First of all, it is worth saying that strict adherence to the required format and careful study of the task itself can help you a lot. In order to understand what the developers want from you, read EVERYTHING that is written in the task. Take your time! From my own experience, I can say that there is a very high probability of missing key points and requirements after reading the essay topics for the first time.

So, in this task you are offered a choice of 3 time periods that coincide with important periods in the history of Russia. Based on them, you need to write an essay, revealing in your work several events and the role of historical figures in them, the choice of which is yours. For example, you are given the following time periods: (1964-1982); (1855-1881); (1237-1242).

The peculiarity here is that you are immediately tested on your knowledge of dates and periods in history, because without this knowledge you will not be able to understand which personalities and events relate to each of the segments, since besides these dates you will have nothing else. Therefore, my first advice to you: in 95% of cases you will be offered 3 time periods exactly according to this principle, which must be memorized like a mantra:

1st period - the reign of the Rurikovichs (+ Time of Troubles)

2nd period - the reign of the Romanovs

3rd period -XX century.

And only the remaining 5% accounts for the probability that you will be given the beginning of the 21st century in one of the segments, and it is very important not to forget about this while preparing for the exam. The complexity of this task also lies in the fact that last year options were introduced in which segments are indicated with specific months, beyond the boundaries of which it is impossible to go. For example: October 1917-December 1922.

II Writing format

It is important to note here that you should not expect special attention to the use of various cliches or writing features on the part of the commission members, however, there are a number of general recommendations for each of the paragraphs. There are supposed to be 5 of them in total. Now we will analyze each of them in detail in accordance with the criteria.

Paragraph 1 should begin with the phrase: “The period being analyzed is very important in the history of Russia.” Here, very little is required of you: just indicate 2 events/processes (preferably with reference to a specific date) that fit within the framework of the period, without detailed disclosure. Result - 1 criterion is already in your pocket!

Paragraph 2 should contain a disclosure of the roles of two historical figures associated with the above events. The algorithm here is as follows: 1 person - role in 1 event; 2nd person - role in 2nd event. At this step, students very often encounter the difficulty of interpreting the very concept of “role in an event.” In fact, everything is very simple: the role of an individual in an event is mainly his specific actions (orders, command of troops, negotiations, etc.), which influenced this event and are directly related to it. For example: the role of Peter I in the Battle of Poltava was that it was he who gave orders to the troops and it was under his direct command that the Russian army defeated the Swedish one. The role itself must be disclosed in as much detail as possible so that the inspectors do not have unnecessary questions.

Paragraph 3 can be called the most difficult in terms of scoring points, and now we will figure out why. It requires you to indicate two cause-and-effect relationships within the selected period. What does it mean? You need to take 2 more new (!) events (processes, phenomena) and indicate the other 2 events that they influenced, indicating the cause-and-effect relationship. Sounds scary, doesn't it? Although in reality everything is very simple! Here is a striking example: we take the 10th Congress of the Russian Communist Party (Bolsheviks) in 1921 as an event and point out that it directly influenced the replacement of the policy of “war communism” with the policy of “NEP”. The cause-and-effect relationship is indicated, and again the point is in your pocket!

Paragraph 4 should contain a historical assessment of the significance of the period based on facts or authoritative opinion. Here you have 2 options: either you generally assess the significance of the period, “throwing” into the text the facts of its impact on other events (in this regard, you can go beyond the upper limits of the period and describe its impact on events in the future), or you memorize in advance opinions of famous historians on all periods and indicate them. In this regard, one can quote such authoritative historians as Klyuchevsky, Solovyov and Karamzin.

In paragraph 5, you conclude the entire essay with the classic “the significance of this period for the history of Russia cannot be overestimated because of...”. And score again!

Overall, following this format will provide you with 7 points out of a possible 11. Another point concerns the use of historical terms and concepts, you will receive 2 points for the absence of factual errors, and the last point for the form of presentation. Finally, the most important thing is not to get lost in a large number of facts and be sure to indicate specific dates and facts that will not cause the slightest doubt in the commission!

By following the link you can view my essay from last year, which meets all the criteria and helped me write the Unified State Examination in history for 100 points.

Methodological development “Historical essay on a given period of reign”

Explanatory note.
In 2016, the Unified State Examination in history, as we know, underwent dramatic changes: the previous tasks were complicated, new tasks were added. Particular difficulty for students is caused by tasks of high complexity with their own detailed argumentation, including an essay on a given period in the history of the Russian state. Teachers and students wondered: how to write an essay? It’s always difficult in the first year until there are any “intelligible” developments or hints. I developed an essay writing algorithm based on the evaluation criteria from the demo version. In addition, you can borrow ideas from a previous assignment - an essay about a historical figure. Below is the algorithm for an essay and a sample essay on the period of the reign of Yaroslav the Wise.

Algorithm for writing the assignment “Historical essay by period”

Evaluation criteria

K1. Events, phenomena, processes are indicated - 2 points
K2. Two historical figures and their role in events - 2 points
K3. Cause-and-effect relationships between events - 2 points
K4. Historical assessment of the period – 1 point
K5. Historical terms, concepts – 1 point
K6. Factual errors – 2 points
K7. Form, style of presentation – 1 point
K1 + K4 = must have at least 4 points for K6 and K7 to be assessed! (total – 11 points.)

This period belongs to the era...
During this period, important processes and phenomena took place (the formation of the Old Russian state, feudal fragmentation, the unification and gathering of Russian lands, the Rebellious Age, Palace coups...).
During this period, the great (outstanding, remarkable) ruler in the Russian state was ... (1st historical figure).
During this period, the great (outstanding, remarkable) statesman (politician, scientist-researcher) in the Russian state was ... (2nd historical figure).
The ruler (prince, emperor) managed to carry out many transformations that seriously influenced the life of the Russian state and society.
Due to the process/appearance, the ruler undertakes:

In domestic policy:

1. reforms of government bodies, state. and local government,
2. peasant reform,
3. education reform, encouragement of scientific research, book publishing,
4. active lawmaking, codification of laws,
5. reform and reorganization of the army,
6. socio-economic transformations, tax reform...

In foreign policy:

1. diplomacy, cultural, trade, economic relations with foreign states,
2. dynastic marriages,
3. expansion of territorial borders,
4. military campaigns, wars, repelling external aggression, foreign intervention.
The role of this ruler/figure is huge, great, indelible (ambiguous).

Activity results:

1. thanks to reforms in the field of management, a more advanced state was created. apparatus of power, reforms contributed to the strengthening and centralization of the state. authorities.
2. there was growth and economic development, economic indicators increased, regional specialization, trade development, and commodity-money relations developed.
3. a strong impetus to the development of science, art, culture, education, the opening of new educational and educational institutions.
4. Russia's international authority has increased, Russia has become one of the leading European powers.
5. The territory of the Russian state expanded significantly, new lands were acquired...

K3. Cause-and-effect relationships between events, processes, phenomena

The causes of the process (baptism, fragmentation, uprising, defeat-victory in war, revolution...) were... .
Because of... a war was started with.... The result, the consequence of the war, was...
Due to... and urgent need, the following reforms were implemented... . As a result of this, ... happened.

K4. Historical assessment of the period based on historical facts and opinions of historians

This period is important, turning point, significant in the history of the Russian state...
Famous historians (such as Solovyov, Klyuchevsky, Lomonosov, Tatishchev...) assess this reign as a complex, ambiguous period that influenced subsequent events and the entire subsequent history of the country. On the one hand, it strengthened/weakened the state, but on the other hand...

Essay on the period 1019-1054.
This period of reign refers to the heyday of the Old Russian state. The ruler of the state during these years was the outstanding Prince Yaroslav the Wise, who made a huge contribution to the development and prosperity of the Russian state. He actively carried out transformative reform activities that seriously influenced the life of the state and society.
The prince's rise to power was not easy. The civil strife between the sons and heirs of Prince Vladimir lasted for many years. In the struggle for the Kiev throne, the main rival of the Novgorod prince Yaroslav was his brother Svyatopolk, nicknamed the Damned for the cruel reprisal against his brothers Boris and Gleb. Yaroslav emerged victorious from this feud and began to rule wisely for many years.
Thus, under his leadership, the 1st set of laws was created for the first time - “Russian Truth” in 1051. The reason for the creation of the code was the need to streamline, systematize the numerous pre-existing customs and rules (some of them were quite barbaric, for example, the custom of blood feud, which was replaced with a more humane one - a fine).
In order to strengthen power, a reform of government bodies was carried out: the positions of mayor and governor were introduced. In foreign policy, Prince Yaroslav tried to expand international relations between Rus' and foreign countries, which was greatly facilitated by dynastic marriages of relatives with Western rulers. So, he married his daughters to the kings of Norway and France, and, as you know, he himself took the daughter of the king of Sweden, Ingegerda, as his wife. Thus, the international authority of Rus' increased, ties and contacts with Western powers expanded. The prince did not forget about protecting his borders and actively repelled the military threat in the form of raids by the steppe nomads, the Pechenegs. Under his leadership, the Pechenegs were completely defeated.
The growing power and authority of Rus' allowed Yaroslav to appoint the first Russian metropolitan for the first time. In 1051, the outstanding writer and activist Hilarion became Metropolitan of Kyiv. He is the author of the religious-journalistic “Tale of the Law and Grace of God.” The role of this statesman and religious figure is great. He contributed to strengthening the position of the Russian Church, spreading literacy and education in humane customs.
Yaroslav, nicknamed the Wise, was indeed an educated, versatile personality of his time. He patronized culture and education, under him Christianity spread, literacy, book publishing, and libraries grew. Beautiful churches were erected - St. Sophia Cathedrals in Kyiv and Novgorod (1037, 1045), the Kiev-Pechersk Monastery. New cities were built - Yaroslavl, Yuryev.
The role of this ruler in the history of the Old Russian state is great. Thanks to his reforms, Rus' increased its authority in the international arena. Power reforms contributed to centralization and its strengthening. This ruler gave a strong impetus to the development of culture, art, chronicle writing, and education. The foundations of Russian legislation were laid.
Many historians, such as Klyuchevsky, Soloviev, evaluate this period as a significant era in the history of our state. These years saw the heyday and power of the Old Russian state. During these years, Rus' grew stronger, acquired a reserve of spirit, perseverance, wisdom and was able to reserve strength to meet the era of fragmentation and new trials.

A lesson conducted using a notebook for practical work by I.I. Novoshinsky, N.S. Novoshinskaya for the textbook Chemistry 8th grade at the municipal educational institution “Secondary School No. 11” in Severodvinsk, Arkhangelsk region, by chemistry teacher O.A. Olkina in 8th grade (on parallel ).

Purpose of the lesson: Formation, consolidation and control of students’ skills in determining the reaction of a solution environment using various indicators, including natural ones, using a notebook for practical work by I.I. Novoshinsky, N.S. Novoshinskaya for the textbook Chemistry 8th grade.

Lesson objectives:

1. Educational. Reinforce the following concepts: indicators, medium reaction (types), pH, filtrate, filtration based on performing practical work tasks. Test students’ knowledge that reflects the relationship “solution of a substance (formula) – pH value (numerical value) – reaction of the medium.” Tell students about ways to reduce the acidity of soils in the Arkhangelsk region.
2. Developmental. To promote the development of logical thinking of students based on the analysis of results obtained during practical work, their generalization, as well as the ability to draw conclusions. Confirm the rule: practice proves or disproves theory. To continue the formation of the aesthetic qualities of the personality of students based on the diverse range of solutions presented, as well as to support the children’s interest in the subject “Chemistry” being studied.
3. Educating. Continue to develop students’ skills in performing practical work tasks, adhering to occupational health and safety rules, including correctly performing filtering and heating processes.

Practical work No. 6 “Determination of pH of the environment.”

Goal for students: Learn to determine the reaction of the environment of solutions of various objects (acids, alkalis, salts, soil solution, some solutions and juices), as well as study plant objects as natural indicators.

Equipment and reagents: rack with test tubes, stopper, glass rod, rack with ring, filter paper, scissors, chemical funnel, glasses, porcelain mortar and pestle, fine grater, clean sand, universal indicator paper, test solution, soil, boiled water, fruits, berries and other plant material, solution of sodium hydroxide and sulfuric acid, sodium chloride.

During the classes

Guys! We have already become acquainted with such concepts as the reaction of the medium of aqueous solutions, as well as indicators.

What types of reactions in aqueous solutions do you know?

• neutral, alkaline and acidic.

What are indicators?

• substances that can be used to determine the reaction of the environment.

What indicators do you know?

• in solutions: phenolphthalein, litmus, methyl orange.

• dry: universal indicator paper, litmus paper, methyl orange paper

How can you determine the reaction of aqueous solutions?

• wet and dry.

What is the pH of the environment?

• pH value of hydrogen ions in solution (pH=– log)

Let's remember which scientist introduced the concept of pH?

• Danish chemist Sorensen.

Well done!!! Now open the notebook for practical work on p. 21 and read task No. 1.

Task No. 1. Determine the pH of the solution using a universal indicator.

Let's remember the rules when working with acids and alkalis!

Complete the experiment from task No. 1.

Draw a conclusion. Thus, if a solution has pH = 7 the environment is neutral, at pH< 7 среда кислотная, при pH >7 alkaline environment.

Task No. 2. Obtain a soil solution and determine its pH using a universal indicator.

Read the task on pp. 21-p. 22, complete the task according to plan, enter the results in the table.

Let's remember the safety rules when working with heating devices (alcohol stove).

What is filtering?

• the process of separating a mixture, which is based on the different throughput of the porous material - the filtrate in relation to the particles that make up the mixture.

What is filtrate?

• it is a clear solution obtained after filtration.

Present the results in table form.

What is the reaction of the soil solution environment?

• Sour

What needs to be done to improve soil quality in our region?

• CaCO 3 + H 2 O + CO 2 = Ca(HCO 3) 2

Application of fertilizers that have an alkaline reaction environment: ground limestone and other carbonate minerals: chalk, dolomite. In the Pinezhsky district of the Arkhangelsk region there are deposits of such a mineral as limestone near karst caves, so it is accessible.

Draw a conclusion. The reaction of the resulting soil solution is pH = 4, slightly acidic, therefore, liming is necessary to improve the quality of the soil.

Task No. 3. Determine the pH of some solutions and juices using a universal indicator.

Read the task on p. 22, complete the task according to the algorithm, enter the results in the table.

Juice source		Juice source
Potato		Silicate glue
Fresh cabbage		Table vinegar
Sauerkraut		Baking soda solution
		Orange
		Fresh beets
		Boiled beets

Draw a conclusion. Thus, different natural objects have different pH values: pH 1–7 – acidic environment (lemon, cranberry, orange, tomato, beetroot, kiwi, apple, banana, tea, potatoes, sauerkraut, coffee, silicate glue).

pH 7–14 alkaline medium (fresh cabbage, baking soda solution).

pH = 7 neutral environment (persimmon, cucumber, milk).

Task No. 4. Research plant indicators.

What plant objects can act as indicators?

• berries: juices, flower petals: extracts, juices of vegetables: roots, leaves.
• substances that can change the color of a solution in different environments.

Read the task on p. 23 and complete it according to plan.

Present the results in a table.

Plant material (natural indicators)	Natural indicator solution color
	Acidic environment	Natural color of the solution (neutral environment)	Alkaline environment
Cranberry (juice)			violet
Strawberry (juice)	orange	peach-pink
Blueberry (juice)		red-violet	blue-violet
Blackcurrant (juice)		red-violet	blue-violet

Draw a conclusion. Thus, depending on the pH of the environment, natural indicators: cranberries (juice), strawberries (juice), blueberries (juice), black currants (juice) acquire the following colors: in an acidic environment - red and orange, in a neutral environment - red, peach – pink and violet colors, in an alkaline environment from pink through blue-violet to violet.

Consequently, the intensity of the color of a natural indicator can be judged by the reaction of the medium of a particular solution.

When finished, tidy up your work area.

Guys! Today was a very unusual lesson! Did you like?! Can the information learned in this lesson be used in everyday life?

Now complete the task given in your practice notebooks.

Control task. Distribute the substances whose formulas are given below into groups depending on the pH of their solutions: HCl, H 2 O, H 2 SO 4, Ca (OH) 2, NaCl, NaOH, KNO 3, H 3 PO 4, KOH.

pH 17 – environment (acidic), have solutions (HCl, H 3 PO 4, H 2 SO 4).

pH 714 environment (alkaline), have solutions (Ca(OH) 2, KOH, NaOH).

pH = 7 environment (neutral), have solutions (NaCl, H 2 O, KNO 3).

Evaluation for work_______________

Chemically, the pH of a solution can be determined using acid-base indicators.

Acid-base indicators are organic substances whose color depends on the acidity of the medium.

The most common indicators are litmus, methyl orange, and phenolphthalein. Litmus turns red in an acidic environment and blue in an alkaline environment. Phenolphthalein is colorless in an acidic environment, but turns crimson in an alkaline environment. Methyl orange turns red in an acidic environment, and yellow in an alkaline environment.

In laboratory practice, a number of indicators are often mixed, selected so that the color of the mixture changes over a wide range of pH values. With their help, you can determine the pH of a solution with an accuracy of one. These mixtures are called universal indicators.

There are special devices - pH meters, with which you can determine the pH of solutions in the range from 0 to 14 with an accuracy of 0.01 pH units.

Hydrolysis of salts

When some salts are dissolved in water, the equilibrium of the water dissociation process is disrupted and, accordingly, the pH of the environment changes. This is because salts react with water.

Hydrolysis of salts – chemical exchange interaction of dissolved salt ions with water, leading to the formation of weakly dissociating products (molecules of weak acids or bases, anions of acid salts or cations of basic salts) and accompanied by a change in the pH of the medium.

Let's consider the process of hydrolysis depending on the nature of the bases and acids that form the salt.

Salts formed by strong acids and strong bases (NaCl, kno3, Na2so4, etc.).

Let's say that when sodium chloride reacts with water, a hydrolysis reaction occurs to form an acid and a base:

NaCl + H 2 O ↔ NaOH + HCl

To get a correct idea of ​​the nature of this interaction, let us write the reaction equation in ionic form, taking into account that the only weakly dissociating compound in this system is water:

Na + + Cl - + HOH ↔ Na + + OH - + H + + Cl -

When canceling identical ions on the left and right sides of the equation, the water dissociation equation remains:

H 2 O ↔ H + + OH -

As you can see, there are no excess H + or OH - ions in the solution compared to their content in water. In addition, no other weakly dissociating or sparingly soluble compounds are formed. From this we conclude that salts formed by strong acids and bases do not undergo hydrolysis, and the reaction of solutions of these salts is the same as in water, neutral (pH = 7).

When composing ion-molecular equations for hydrolysis reactions, it is necessary:

1) write down the salt dissociation equation;

2) determine the nature of the cation and anion (find the cation of a weak base or the anion of a weak acid);

3) write down the ionic-molecular equation of the reaction, taking into account that water is a weak electrolyte and that the sum of charges should be the same on both sides of the equation.

Salts formed by a weak acid and a strong base

(Na 2 CO 3 , K 2 S,CH 3 COONa And etc. .)

Consider the hydrolysis reaction of sodium acetate. This salt in solution breaks down into ions: CH 3 COONa ↔ CH 3 COO - + Na + ;

Na + is the cation of a strong base, CH 3 COO - is the anion of a weak acid.

Na + cations cannot bind water ions, since NaOH, a strong base, completely disintegrates into ions. Anions of weak acetic acid CH 3 COO - bind hydrogen ions to form slightly dissociated acetic acid:

CH 3 COO - + HON ↔ CH 3 COOH + OH -

It can be seen that as a result of the hydrolysis of CH 3 COONa, an excess of hydroxide ions was formed in the solution, and the reaction of the medium became alkaline (pH > 7).

Thus we can conclude that salts formed by a weak acid and a strong base hydrolyze at the anion ( An n - ). In this case, the salt anions bind H ions + , and OH ions accumulate in the solution - , which causes an alkaline environment (pH>7):

An n - + HOH ↔ Han (n -1)- + OH - , (at n=1 HAn is formed – a weak acid).

Hydrolysis of salts formed by di- and tribasic weak acids and strong bases proceeds stepwise

Let's consider the hydrolysis of potassium sulfide. K 2 S dissociates in solution:

K 2 S ↔ 2K + + S 2- ;

K + is the cation of a strong base, S 2 is the anion of a weak acid.

Potassium cations do not take part in the hydrolysis reaction; only weak hydrosulfide anions interact with water. In this reaction, the first step is the formation of weakly dissociating HS - ions, and the second step is the formation of a weak acid H 2 S:

1st stage: S 2- + HOH ↔ HS - + OH - ;

2nd stage: HS - + HOH ↔ H 2 S + OH - .

The OH ions formed in the first stage of hydrolysis significantly reduce the likelihood of hydrolysis in the next stage. As a result, a process that occurs only in the first stage is usually of practical importance, which, as a rule, is limited to when assessing the hydrolysis of salts under normal conditions.

Remember:

A neutralization reaction is a reaction between an acid and a base that produces salt and water;

By pure water, chemists understand chemically pure water that does not contain any impurities or dissolved salts, i.e. distilled water.

Acidity of the environment

For various chemical, industrial and biological processes, a very important characteristic is the acidity of solutions, which characterizes the content of acids or alkalis in solutions. Since acids and alkalis are electrolytes, the content of H+ or OH - ions is used to characterize the acidity of the medium.

In pure water and in any solution, along with particles of dissolved substances, H+ and OH - ions are also present. This occurs due to the dissociation of the water itself. And although we consider water to be a non-electrolyte, it can nevertheless dissociate: H 2 O ^ H+ + OH - . But this process occurs to a very small extent: in 1 liter of water only 1 ion breaks down into ions. 10 -7 mol molecules.

In acid solutions, as a result of their dissociation, additional H+ ions appear. In such solutions there are significantly more H+ ions than OH - ions formed during slight dissociation of water, therefore these solutions are called acidic (Fig. 11.1, left). It is commonly said that such solutions have an acidic environment. The more H+ ions contained in the solution, the more acidic the medium.

In alkali solutions, as a result of dissociation, on the contrary, OH - ions predominate, and H + cations are almost absent due to the insignificant dissociation of water. The environment of such solutions is alkaline (Fig. 11.1, right). The higher the concentration of OH - ions, the more alkaline the solution environment is.

In a solution of table salt, the number of H+ and OH ions is the same and equal to 1. 10 -7 mol in 1 liter of solution. Such a medium is called neutral (Fig. 11.1, center). In fact, this means that the solution contains neither acid nor alkali. A neutral environment is characteristic of solutions of some salts (formed by alkali and strong acid) and many organic substances. Pure water also has a neutral environment.

pH value

If we compare the taste of kefir and lemon juice, we can safely say that lemon juice is much more acidic, i.e. the acidity of these solutions is different. You already know that pure water also contains H+ ions, but the sour taste of the water is not felt. This is due to the too low concentration of H+ ions. Often it is not enough to say that a medium is acidic or alkaline, but it is necessary to characterize it quantitatively.

The acidity of the environment is quantitatively characterized by the hydrogen indicator pH (pronounced “p-ash”), associated with the concentration

Hydrogen ions. The pH value corresponds to a certain content of Hydrogen cations in 1 liter of solution. Pure water and neutral solutions contain 1 liter in 1 liter. 10 7 mol of H+ ions, and the pH value is 7. In acid solutions, the concentration of H+ cations is greater than in pure water, and in alkaline solutions it is less. In accordance with this, the value of the pH value changes: in an acidic environment it ranges from 0 to 7, and in an alkaline environment it ranges from 7 to 14. The Danish chemist Peder Sørensen first proposed using the pH value.

You may have noticed that the pH value is related to the concentration of H+ ions. Determining pH is directly related to calculating the logarithm of a number, which you will study in 11th grade math classes. But the relationship between the content of ions in the solution and the pH value can be traced according to the following scheme:

The pH value of aqueous solutions of most substances and natural solutions is in the range from 1 to 13 (Fig. 11.2).

Rice. 11.2. pH value of various natural and artificial solutions

Søren Peder Laurits Sørensen

Danish physical chemist and biochemist, President of the Royal Danish Society. Graduated from the University of Copenhagen. At the age of 31 he became a professor at the Danish Polytechnic Institute. He headed the prestigious physicochemical laboratory at the Carlsberg brewery in Copenhagen, where he made his main scientific discoveries. His main scientific activity was devoted to the theory of solutions: he introduced the concept of pH value and studied the dependence of enzyme activity on the acidity of solutions. For his scientific achievements, Sørensen was included in the list of “100 outstanding chemists of the 20th century,” but in the history of science he remained primarily as the scientist who introduced the concepts of “pH” and “pH-metry.”

Determination of medium acidity

To determine the acidity of a solution in laboratories, a universal indicator is most often used (Fig. 11.3). By its color, you can determine not only the presence of acid or alkali, but also the pH value of the solution with an accuracy of 0.5. To more accurately measure pH, there are special devices - pH meters (Fig. 11.4). They allow you to determine the pH of a solution with an accuracy of 0.001-0.01.

Using indicators or pH meters, you can monitor how chemical reactions are progressing. For example, if chloride acid is added to a solution of sodium hydroxide, a neutralization reaction will occur:

Rice. 11.3. A universal indicator determines the approximate pH value

Rice. 11.4. To measure the pH of solutions, special devices are used - pH meters: a - laboratory (stationary); b - portable

In this case, solutions of reagents and reaction products are colorless. If a pH meter electrode is placed in the initial alkali solution, then the complete neutralization of the alkali by the acid can be judged by the pH value of the resulting solution.

Application of pH indicator

Determining the acidity of solutions is of great practical importance in many areas of science, industry and other areas of human life.

Ecologists regularly measure the pH of rainwater, rivers and lakes. A sharp increase in the acidity of natural waters may be a consequence of atmospheric pollution or the entry of industrial waste into water bodies (Fig. 11.5). Such changes entail the death of plants, fish and other inhabitants of water bodies.

The hydrogen index is very important for studying and observing processes occurring in living organisms, since numerous chemical reactions take place in cells. In clinical diagnostics, the pH of blood plasma, urine, gastric juice, etc. is determined (Fig. 11.6). Normal blood pH is between 7.35 and 7.45. Even a small change in the pH of human blood causes serious illness, and at pH = 7.1 and below, irreversible changes begin that can lead to death.

For most plants, soil acidity is important, so agronomists conduct soil analyzes in advance, determining their pH (Fig. 11.7). If the acidity is too high for a particular crop, the soil is limed by adding chalk or lime.

In the food industry, acid-base indicators are used to control the quality of food products (Fig. 11.8). For example, the normal pH for milk is 6.8. Deviation from this value indicates either the presence of foreign impurities or its souring.

Rice. 11.5. The influence of the pH level of water in reservoirs on the vital activity of plants in them

The pH value for cosmetics that we use in everyday life is important. The average pH for human skin is 5.5. If the skin comes into contact with products whose acidity differs significantly from this value, this will lead to premature skin aging, damage or inflammation. It was noticed that laundresses who used ordinary laundry soap (pH = 8-10) or washing soda (Na 2 CO 3, pH = 12-13) for a long time for washing, the skin of their hands became very dry and covered with cracks. Therefore, it is very important to use various cosmetics (gels, creams, shampoos, etc.) with a pH close to the natural pH of the skin.

LABORATORY EXPERIMENTS No. 1-3

Equipment: rack with test tubes, pipette.

Reagents: water, chloride acid, NaCl, NaOH solutions, table vinegar, universal indicator (solution or indicator paper), food and cosmetic products (for example, lemon, shampoo, toothpaste, washing powder, carbonated drinks, juices, etc. .).

Safety regulations:

For experiments, use small amounts of reagents;

Be careful not to get reagents on your skin or eyes; If a caustic substance gets in, wash it off with plenty of water.

Determination of Hydrogen ions and hydroxide ions in solutions. Establishing the approximate pH value of water, alkaline and acidic solutions

1. Pour 1-2 ml into five test tubes: into test tube No. 1 - water, No. 2 - chloride acid, No. 3 - sodium chloride solution, No. 4 - sodium hydroxide solution and No. 5 - table vinegar.

2. Add 2-3 drops of a universal indicator solution to each test tube or lower the indicator paper. Determine the pH of solutions by comparing the color of the indicator on a standard scale. Draw conclusions about the presence of Hydrogen cations or hydroxide ions in each test tube. Write dissociation equations for these compounds.

Study of pH of food and cosmetic products

Test samples of food and cosmetic products with a universal indicator. To study dry substances, for example, washing powder, they must be dissolved in a small amount of water (1 spatula of dry substance per 0.5-1 ml of water). Determine the pH of solutions. Draw conclusions about the acidity of the environment in each of the studied products.

Key idea

Control questions

130. The presence of what ions in a solution determines its acidity?

131. What ions are found in excess in acid solutions? in alkaline?

132. What indicator quantitatively describes the acidity of solutions?

133. What is the pH value and the content of H+ ions in solutions: a) neutral; b) weakly acidic; c) slightly alkaline; d) strongly acidic; d) highly alkaline?

Assignments for mastering the material

134. An aqueous solution of a certain substance has an alkaline medium. Which ions are more present in this solution: H+ or OH -?

135. Two test tubes contain solutions of nitrate acid and potassium nitrate. What indicators can be used to determine which test tube contains a salt solution?

136. Three test tubes contain solutions of barium hydroxide, nitrate acid and calcium nitrate. How to recognize these solutions using one reagent?

137. From the list above, write down separately the formulas of substances whose solutions have a medium: a) acidic; b) alkaline; c) neutral. NaCl, HCl, NaOH, HNO 3, H 3 PO 4, H 2 SO 4, Ba(OH) 2, H 2 S, KNO 3.

138. Rain water has pH = 5.6. What does this mean? What substance contained in the air, when dissolved in water, determines the acidity of the environment?

139. What kind of environment (acidic or alkaline): a) in a shampoo solution (pH = 5.5);

b) in the blood of a healthy person (pH = 7.4); c) in human gastric juice (pH = 1.5); d) in saliva (pH = 7.0)?

140. Coal used in thermal power plants contains compounds of Nitrogen and Sulfur. The release of coal combustion products into the atmosphere leads to the formation of so-called acid rain containing small amounts of nitrate or sulfite acids. What pH values ​​are typical for such rainwater: more than 7 or less than 7?

141. Does the pH of a solution of a strong acid depend on its concentration? Justify your answer.

142. A solution of phenolphthalein was added to a solution containing 1 mol of potassium hydroxide. Will the color of this solution change if chloride acid is added to it in the amount of substance: a) 0.5 mol; b) 1 mol;

c) 1.5 mol?

143. Three unlabeled test tubes contain colorless solutions of sodium sulfate, sodium hydroxide and sulfate acid. The pH value was measured for all solutions: in the first test tube - 2.3, in the second - 12.6, in the third - 6.9. Which test tube contains which substance?

144. The student bought distilled water at the pharmacy. The pH meter showed that the pH value of this water was 6.0. The student then boiled this water for a long time, filled the container to the top with hot water and closed the lid. When the water cooled to room temperature, the pH meter detected a value of 7.0. After this, the student passed air through the water with a straw, and the pH meter again showed 6.0. How can the results of these pH measurements be explained?

145. Why do you think two bottles of vinegar from the same manufacturer may contain solutions with slightly different pH values?

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