P. P., Kriklya L. S., Chemistry: Pdruch. for 7 cells. zahalnosvit. navch. zakl. - K .: Exhibition Center "Academy", 2008. - 136 p.: il.
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