Structural formula

True, empirical, or gross formula: H 2 O

Chemical Composition of Water

Molecular Weight: 18.015

Water (hydrogen oxide) is a binary inorganic compound with the chemical formula H 2 O. A water molecule consists of two hydrogen atoms and one oxygen atom, which are interconnected by a covalent bond. Under normal conditions, it is a transparent liquid, has no color (in a small volume), smell and taste. In a solid state it is called ice (ice crystals can form snow or hoarfrost), and in a gaseous state - water vapor. Water can also exist in the form of liquid crystals (on hydrophilic surfaces). It makes up about 0.05% of the Earth’s mass.

It is a good strongly polar solvent. Under natural conditions, it always contains dissolved substances (salts, gases).

Water under normal conditions is in a liquid state, while similar hydrogen compounds of other elements are gases (H 2 S, CH 4, HF). Hydrogen atoms are attached to the oxygen atom, forming an angle of 104.45 ° (104 ° 27 ′). Due to the large difference in the electronegativity of the hydrogen and oxygen atoms, the electron clouds are strongly biased towards oxygen. For this reason, the water molecule has a large dipole moment (p \u003d 1.84 D, second only to hydrocyanic acid). Each water molecule forms up to four hydrogen bonds - two of them form an oxygen atom and two - hydrogen atoms. The number of hydrogen bonds and their branched structure determine the high boiling point of water and its specific heat of vaporization. If there were no hydrogen bonds, water, based on the place of oxygen in the periodic table and the boiling points of hydrides of elements similar to oxygen (sulfur, selenium, tellurium), would boil at −80 ° C, and freeze at −100 ° C.

Upon transition to a solid state, water molecules are ordered, while the volumes of voids between the molecules increase, and the total density of water decreases, which explains the lower density (larger volume) of water in the ice phase. During evaporation, on the contrary, all hydrogen bonds break. Breaking bonds requires a lot of energy, which is why water has the largest specific heat capacity among other liquids and solids. In order to heat one liter of water per degree, it is required to spend 4.1868 kJ of energy. Due to this property, water is often used as a coolant. In addition to a large specific heat, water also has large values \u200b\u200bof specific heat of fusion (333.55 kJ / kg at 0 ° C) and vaporization (2250 kJ / kg).

Definition

Water   - hydrogen oxide is a binary compound of inorganic nature.

The formula is H 2 O. Molar mass is 18 g / mol. It can exist in three states of aggregation - liquid (water), solid (ice) and gaseous (water vapor).

Chemical properties of water

Water is the most common solvent. There is equilibrium in the water solution, so water is called an ampholyte:

H 2 O ↔ H + + OH - ↔ H 3 O + + OH -.

Under the influence of an electric current, water decomposes into hydrogen and oxygen:

H 2 O \u003d H 2 + O 2.

At room temperature, water dissolves the active metals with the formation of alkalis, while hydrogen is also released:

2H 2 O + 2Na \u003d 2NaOH + H 2.

Water is able to interact with fluorine and interhaloid compounds, and in the second case, the reaction proceeds at low temperatures:

2H 2 O + 2F 2 \u003d 4HF + O 2.

3H 2 O + IF 5 \u003d 5HF + HIO 3.

Salts formed by a weak base and a weak acid undergo hydrolysis when dissolved in water:

Al 2 S 3 + 6H 2 O \u003d 2Al (OH) 3 ↓ + 3H 2 S.

Water is able to dissolve certain substances metals and non-metals when heated:

4H 2 O + 3Fe \u003d Fe 3 O 4 + 4H 2;

H 2 O + C ↔ CO + H 2.

Water, in the presence of sulfuric acid, enters into a reaction of interaction (hydration) with unsaturated hydrocarbons - alkenes with the formation of saturated monohydric alcohols:

CH 2 \u003d CH 2 + H 2 O → CH 3 —CH 2 —OH.

Physical properties of water

Water is a clear liquid (n.o.). The dipole moment is 1.84 D (due to the strong difference in the electronegativities of oxygen and hydrogen). Water has the highest specific heat capacity among all substances in the liquid and solid state. The specific heat of water melting is 333.25 kJ / kg (0 C), the vaporization rate is 2250 kJ / kg. Water is capable of dissolving polar substances. Water has a high surface tension and negative electrical surface potential.

Getting water

Water is obtained by a neutralization reaction, i.e. reactions of interaction between acids and alkalis:

H 2 SO 4 + 2KOH \u003d K 2 SO 4 + H 2 O;

HNO 3 + NH 4 OH \u003d NH 4 NO 3 + H 2 O;

2CH 3 COOH + Ba (OH) 2 \u003d (CH 3 COO) 2 Ba + H 2 O.

One of the ways to obtain water is the reduction of metals with hydrogen from their oxides:

CuO + H 2 \u003d Cu + H 2 O.

Examples of solving problems

EXAMPLE 1

The task	How much water should I take to prepare a 5% solution from a 20% solution of acetic acid?
Decision	According to the determination of the mass fraction of the substance, a 20% solution of acetic acid represents 80 ml of solvent (water) 20 g of acid, and a 5% solution of acetic acid represents 95 ml of solvent (water) 5 g of acid. We compose the proportion: x \u003d 20 × 95/5 \u003d 380. Those. in a new solution (5%) contains 380 ml of solvent. The initial solution is known to contain 80 ml of solvent. Therefore, to get a 5% solution of acetic acid from a 20% solution you need to add: 380-80 \u003d 300 ml of water.
Answer	You need 300 ml of water.

EXAMPLE 2

The task	During the combustion of organic matter weighing 4.8 g, 3.36 l of carbon dioxide (n.o.) and 5.4 g of water were formed. The density of organic matter in hydrogen is 16. Determine the formula for organic matter.
Decision	The molar masses of carbon dioxide and water calculated using the table of chemical elements Mendeleev - 44 and 18 g / mol, respectively. Calculate the amount of substance of the reaction products: n (CO 2) \u003d V (CO 2) / V m; n (H 2 O) \u003d m (H 2 O) / M (H 2 O); n (CO 2) \u003d 3.36 / 22.4 \u003d 0.15 mol; n (H 2 O) \u003d 5.4 / 18 \u003d 0.3 mol. Considering that there is one carbon atom in the composition of the CO 2 molecule, and 2 hydrogen atoms in the composition of the H 2 O molecule, the amount of substance and mass of these atoms will be equal to: n (C) \u003d 0.15 mol; n (H) \u003d 2 × 0.3 mol; m (C) \u003d n (C) × M (C) \u003d 0.15 × 12 \u003d 1.8 g; m (H) \u003d n (H) × M (H) \u003d 0.3 × 1 \u003d 0.3 g. Let us determine if oxygen is present in the composition of organic matter: m (O) \u003d m (C x H y O z) - m (C) - m (H) \u003d 4.8 - 0.6 - 1.8 \u003d 2.4 g. The number of substances of oxygen atoms: n (O) \u003d 2.4 / 16 \u003d 0.15 mol. Then, n (C): n (H): n (O) \u003d 0.15: 0.6: 0.15. Divide by the smallest value, we get n (C): n (H): n (O) \u003d 1: 4: 1. Therefore, the formula of the organic substance is CH 4 O. The molar mass of the organic substance calculated using the table of chemical elements Mendeleev - 32 g / mol. The molar mass of organic matter calculated using its hydrogen density: M (C x H y O z) \u003d M (H 2) × D (H 2) \u003d 2 × 16 \u003d 32 g / mol. If the formulas of the organic matter removed by the combustion products and using the density of hydrogen are different, then the ratio of the molar masses will be more than 1. Check this: M (C x H y O z) / M (CH 4 O) \u003d 1. Therefore, the formula of organic matter is CH 4 O.
Answer	Formula of the organic substance CH 4 O.

Water (hydrogen oxide) is a transparent liquid that does not have color (in a small volume), odor and taste. Chemical formula: H2O. In a solid state it is called ice or snow, and in a gaseous state it is called water vapor. About 71% of the Earth’s surface is covered by water (oceans, seas, lakes, rivers, ice at the poles).

It is a good strongly polar solvent. Under natural conditions, it always contains dissolved substances (salts, gases). Water is of key importance in creating and maintaining life on Earth, in the chemical structure of living organisms, in the formation of climate and weather.

Almost 70% of the surface of our planet is occupied by oceans and seas. Hard water - snow and ice - covers 20% of the land. Of the total amount of water on Earth equal to 1 billion 386 million cubic kilometers, 1 billion 338 million cubic kilometers is accounted for by the saline waters of the World Ocean, and only 35 million cubic kilometers are accounted for by fresh water. The total amount of ocean water would be enough to cover the globe with a layer of more than 2.5 kilometers. For each inhabitant of the Earth, approximately 0.33 cubic kilometers of sea water and 0.008 cubic kilometers of fresh water are accounted for. But the difficulty is that the vast majority of fresh water on Earth is in a state that makes it difficult to access for humans. Almost 70% of fresh water is contained in the ice sheets of the polar countries and in mountain glaciers, 30% is in the aquifers underground, and in the channels of all rivers only 0.006% of fresh water is contained at the same time. Water molecules are found in interstellar space. Water is part of comets, most planets of the solar system and their satellites.

Water composition (by weight): 11.19% hydrogen and 88.81% oxygen. Pure water is clear, odorless and tasteless. It has the highest density at 0 ° C (1 g / cm3). The density of ice is less than the density of liquid water, so ice floats to the surface. Water freezes at 0 ° C and boils at 100 ° C at a pressure of 101 325 Pa. It conducts heat poorly and conducts electricity very poorly. Water is a good solvent. The water molecule has an angular shape, the hydrogen atoms with respect to oxygen form an angle equal to 104.5 °. Therefore, the water molecule is a dipole: that part of the molecule where hydrogen is located is positively charged, and the part where oxygen is located is negatively charged. Due to the polarity of water molecules, the electrolytes in it dissociate into ions.

In liquid water, along with the usual Н20 molecules, associated molecules are contained, i.e., they are connected into more complex aggregates (Н2О) x due to the formation of hydrogen bonds. The presence of hydrogen bonds between water molecules explains the anomalies of its physical properties: maximum density at 4 ° С, high boiling point (in the series Н20-Н2S - Н2Sе) anomalously high heat capacity. With increasing temperature, hydrogen bonds break, and a complete break occurs when water passes into steam.

Water is a highly reactive substance. Under ordinary conditions, it interacts with many basic and acid oxides, as well as with alkali and alkaline earth metals. Water forms numerous compounds - crystalline hydrates.

Obviously, water binding compounds can serve as desiccants. Other drying agents include P205, CaO, BaO, metallic Ma (they also chemically interact with water), as well as silica gel. An important chemical property of water is its ability to enter into hydrolytic decomposition reactions.

Physical properties of water.

Water has a number of unusual features:

1. When ice melts, its density increases (from 0.9 to 1 g / cm³). In almost all other substances, the density decreases during melting.

2. When heated from 0 ° C to 4 ° C (more precisely, 3.98 ° C), the water is compressed. Accordingly, when cooling, the density drops. Thanks to this, fish can live in freezing waters: when the temperature drops below 4 ° C, colder water, as less dense, remains on the surface and freezes, and a positive temperature remains under the ice.

3. High temperature and specific heat of fusion (0 ° C and 333.55 kJ / kg), boiling point (100 ° C) and specific heat of vaporization (2250 KJ / kg), in comparison with hydrogen compounds with a similar molecular weight.

4. High heat capacity of liquid water.

5. High viscosity.

6. High surface tension.

7. Negative electric potential of the surface of the water.

All these features are associated with the presence of hydrogen bonds. Due to the large difference in the electronegativity of the hydrogen and oxygen atoms, the electron clouds are strongly biased towards oxygen. Because of this, and also because the hydrogen ion (proton) has no internal electron layers and is small in size, it can penetrate into the electron shell of a negatively polarized atom of a neighboring molecule. Due to this, each oxygen atom is attracted to the hydrogen atoms of other molecules and vice versa. A certain role is played by the proton exchange interaction between molecules and inside water molecules. Each water molecule can participate in a maximum of four hydrogen bonds: 2 hydrogen atoms - each in one, and an oxygen atom in two; In this state, the molecules are in an ice crystal. When the ice melts, some of the bonds break, which allows you to put the water molecules denser; when water is heated, bonds continue to break, and its density increases, but at temperatures above 4 ° C this effect becomes weaker than thermal expansion. Upon evaporation, all remaining bonds are torn. Breaking bonds requires a lot of energy, hence the high temperature and specific heat of melting and boiling and high heat capacity. The viscosity of water is due to the fact that hydrogen bonds prevent water molecules from moving at different speeds.

For similar reasons, water is a good solvent for polar substances. Each molecule of the solute is surrounded by water molecules, moreover, positively charged sections of the molecule of the soluble substance attract oxygen atoms, and negatively charged ones attract hydrogen atoms. Because a water molecule is small, many water molecules can surround each molecule of a solute.

This property of water is used by living things. In a living cell and in the intercellular space, solutions of various substances in water interact. Water is necessary for the life of all, without exception, unicellular and multicellular living beings on Earth.

Pure (impurity-free) water is a good insulator. Under normal conditions, water is weakly dissociated and the concentration of protons (more precisely, hydroxonium ions H3O +) and hydroxyl ions HO– is 0.1 μmol / L. But since water is a good solvent, one or another salt is almost always dissolved in it, that is, positive and negative ions are present in the water. Thanks to this, water conducts electricity. The conductivity of water can determine its purity.

Water has a refractive index of n \u003d 1.33 in the optical range. However, it strongly absorbs infrared radiation, and therefore water vapor is the main natural greenhouse gas, responsible for more than 60% of the greenhouse effect. Due to the large dipole moment of the molecules, water also absorbs microwave radiation, on which the principle of operation of a microwave oven is based.

State of aggregation.

1. As distinguished:

2. Hard - ice

3. Liquid - water

4. Gaseous - water vapor

Fig. 1 “Types of snowflakes”

At atmospheric pressure, water freezes (turns into ice) at a temperature of 0 ° C and boils (turns into water vapor) at a temperature of 100 ° C. With a decrease in pressure, the melting point of water slowly increases, and the boiling point drops. At a pressure of 611.73 Pa (about 0.006 atm), the boiling and melting points coincide and become equal to 0.01 ° C. Such pressure and temperature are called the triple point of water. At a lower pressure, water cannot be in a liquid state, and ice turns directly into steam. The temperature of sublimation of ice decreases with a decrease in pressure.

With increasing pressure, the boiling point of water increases, the density of water vapor at the boiling point also increases, and liquid water decreases. At a temperature of 374 ° C (647 K) and a pressure of 22.064 MPa (218 atm), water passes a critical point. At this point, the density and other properties of liquid and gaseous water coincide. At higher pressures, there is no difference between liquid water and water vapor, therefore, there is no boiling or evaporation.

Metastable states are also possible - supersaturated steam, superheated liquid, supercooled liquid. These states can exist for a long time, however, they are unstable and, in contact with a more stable phase, a transition occurs. For example, it is not difficult to obtain supercooled liquid by cooling clean water in a clean vessel below 0 ° C, but when a crystallization center appears, liquid water quickly turns into ice.

Isotopic modifications of water.

Both oxygen and hydrogen have natural and artificial isotopes. Depending on the type of isotopes that make up the molecule, the following types of water are distinguished:

1. Light water (just water).

2. Heavy water (deuterium).

3. Superheavy water (tritium).

Chemical properties of water.

Water is the most common solvent on Earth, largely determining the nature of terrestrial chemistry as a science. Most of chemistry, when it was born as a science, began precisely as the chemistry of aqueous solutions of substances. It is sometimes considered as an ampholyte - both acid and base at the same time (cation H + anion OH-). In the absence of foreign substances in water, the concentration of hydroxide ions and hydrogen ions (or hydroxonium ions) is the same, pKa ≈ approx. 16.

Water is one of the main substances that ensure the existence of the planet and humanity. This is a completely unique element, without which the life of any living creature is impossible. Some chemical and physical properties of water are unique.

The importance of this substance is difficult to overestimate. Water occupies most of the planet, forms oceans, seas, rivers and other bodies of water. She is directly involved in the formation of climate and weather, thereby ensuring certain conditions of existence in a particular corner of the planet.

For many organisms, it serves as a habitat. In addition, almost every living creature in one way or another consists precisely of water. For example, its content in the human body is from 70 to 90 percent.

Physical properties of water: a brief description

The water molecule is unique. Its formula is probably known to everyone: H2O. But here are some of the physical properties of water that directly depend on the structure of its molecule.

In nature, water exists in three at once. Under normal conditions, it is colorless, odorless, and tasteless. When the temperature drops, water crystallizes and turns into ice. With increasing temperature, the liquid passes into a gaseous state - water vapor.

Water is characterized by a high density, which is about 1 gram per cubic centimeter. Boiling water occurs when the temperature rises to one hundred degrees Celsius. But when the temperature drops to 0 degrees, the liquid turns into ice.

Interestingly, a decrease in atmospheric pressure causes a change in these indicators - water boils at a lower temperature.

The thermal conductivity of water is approximately 0.58 W / (m * K). Another important indicator is its high, which is almost equal to the corresponding indicator for mercury.

Unique physical properties of water

As already mentioned, it is water that ensures the normal existence of the planet, affecting the climate and vital functions of organisms. But this substance is actually unique. It is these amazing properties of water that provide life.

Take, for example, the density of ice and water. In most cases, when freezing, the molecules of substances are closer to each other, their structure becomes more compact and denser. But this scheme does not work with water. This amazing property was first described by Galileo.

If you slowly lower the temperature and follow, then at first the scheme will be quite standard - the substance will become denser and more compact. Changes will occur after the temperature reaches +4 degrees. With this indicator, water suddenly becomes lighter. That is why ice floats on the surface of the water, but does not sink. By the way, this feature ensures the survival of aquatic life - water rarely freezes completely, preserving the life of its inhabitants.

By the way, when freezing, the substance expands by about 9%. This feature of water causes natural corrosion of rocks. On the other hand, the water pipes are therefore burst when unexpected cooling.

But this is not all. Another one of its unique features is its abnormally high heat capacity. For example, the amount of heat needed to heat one gram of water per degree is enough to heat about 10 g of copper or 9 g of iron.

The entire oceans is a global thermostat that smoothes temperature fluctuations, both daily and annual. By the way, it is endowed with the same properties and which is contained in the atmosphere. It's no secret that the desert is characterized by sharp temperature changes - it is too hot during the day and very cold at night. This is due precisely to dry air and the lack of the required amount of water vapor.

1,000 g / cm3 (3.98 ° C), mp 0 ° C, bk 100 ° C; forms ice when freezing. One of the most common compounds in nature (the amount of water on the Earth’s surface is 1.39 × 1018 tons, in the atmosphere 1.3 × 1013 tons); Isotopic varieties are known (see Heavy Water). Water is part of many minerals and rocks, all living organisms (45 - 98%, including about 60% of body weight in the human body), is present in the soil. A mandatory component of almost all technological processes in industry and agriculture. High purity water is necessary in the production of food products, semiconductors, phosphors, in nuclear engineering, chemical analysis, etc. How medicinal waters use natural waters containing an increased amount of mineral salts, gases, and some chemical elements (see Mineral waters).

Modern Encyclopedia. 2000 .

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