Substances that do not dissolve in water. Lesson "The ability of water to dissolve solids (salt, sugar, etc.)
Amanbayeva Zhanar Zhumabekovna
Aktobe region Shalkar
Secondary School No. 5
Subject: Primary school
Subject: Water is a solvent. Substances soluble and insoluble in water.
Lesson objectives: to give an idea about water as a solvent, about soluble and insoluble substances; introduce the concept of "filter", with the simplest ways to determine soluble and insoluble substances; prepare a report on the topic “Water is a solvent”.
Equipment and visual aids: textbooks, readers, notebooks for independent work; sets: glasses empty and with boiled water; boxes with table salt, sugar, river sand, clay; teaspoons, funnels, paper napkin filters; gouache (watercolors), brushes and sheets for reflection; presentation made in Power Point, multimedia projector, screen.
DURING THE CLASSES
I. Organizational moment
U. Good morning everyone! (Slide 1)
I invite you to the third meeting of the school science club "We and the world around us."
II. Message about the topic and purpose of the lesson
Teacher. Today we have guests, teachers from other schools who came to the meeting of the club. I propose to the chairman of the club, Poroshina Anastasia, to open the meeting.
Chairman. Today we have gathered for a club meeting on the topic “Water is a solvent”. The task for all those present is to prepare a report on the topic “Water is a solvent”. In this lesson, you will again become researchers of the properties of water. You will study these properties in your laboratories, with the help of "consultants" - Mikhail Makarenkov, Olesya Starkova and Yulia Stenina. Each laboratory will have to perform the following task: to conduct experiments and observations, and at the end of the meeting, discuss the plan for the message "Water - solvent".
III. Learning new material
U. With the permission of the chairman, I would like to make the first announcement. (Slide 2) The same session on the topic “Water is a solvent” was recently held by students from the village of Mirny. The meeting was opened by Kostya Pogodin, who reminded everyone present of another amazing property of water: many substances in water can break down into invisible tiny particles, that is, dissolve. Therefore, water is a good solvent for many substances. After that, Masha proposed to conduct experiments and identify ways by which it would be possible to get an answer to the question of whether a substance dissolves in water or not.
U. I suggest that you at a club meeting determine the solubility in water of substances such as table salt, sugar, river sand and clay.
Let's assume which substance, in your opinion, will dissolve in water, and which will not. Express your assumptions, guesses and continue the statement: (Slide 3)
U. Let's think together what hypotheses we will confirm. (Slide 3)
Suppose ... (salt will dissolve in water)
Let's say ... (sugar will dissolve in water)
Perhaps ... (sand will not dissolve in water)
What if... (clay won't dissolve in water)
U. Let's, and we will conduct experiments that will help us figure it out. Before work, the chairman will remind you of the rules for conducting experiments and distribute cards on which these rules are printed. (Slide 4)
P. Look at the screen where the rules are written.
"Rules for conducting experiments"
All equipment must be handled with care. They can not only be broken, they can also get hurt.
During work, you can not only sit, but also stand.
The experiment is conducted by one of the students (the speaker), the rest silently observe or, at the request of the speaker, help him.
The exchange of opinions on the results of the experiment begins only after the speaker allows it to begin.
You need to talk to each other quietly, without disturbing the others.
Approaching the table and changing laboratory equipment is possible only with the permission of the chairman.
IV. Practical work
U. I suggest that the chairman choose a "consultant" who will read aloud from the textbook the procedure for conducting the first experiment. (Slide 5)
1) P. Experiment with table salt. Check if table salt dissolves in water.
A "consultant" from each laboratory takes one of the prepared sets and conducts an experiment with table salt. Boiled water is poured into a transparent glass. Pour a small amount of table salt into the water. The group observes what happens to the salt crystals and tastes the water.
The chairman (as in the KVN game) reads the same question to each group, and representatives from the laboratories answer them.
P. (Slide 6) Has the transparency of the water changed? (Transparency has not changed)
Has the color of the water changed? (Color has not changed)
Has the taste of water changed? (Water turned salty)
Can we say that the salt has disappeared? (Yes, she disappeared, disappeared, she is not visible)
U. Make a conclusion. (Salt dissolved) (Slide 6)
P. I ask everyone to proceed with the second experiment, for which it is necessary to use filters.
U. What is a filter? (A device, device or structure for purifying liquids, gases from solid particles, impurities.) (Slide 7)
U. Read aloud the procedure for performing the experiment with the filter. (Slide 8)
Students pass water with salt through a filter, observe and examine the taste of water.
P. (Slide 9) Is there any salt left on the filter? (There is no edible salt left on the filter)
Have you been able to remove the salt from the water? (Table salt passed through the filter with water)
U. Make a conclusion from your observations. (Salt dissolved in water) (Slide 9)
U. Was your hypothesis confirmed?
U. All right! Well done!
U. Write the results of the experiment in writing in the Notebook for independent work (p. 30). (Slide 10)
2) P. (Slide 11) Let's do the same experiment again, but instead of salt, put a teaspoon of granulated sugar.
A "consultant" from each laboratory takes a second set and runs an experiment with sugar. Boiled water is poured into a transparent glass. Pour a small amount of sugar into the water. The group observes what is happening and examines the taste of the water.
P. (Slide 12) Has the transparency of the water changed? (The transparency of the water has not changed)
Has the color of the water changed? (The color of the water has not changed)
Has the taste of water changed? (Water turned sweet)
Can we say that sugar is gone? (Sugar became invisible in water, water dissolved it)
U. Make a conclusion. (Sugar dissolved) (Slide 12)
U. Pass water with sugar through a paper filter. (Slide 13)
Students pass water with sugar through a filter, observe and examine the taste of water.
P. (Slide 14) Is there any sugar left on the filter? (Sugar is not visible on the filter)
Has the taste of water changed? (The taste of the water has not changed)
Have you managed to clear the water of sugar? (Water could not be purified from sugar, along with water it passed through the filter)
U. Make a conclusion. (Sugar dissolved in water) (Slide 14)
U. Was the hypothesis confirmed?
W. Right. Well done!
U. Write the results of the experiment in writing in a notebook for independent work. (Slide 15)
3) P. (Slide 16) Let's check the statements and conduct an experiment with river sand.
U. Read in the textbook the procedure for conducting the experiment.
Experiment with river sand. Stir a teaspoon of river sand in a glass of water. Let the mixture stand. Observe what happens to the grains of sand and water.
P. (Slide 17) Has the transparency of the water changed? (Water became cloudy, dirty)
Has the color of the water changed? (The color of the water has changed)
Are the grains gone? (Heavier grains of sand sink to the bottom, while smaller ones float in the water, making it cloudy)
U. Make a conclusion. (Sand did not dissolve) (Slide 17)
U. (Slide 18) Pass the contents of the glass through a paper filter.
Students pass water with sugar through a filter, observe.
P. (Slide 19) What passes through the filter and what remains on it? (Water passes through the filter, but the river sand remains on the filter and the grains of sand are clearly visible)
Was the water cleared of sand? (The filter helps to clean the water of particles that do not dissolve in it)
U. Make a conclusion. (River sand did not dissolve in water) (Slide 19)
U. Was your assumption about the solubility of sand in water correct?
U. Great! Well done!
U. Write the results of the experiment in writing in a notebook for independent work. (Slide 20)
4) P. (Slide 21) Do the same experiment with a piece of clay.
Experiment with clay. Stir a piece of clay in a glass of water. Let the mixture stand. Observe what happens to clay and water.
P. (Slide 22) Has the transparency of the water changed? (Water turned cloudy)
Has the color of the water changed? (Yes)
Did the clay particles disappear? (Heavier particles sink to the bottom, while smaller ones float in the water, making it cloudy)
U. Make a conclusion. (Clay did not dissolve in water) (Slide 22)
U. (Slide 23) Pass the contents of the glass through a paper filter.
P. (Slide 24) What passes through the filter and what remains on it? (Water passes through the filter, and undissolved particles remain on the filter.)
Has the water been cleared of clay? (The filter helped clear the water of particles that didn't dissolve in the water)
U. Make a conclusion. (Clay does not dissolve in water) (Slide 24)
U. Was the hypothesis confirmed?
U. Well done! Everything is correct!
U. I ask one of the group members to read the conclusions written in the notebook to all those present.
U. Does anyone have any additions, clarifications?
U. Let's draw conclusions from the experiments. (Slide 25)
Are all substances soluble in water? (Salt, granulated sugar dissolved in water, but sand and clay did not dissolve.)
Is it always possible to use a filter to determine whether a substance is soluble in water or not? (Substances dissolved in water pass through the filter along with water, while particles that do not dissolve remain on the filter)
D. Read about the solubility of substances in water in the textbook (p. 87).
U. Draw a conclusion about the property of water as a solvent. (Water is a solvent, but not all substances dissolve in it) (Slide 25)
U. I advise club members to read the story in the reader “Water is a solvent” (p. 46). (Slide 26)
Why have scientists not yet been able to obtain absolutely pure water? (Because hundreds, maybe thousands of different substances are dissolved in water)
U. How do people use the property of water to dissolve certain substances?
(Slide 27) Tasteless water becomes sweet or salty due to sugar or salt, as water dissolves and acquires their taste. A person uses this property when preparing food: brews tea, cooks compote, soups, salts and preserves vegetables, prepares jam.
(Slide 28) When we wash our hands, wash or bathe, when we wash clothes, we use liquid water and its solvent property.
(Slide 29) Gases, in particular oxygen, also dissolve in water. Thanks to this, fish and others live in rivers, lakes, seas. In contact with air, water dissolves oxygen, carbon dioxide and other gases that are in it. For living organisms that live in water, such as fish, oxygen dissolved in water is very important. They need it to breathe. If oxygen did not dissolve in water, then water bodies would be lifeless. Knowing this, people do not forget to oxygenate the water in the aquarium where the fish live, or cut holes in the ponds in winter to improve life under the ice.
(Slide 30) When we paint with watercolors or gouache.
U. Pay attention to the task written on the board. (Slide 31) I propose to draw up a collective speech plan on the topic “Water is a solvent”. Discuss it in your laboratories.
Listening to plans on the topic “Water is a solvent” compiled by students.
U. Let's all formulate a speech plan together. (Slide 31)
Approximate speech plan on the topic “Water is a solvent”
Introduction.
Dissolution of substances in water.
Conclusions.
People use the property of water to dissolve certain substances.
Excursion to the "Exhibition Hall". (Slide 32)
U. When preparing a report, you can use additional literature selected by the guys, assistant speakers on the topic of our meeting. (Draw students' attention to the exhibition of books, Internet pages)
V. Summary of the lesson
What property of water was investigated at a club meeting? (Property of water as a solvent)
What conclusion did we come to by examining this property of water? (Water is a good solvent for some substances.)
Do you think it's hard to be explorers?
What seemed the most difficult, interesting?
Will the knowledge acquired during the study of this property of water be useful to you in later life? (Slide 33) (It is very important to remember that water is a solvent. Water dissolves salts, among which there are both beneficial and harmful to humans. Therefore, you cannot drink water from a source if you do not know whether it is clean. Do not in vain there is a proverb among the people: "Not all water is suitable for drinking.")
VI. Reflection
How do we use the property of water to dissolve certain substances in art classes? (When we paint with watercolors or gouache)
I suggest you, using this property of water, paint the water in a glass in a color that best suits your mood. (Slide 34)
"Yellow color" - joyful, bright, good mood.
"Green color" - calm, balanced.
"Blue color" - a sad, sad, dreary mood.
Show your sheets of colored water in a glass.
VII. Evaluation
I would like to thank the chairman, the "consultants" and all participants of the meeting for their active work.
VIII. Homework
The secret is simple but brilliant. The water molecule itself is electrically neutral. However, the electric charge inside the molecule is distributed very unevenly. The area of hydrogen atoms has a positively tuned "character", and the "residence" of oxygen is famous for its expressive negative charge.
If the energy of attraction of water molecules to the molecules of a substance prevails compared to the energy of attraction between water molecules, then the substance dissolves. If such a condition is not met, then the “miracle” also, respectively, does not occur.
The main "traffic light" with a lit red color for water is fats. That is why, if we suddenly “reward” clothes with an expressive oily stain, the phrase “Just add water” in this situation will not be saving.
Although, due to the fact that we are subconsciously accustomed to seeing water as a universal solvent, which practically can handle any problem, we often still try to solve the problem with water. And when nothing works out for us, then most often we get angry, but in fact, we should ... rejoice. Yes, just rejoice!
Indeed, for the reason that water is unable to dissolve fats, we can ... live. Because it is precisely due to the fact that fats are included in the “black list” for water that we ourselves do not dissolve.
But salts, alkalis and acids for water are a real “delicacy”. By the way, such chemical properties, again, are very beneficial to a person. After all, if this were not so, then the decay products would create a real dump in the body, and the blood would automatically thicken. Therefore, if a person is deprived of water, then on the 5th day he dies. In addition, of course, if you do not regularly receive the required amount (the "average" norm is 2-3 liters per day), undissolved salts significantly increase the risk of kidney stones, as well as the bladder.
However, of course, it is precisely because water dissolves, for example, the same salts that it is not worth turning into an uncontrolled “water drink”, setting impudent “records”, simply because some dispute has obliged it. After all, this can greatly disrupt the mineral balance of the body.
By the way, passing through oneself (both literally and figuratively) and understanding the physical and chemical essence of this phenomenon, it is easy to understand the role of water as a solvent in many other areas of both domestic and industrial plans.
Solution is called a thermodynamically stable homogeneous (single-phase) system of variable composition, consisting of two or more components (chemicals). The components that make up a solution are a solvent and a solute. Typically, a solvent is considered to be a component that exists in its pure form in the same state of aggregation as the resulting solution (for example, in the case of an aqueous salt solution, the solvent is, of course, water). If both components before dissolution were in the same state of aggregation (for example, alcohol and water), then the component that is in a larger amount is considered the solvent.
Solutions are liquid, solid and gaseous.
Liquid solutions are solutions of salts, sugar, alcohol in water. Liquid solutions may be aqueous or non-aqueous. Aqueous solutions are solutions in which the solvent is water. Non-aqueous solutions are solutions in which organic liquids (benzene, alcohol, ether, etc.) are solvents. Solid solutions are metal alloys. Gaseous solutions - air and other mixtures of gases.
Dissolution process. Dissolution is a complex physical and chemical process. During the physical process, the structure of the dissolved substance is destroyed and its particles are distributed between the solvent molecules. A chemical process is the interaction of solvent molecules with solute particles. As a result of this interaction, solvates. If the solvent is water, then the resulting solvates are called hydrates. The process of formation of solvates is called solvation, the process of formation of hydrates is called hydration. When aqueous solutions are evaporated, crystalline hydrates are formed - these are crystalline substances, which include a certain number of water molecules (water of crystallization). Examples of crystalline hydrates: CuSO 4 . 5H 2 O - copper (II) sulfate pentahydrate; FeSO4 . 7H 2 O - iron sulfate heptahydrate (II).
The physical process of dissolution proceeds with takeover energy, chemical highlighting. If as a result of hydration (solvation) more energy is released than it is absorbed during the destruction of the structure of a substance, then dissolution - exothermic process. Energy is released during the dissolution of NaOH, H 2 SO 4 , Na 2 CO 3 , ZnSO 4 and other substances. If more energy is needed to destroy the structure of a substance than it is released during hydration, then dissolution - endothermic process. Energy absorption occurs when NaNO 3 , KCl, NH 4 NO 3 , K 2 SO 4 , NH 4 Cl and some other substances are dissolved in water.
The amount of energy released or absorbed during dissolution is called thermal effect of dissolution.
Solubility substance is its ability to be distributed in another substance in the form of atoms, ions or molecules with the formation of a thermodynamically stable system of variable composition. The quantitative characteristic of solubility is solubility factor, which shows what is the maximum mass of a substance that can be dissolved in 1000 or 100 g of water at a given temperature. The solubility of a substance depends on the nature of the solvent and substance, on temperature and pressure (for gases). The solubility of solids generally increases with increasing temperature. The solubility of gases decreases with increasing temperature, but increases with increasing pressure.
According to their solubility in water, substances are divided into three groups:
1. Highly soluble (p.). The solubility of substances is more than 10 g in 1000 g of water. For example, 2000 g of sugar dissolves in 1000 g of water, or 1 liter of water.
2. Slightly soluble (m.). The solubility of substances is from 0.01 g to 10 g in 1000 g of water. For example, 2 g of gypsum (CaSO 4 . 2 H 2 O) dissolves in 1000 g of water.
3. Practically insoluble (n.). The solubility of substances is less than 0.01 g in 1000 g of water. For example, in 1000 g of water, 1.5 . 10 -3 g AgCl.
When substances are dissolved, saturated, unsaturated and supersaturated solutions can be formed.
saturated solution is the solution that contains the maximum amount of solute under given conditions. When a substance is added to such a solution, the substance no longer dissolves.
unsaturated solution A solution that contains less solute than a saturated solution under given conditions. When a substance is added to such a solution, the substance still dissolves.
Sometimes it is possible to obtain a solution in which the solute contains more than in a saturated solution at a given temperature. Such a solution is called supersaturated. This solution is obtained by carefully cooling the saturated solution to room temperature. Supersaturated solutions are very unstable. Crystallization of a substance in such a solution can be caused by rubbing the walls of the vessel in which the solution is located with a glass rod. This method is used when performing some qualitative reactions.
The solubility of a substance can also be expressed by the molar concentration of its saturated solution (section 2.2).
Solubility constant. Let us consider the processes that occur during the interaction of a poorly soluble but strong electrolyte of barium sulfate BaSO 4 with water. Under the action of water dipoles, Ba 2+ and SO 4 2 - ions from the crystal lattice of BaSO 4 will pass into the liquid phase. Simultaneously with this process, under the influence of the electrostatic field of the crystal lattice, part of the Ba 2+ and SO 4 2 - ions will again precipitate (Fig. 3). At a given temperature, an equilibrium will finally be established in a heterogeneous system: the rate of the dissolution process (V 1) will be equal to the rate of the precipitation process (V 2), i.e.
BaSO 4 ⇄ Ba 2+ + SO 4 2 -
solid solution
Rice. 3. Saturated barium sulfate solution
A solution in equilibrium with the BaSO 4 solid phase is called rich relative to barium sulfate.
A saturated solution is an equilibrium heterogeneous system, which is characterized by a chemical equilibrium constant:
, (1)
where a (Ba 2+) is the activity of barium ions; a(SO 4 2-) - activity of sulfate ions;
a (BaSO 4) is the activity of barium sulfate molecules.
The denominator of this fraction - the activity of crystalline BaSO 4 - is a constant value equal to one. The product of two constants gives a new constant called thermodynamic solubility constant and denote K s °:
K s ° \u003d a (Ba 2+) . a(SO 4 2-). (2)
This value was previously called the solubility product and was designated PR.
Thus, in a saturated solution of a poorly soluble strong electrolyte, the product of the equilibrium activities of its ions is a constant value at a given temperature.
If we accept that in a saturated solution of a sparingly soluble electrolyte, the activity coefficient f~1, then the activity of ions in this case can be replaced by their concentrations, since a( X) = f (X) . FROM( X). The thermodynamic solubility constant K s ° will turn into the concentration solubility constant K s:
K s \u003d C (Ba 2+) . C(SO 4 2-), (3)
where C(Ba 2+) and C(SO 4 2 -) are the equilibrium concentrations of Ba 2+ and SO 4 2 - ions (mol / l) in a saturated solution of barium sulfate.
To simplify calculations, the concentration solubility constant K s is usually used, taking f(X) = 1 (Appendix 2).
If a poorly soluble strong electrolyte forms several ions during dissociation, then the expression K s (or K s °) includes the corresponding powers equal to the stoichiometric coefficients:
PbCl 2 ⇄ Pb 2+ + 2 Cl-; K s \u003d C (Pb 2+) . C 2 (Cl -);
Ag3PO4 ⇄ 3 Ag + + PO 4 3 - ; K s \u003d C 3 (Ag +) . C (PO 4 3 -).
In general, the expression for the concentration solubility constant for the electrolyte A m B n ⇄ m A n+ + n B m - has the form
K s \u003d C m (A n+) . C n (B m -),
where C are the concentrations of A n+ and B m ions in a saturated electrolyte solution in mol/l.
The value of K s is usually used only for electrolytes, the solubility of which in water does not exceed 0.01 mol/l.
Precipitation conditions
Suppose c is the actual concentration of ions of a sparingly soluble electrolyte in solution.
If C m (A n +) . With n (B m -) > K s , then a precipitate will form, because the solution becomes supersaturated.
If C m (A n +) . C n (B m -)< K s , то раствор является ненасыщенным и осадок не образуется.
Solution properties. Below we consider the properties of nonelectrolyte solutions. In the case of electrolytes, a correction isotonic coefficient is introduced into the above formulas.
If a non-volatile substance is dissolved in a liquid, then the saturation vapor pressure over the solution is less than the saturation vapor pressure over the pure solvent. Simultaneously with the decrease in vapor pressure over the solution, a change in its boiling and freezing point is observed; the boiling points of solutions increase, and the freezing points decrease in comparison with the temperatures characterizing pure solvents.
The relative decrease in the freezing point or the relative increase in the boiling point of a solution is proportional to its concentration.
The concept of solubility is used in chemistry to describe the properties of a solid that mixes with and dissolves in a liquid. Only ionic (charged) compounds are completely soluble. For practical needs, it is enough to remember a few rules or be able to find them in order to use them if necessary and find out whether certain ionic substances will dissolve or not in water. In fact, some number of atoms dissolves in any case, even if the changes are not noticeable, so in order to conduct accurate experiments, it is sometimes necessary to calculate this number.
Steps
Using Simple Rules
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Learn more about ionic compounds. In the normal state, each atom has a certain number of electrons, but sometimes it can capture an extra electron or lose one. As a result, a and he, which has an electrical charge. If an ion with a negative charge (an extra electron) meets an ion with a positive charge (no electron), they bind together, like the opposite poles of two magnets. As a result, an ionic compound is formed.
- Ions with a negative charge are called anions, and ions with a positive charge - cations.
- In the normal state, the number of electrons in an atom is equal to the number of protons, as a result of which the atom is electrically neutral.
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Learn more about solubility. Water molecules (H 2 O) have a peculiar structure that makes them look like a magnet: they have a positive charge on one end and a negative charge on the other. When an ionic compound is placed in water, these water "magnets" gather around its molecules and tend to pull the positive and negative ions away from each other. The molecules of some ionic compounds are not very strong, and such substances soluble in water, because the water molecules pull the ions away from each other and dissolve them. In other compounds, the ions are more tightly bound, and they insoluble, since water molecules are not able to pull the ions apart.
- In the molecules of some compounds, internal bonds are comparable in strength to the action of water molecules. Such connections are called slightly soluble, since a significant part of their molecules dissociate, although others remain undissolved.
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Learn the rules of solubility. Since the interaction between atoms is described by rather complex laws, it is not always possible to immediately tell which substances dissolve and which do not. Find one of the compound's ions in the description below of how various substances typically behave. After that, pay attention to the second ion and check if this substance is not an exception due to the unusual interaction of ions.
- Suppose you are dealing with strontium chloride (SrCl 2). Locate the steps below (in bold) for the Sr and Cl ions. Cl "usually soluble"; after that, look at the exceptions below. Sr ions are not mentioned there, so the SrCl compound must be soluble in water.
- Below the relevant rules are the most common exceptions. There are other exceptions, but you are unlikely to encounter them in chemistry class or in the lab.
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Compounds are soluble if they contain alkali metal ions, that is, Li + , Na + , K + , Rb + and Cs + . These are the elements of group IA of the periodic table: lithium, sodium, potassium, rubidium and cesium. Almost all simple compounds of these elements are soluble.
- Exception: the Li 3 PO 4 compound is insoluble.
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Compounds of NO 3 -, C 2 H 3 O 2 -, NO 2 -, ClO 3 - and ClO 4 - ions are soluble. They are called nitrate, acetate, nitrite, chlorate, and perchlorate ions, respectively. The acetate ion is often abbreviated as OAc.
- Exceptions: Ag(OAc) (silver acetate) and Hg(OAc) 2 (mercury acetate) are insoluble.
- AgNO 2 - and KClO 4 - are only "slightly soluble".
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Compounds of ions Cl - , Br - and I - are usually soluble. Ions of chlorine, bromine and iodine form respectively chlorides, borides and iodides, which are called halogen salts. These salts are almost always soluble.
- Exception: if the second ion in the pair is a silver ion Ag + , mercury Hg 2 2+ or lead Pb 2+ , the salt is insoluble. The same is true for less common halogens with copper ions Cu + and thallium Tl + .
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Compounds of the SO 4 2- ion (sulfates) are usually soluble. As a rule, sulfates dissolve in water, but there are a few exceptions.
- Exceptions: sulfates of the following ions are insoluble: strontium Sr 2+, barium Ba 2+, lead Pb 2+, silver Ag +, calcium Ca 2+, radium Ra 2+ and bivalent silver Hg 2 2+. Note that silver sulfate and calcium sulfate are still slightly soluble in water and are sometimes considered slightly soluble.
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The OH - and S 2- compounds are insoluble in water. These are hydroxide and sulfide ions, respectively.
- Exceptions: remember the alkali metals (group IA) and how almost all of their compounds are soluble? So, Li + , Na + , K + , Rb + and Cs + ions form soluble hydroxides and sulfides. In addition, calcium salts Ca 2+ , strontium Sr 2+ and barium Ba 2+ (group IIA) are soluble. Keep in mind that a significant part of the hydroxide molecules of these elements still does not dissolve, therefore they are sometimes considered "poorly soluble".
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Compounds of CO 3 2- and PO 4 3- ions are insoluble. These ions form carbonates and phosphates, which are usually insoluble in water.
- Exceptions: these ions form soluble compounds with alkali metal ions: Li + , Na + , K + , Rb + and Cs + , as well as with ammonium NH 4 + .
Using the solubility product K sp
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Find the solubility product K sp (this is a constant). Each compound has its own constant K sp . Its values for various substances are given in reference books and on the website (in English). The values of the solubility product are determined experimentally and they can differ significantly from each other in different sources, so it is better to use the table for K sp in your chemistry textbook, if such a table is available. Unless otherwise noted, most tables give the solubility product at 25ºC.
- For example, if you are dissolving lead iodide PbI 2 , find the solubility product for it. The website bilbo.chm.uri.edu lists a value of 7.1×10–9.
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Write down the chemical equation. First, determine which ions the molecule of the substance will decompose into when dissolved. Then write an equation with K sp on one side and the corresponding ions on the other.
- In our example, the PbI 2 molecule is split into a Pb 2+ ion and two I - ions. In this case, it is sufficient to establish the charge of only one ion, since the solution as a whole will be neutral.
- Write down the equation: 7.1 × 10 -9 \u003d 2.
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Transform the equation to solve it. Rewrite the equation in simple algebraic form. Use what you know about the number of molecules and ions. Substitute the unknown value x for the number of atoms of the dissolved compound and express the number of ions in terms of x.
- In our example, it is necessary to rewrite the following equation: 7.1 × 10 -9 \u003d 2.
- Since there is only one lead atom (Pb) in the compound, the number of dissolved molecules will equal the number of free lead ions. So we can equate x as well.
- Since there are two iodine (I) ions for every lead ion, the number of iodine atoms should be equal to 2x.
- The result is the equation 7.1×10 -9 = (x)(2x) 2 .
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Allow for common ions if necessary. Skip this step if the substance is soluble in pure water. However, if you are using a solution that already contains one or more of the ions of interest ("total ions"), the solubility may be significantly reduced. The effect of common ions is especially noticeable for poorly soluble substances, and in such cases it can be assumed that the vast majority of dissolved ions were already present in the solution earlier. Rewrite the equation and take into account the known molar concentrations (moles per liter, or M) of already dissolved ions. Correct the unknown x values for these ions.
- For example, if lead iodide is already present in the solution at a concentration of 0.2M, the equation should be rewritten as follows: 7.1×10 -9 = (0.2M+x)(2x) 2 . Since 0.2M is much larger than x, the equation can be written as 7.1×10 –9 = (0.2M)(2x) 2 .
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Solve the equation. Find the x value to find out how soluble this compound is. In view of the definition of the solubility product, the answer will be expressed in moles of solute per liter of water. You may need a calculator to calculate the final result.
- For dissolution in pure water, that is, in the absence of common ions, we find:
- 7.1×10 –9 = (x)(2x) 2
- 7.1×10 -9 = (x)(4x2)
- 7.1x10 -9 = 4x3
- (7.1 × 10 -9) / 4 \u003d x 3
- x = ∛((7.1×10 –9)/4)
- x= 1.2 x 10 -3 moles per liter of water. This is a very small amount, so this substance is practically insoluble.
Objective: To learn by experience which solids dissolve in water and which do not dissolve in water.
Educational:
- To acquaint students with the concepts: soluble and insoluble substances.
- Learn to prove empirically the correctness of assumptions about the solubility (insolubility) of solids.
Corrective:
- Develop speech through the explanation of the work being done.
Learn how to use laboratory equipment and conduct experiments.
Educational:
- Develop the ability to communicate and work in groups.
Cultivate perseverance.
Type of lesson: laboratory work.
Teaching aids: textbook "Natural science" N.V. Koroleva, E.V. Makarevich
Equipment for laboratory work: beakers, filters, instructions. Solids: salt, sugar, soda, sand, coffee, starch, earth, chalk, clay.
During the classes
I. Organizational moment
W: Hello guys. Greet each other with your eyes. Nice to see you, have a seat.
. Repetition of the pastT: Let's repeat what we already know about water:
What happens to water when heated?
What happens to water when it cools?
What happens to water when it freezes?
What are the three states in which water occurs in nature?
W: What good fellows you are! Everyone knows!
III. Learning new material
(I agree with the students in advance on the groups they will work with, the guys themselves choose the head of the laboratory (another child can be selected at another laboratory lesson), who writes the experience indicators in a table and gives oral comments when filling out the final part of the table - the result.)
U: Guys, today in the laboratory work we will find out which substances water can dissolve and which cannot. Open a notebook, write down the date and the topic of the lesson “Soluble and insoluble substances in water”. ( I'm attaching to the board.) What is the goal of today's lesson?
R: Find out which substances dissolve in water and which do not. ( I'm attaching to the board.)
U: All substances in nature can be divided into two groups: soluble and insoluble. What substances can be called soluble? (Check textbook p.80:2) Water-soluble substances are those that, when placed in water, become invisible and do not settle on the filter during filtration.. (Attached to the board.)
T: And what substances can be named insoluble? (check textbook p.47-2) Water-insoluble substances - those that do not dissolve in water and settle on the filter (attach to the board).
T: Guys, what do you think we need to complete the laboratory work?
R: Water, some substances, beakers, filter ( I show the water in the decanter; beakers filled with substances: salt, sugar, soda, sand, coffee, starch, chalk, clay; empty beakers, filter).
Q: What is a filter?
R: A device for purifying liquids from substances insoluble in it that settle on it.
U: And what improvised means can be used to make a filter? Well done! And we will use cotton wool ( I put a piece of cotton in the funnel).
U: But before starting the laboratory work, let's fill in the table (the table is drawn on the blackboard, I use two colors of crayons, if the students assume that the substance is completely soluble in water, then I mark "+" in the second column; if the students assume that the substance remains on the filter, then “+” in the third column, and vice versa; with colored chalk I fix the expected result in the fourth column - P (soluble) or H (insoluble))
| Our Assumptions | Result | ||
| Solubility | Filtration | ||
| 1. Water + sand | – | + | H |
| 2. Water + clay | |||
| 3. Water + coffee | |||
| 4. Water + starch | |||
| 5. Water + soda | |||
| 6. Water + earth | |||
| 7. Water + sugar | |||
| 8. Water + chalk | |||
U: And after doing the laboratory work, we will compare our assumptions with the results obtained.
T: Each lab will test two solids, all results will be recorded in the Water Soluble and Insoluble Substances report. Attachment 1
U: Guys, this is your first independent laboratory work and before you start doing it, listen to the procedure or instructions. ( I distribute to each laboratory, after reading we discuss.)
Laboratory work
(I help if necessary. It may be difficult to filter the coffee solution, because the filter will be stained. To facilitate filling out the reports, I suggest using the phrases that I attach to the board. Appendix 3.)
T: Now let's check our assumptions. Heads of laboratories, check if your report is signed and comment on the results obtained by experience. (The head of the laboratory reports, I fix the result with a piece of chalk of a different color)
U: Guys, what substances for research turned out to be soluble? What are not? How many matches were there? Well done. Almost all of our assumptions were confirmed.
VI. Questions for consolidation
U: Guys, where does a person use a solution of salt, sugar, soda, sand, coffee, starch, clay?
VII. Lesson summary
T: What is our goal today? Did you complete it? Are we great? I am very satisfied with you! And I give everyone "excellent".
VIII. Homework
T: Read the text for extracurricular reading on page 43, answer the questions.
Stand up, please, those guys who did not like our lesson. Thank you for your honesty. And now those who liked our work. Thank you. Goodbye everyone.
