Why does ice float in water? Why is water able to dissolve so many different substances? Why is a towel able to absorb water from bottom to top, contrary to the laws of gravity? If we assume that water came to us from another world, these and other mysteries surrounding water will seem less difficult to understand.

If water behaved like all other substances on earth, you and I would not exist.

Water is something so simple that we rarely think about it. However, there is nothing more mysterious than plain water. The biggest mystery of water: why ice floats. Any other substance, passing from a liquid to a solid state, becomes heavier as the density of the substance increases.

Water, passing from a liquid to a solid state, on the contrary, becomes lighter.

In the structure of ice, water particles are arranged in a very orderly manner, with a lot of free space between the particles. The volume of ice is greater than the volume of water from which it was formed. The volume is larger, the density is lower - ice is lighter than water, so it does not sink in water. Huge blocks of ice and icebergs do not sink in water.

• When the ice turns back into water, the particles become hundreds of thousands of times more active, and the free space is filled.

The liquid form of water is denser and heavier than the solid form. Water becomes heaviest at a temperature of + 4°C. As the temperature rises, water particles become more active, which leads to a decrease in its density.

No matter how cold the winter is over the reservoir, the water temperature at the bottom is constant: +4°C. Anything that lives on the bottom can survive long winters under the ice. Ice is lighter than water. With its shell on the surface of the water, it protects the bottom of the reservoir from freezing.

Everyone knows that ice is frozen water, or rather, it is in a solid state of aggregation. But Why does ice not sink in water, but float on its surface?

Water is an unusual substance with rare, even anomalous properties. In nature, most substances expand when heated and contract when cooled. For example, mercury in a thermometer rises through a narrow tube and shows an increase in temperature. Because mercury freezes at -39ºC, it is not suitable for thermometers used in harsh temperature environments.

Water also expands when heated and contracts when cooled. However, in the cooling range from approximately +4 ºC to 0 ºC it expands. This is why water pipes can burst in winter if the water in them has frozen and large masses of ice have formed. The ice pressure on the pipe walls is enough to cause them to burst.

Water expansion

Since water expands when cooled, the density of ice (i.e. its solid form) is less than that of liquid water. In other words, a given volume of ice weighs less than the same volume of water. This is reflected by the formula m = ρV, where V is the volume of the body, m is the mass of the body, ρ is the density of the substance. There is an inversely proportional relationship between density and volume (V = m/ρ), i.e., with increasing volume (as water cools), the same mass will have a lower density. This property of water leads to the formation of ice on the surface of reservoirs - ponds and lakes.

Let's assume that the density of water is 1. Then the ice will have a density of 0.91. Thanks to this figure, we can find out the thickness of the ice floe that floats on the water. For example, if an ice floe has a height above water of 2 cm, then we can conclude that its underwater layer is 9 times thicker (i.e. 18 cm), and the thickness of the entire ice floe is 20 cm.

In the area of ​​the Earth's North and South Poles, water freezes and forms icebergs. Some of these floating ice mountains are enormous. The largest iceberg known to man is considered to be with a surface area of ​​31,000 square meters. kilometers, which was discovered in 1956 in the Pacific Ocean.

How does water in its solid state increase its volume? By changing its structure. Scientists have proven that ice has an openwork structure with cavities and voids, which, when melted, are filled with water molecules.

Experience shows that the freezing point of water decreases with increasing pressure by approximately one degree for every 130 atmospheres.

It is known that in the oceans at great depths the water temperature is below 0 ºС, and yet it does not freeze. This is explained by the pressure created by the upper layers of water. A layer of water one kilometer thick presses with a force of about 100 atmospheres.

Comparison of the densities of water and ice

Can the density of water be less than the density of ice and does this mean that he will drown in it? The answer to this question is affirmative, which is easy to prove with the following experiment.

Take from the freezer, where the temperature is -5 ºС, a piece of ice the size of a third of a glass or a little more. Let's put it in a bucket of water at a temperature of +20 ºС. What are we observing? The ice quickly sinks and sinks, gradually beginning to melt. This happens because water at a temperature of +20 ºС has a lower density compared to ice at a temperature of -5 ºС.

There are modifications of ice (at high temperatures and pressures), which, due to their greater density, will sink in water. We are talking about the so-called “heavy” ice - deuterium and tritium (saturated with heavy and superheavy hydrogen). Despite the presence of the same voids as in protium ice, it will sink in water. In contrast to “heavy” ice, protium ice is devoid of heavy hydrogen isotopes and contains 16 milligrams of calcium per liter of liquid. The process of its preparation involves purification from harmful impurities by 80%, due to which protium water is considered the most optimal for human life.

Meaning in nature

The fact that ice floats on the surface of bodies of water plays an important role in nature. If the water did not have this property and the ice sank to the bottom, this would lead to freezing of the entire reservoir and, as a result, the death of the living organisms inhabiting it.

When cold weather occurs, first at temperatures above +4 ºС, colder water from the surface of the reservoir sinks down, and warm (lighter) water rises. This process is called vertical circulation (mixing) of water. When it reaches +4 ºС throughout the entire reservoir, this process stops, since from the surface the water already at +3 ºС becomes lighter than that which is below. Water expands (its volume increases by approximately 10%) and its density decreases. As a consequence of the fact that the colder layer is on top, water freezes on the surface and an ice cover appears. Due to its crystalline structure, ice has poor thermal conductivity, meaning it retains heat. The ice layer acts as a kind of heat insulator. And the water under the ice retains its heat. Thanks to the thermal insulating properties of ice, the transfer of “cold” to the lower layers of water is sharply reduced. Therefore, at least a thin layer of water almost always remains at the bottom of a reservoir, which is extremely important for the life of its inhabitants.

Thus, +4 ºС - the temperature of maximum density of water - is the temperature of survival of living organisms in a reservoir.

Use in everyday life

Mentioned above was the possibility of water pipes bursting when water freezes. To avoid damage to the water supply system at low temperatures, there should be no interruptions in the supply of warm water that flows through the heating pipes. A vehicle is exposed to a similar danger if water is left in the radiator in cold weather.

Now let's talk about the pleasant side of the unique properties of water. Ice skating is great fun for children and adults. Have you ever wondered why ice is so slippery? For example, glass is also slippery, and also smoother and more attractive than ice. But skates don't glide on it. Only ice has such a specific delightful property.

The fact is that under the weight of our weight there is pressure on the thin blade of the skate, which, in turn, causes pressure on the ice and its melting. In this case, a thin film of water is formed, against which the steel blade of the skate slides.

Difference in freezing of wax and water

Experiments show that the surface of an ice cube forms a certain bulge. This is due to the fact that freezing in the middle occurs last. And expanding during the transition to a solid state, this bulge rises even more. This can be counteracted by the hardening of wax, which, on the contrary, forms a depression. This is explained by the fact that the wax contracts after turning into a solid state. Liquids that contract uniformly when frozen form a somewhat concave surface.

To freeze water, it is not enough to cool it to the freezing point of 0 ºС; this temperature must be maintained through constant cooling.

Water mixed with salt

Adding table salt to water lowers its freezing point. It is for this reason that roads are sprinkled with salt in winter. Salt water freezes at -8°C and below, so until the temperature drops to at least this point, freezing does not occur.

The ice-salt mixture is sometimes used as a “cooling mixture” for low-temperature experiments. When ice melts, it absorbs the latent heat required for the transformation from its surroundings, thereby cooling it. This absorbs so much heat that the temperature can drop below -15 °C.

Universal solvent

Pure water (molecular formula H 2 0) has no color, no taste, no smell. The water molecule consists of hydrogen and oxygen. When other substances (soluble and insoluble in water) get into the water, it becomes polluted, which is why there is no absolutely pure water in nature. All substances that occur in nature can be dissolved in water to varying degrees. This is determined by their unique properties - solubility in water. Therefore, water is considered a “universal solvent.”

Guarantor of stable air temperature

Water heats up slowly due to its high heat capacity, but, nevertheless, the cooling process occurs much more slowly. This makes it possible for the oceans and seas to accumulate heat in the summer. The release of heat occurs in winter, due to which there is no sharp change in air temperature on the territory of our planet throughout the year. Oceans and seas are the original and natural heat accumulator on the Earth.

Surface tension

Conclusion

The fact that ice does not sink, but floats on the surface, is explained by its lower density compared to water (the specific density of water is 1000 kg/m³, of ice - about 917 kg/m³). This thesis is true not only for ice, but also for any other physical body. For example, the density of a paper boat or an autumn leaf is much lower than the density of water, which ensures their buoyancy.

However, the property of water to have a lower density in the solid state is very rare in nature, an exception to the general rule. Only metal and cast iron (an alloy of the metal iron and the nonmetal carbon) have similar properties.

Polar ice blocks and icebergs drift in the ocean, and even in drinks the ice never sinks to the bottom. We can conclude that ice does not sink in water. Why? If you think about it, this question may seem a little strange, because ice is solid and - intuitively - should be heavier than liquid. Although this statement is true for most substances, water is an exception to the rule. What distinguishes water and ice are hydrogen bonds, which make ice lighter in its solid state than when it is in its liquid state.

Scientific question: why does ice not sink in water?

Let's imagine that we are in a lesson called “The world around us” in 3rd grade. “Why doesn’t ice sink in water?” the teacher asks the children. And kids, without deep knowledge of physics, begin to reason. “Perhaps this is magic?” - says one of the children.

Indeed, the ice is extremely unusual. There are practically no other natural substances that, in a solid state, could float on the surface of a liquid. This is one of the properties that makes water such an unusual substance and, frankly, it is what changes the path of planetary evolution.

There are some planets that contain huge amounts of liquid hydrocarbons such as ammonia - however, when this material freezes, it sinks to the bottom. The reason why ice does not sink in water is that when water freezes, it expands, and at the same time its density decreases. Interestingly, the expansion of ice can break the stones - the process of glaciation of water is so unusual.

Scientifically speaking, the freezing process sets up rapid weathering cycles and certain chemicals released on the surface can dissolve minerals. In general, the freezing of water is associated with processes and possibilities that the physical properties of other liquids do not suggest.

Density of ice and water

Thus, the answer to the question of why ice does not sink in water but floats on the surface is that it has a lower density than liquid - but this is a first-level answer. To better understand, you need to know why ice has low density, why things float in the first place, and how density causes float.

Let's remember the Greek genius Archimedes, who found out that after immersing a certain object in water, the volume of water increases by a number equal to the volume of the immersed object. In other words, if you place a deep dish on the surface of water and then place a heavy object in it, the volume of water that pours into the dish will be exactly equal to the volume of the object. It does not matter whether the object is fully or partially immersed.

Properties of water

Water is an amazing substance that mainly nourishes life on earth, because every living organism needs it. One of the most important properties of water is that it is at its highest density at 4°C. Thus, hot water or ice is less dense than cold water. Less dense substances float on top of denser substances.

For example, when preparing a salad, you may notice that the oil is on the surface of the vinegar - this can be explained by the fact that it has a lower density. The same law is also valid to explain why ice does not sink in water, but does sink in gasoline and kerosene. It’s just that these two substances have a lower density than ice. So, if you throw an inflatable ball into a pool, it will float on the surface, but if you throw a stone into the water, it will sink to the bottom.

What changes happen to water when it freezes?

The reason why ice does not sink in water is due to hydrogen bonds, which change when water freezes. As you know, water consists of one oxygen atom and two hydrogen atoms. They are attached by covalent bonds that are incredibly strong. However, another type of bond that forms between different molecules, called a hydrogen bond, is weaker. These bonds form because positively charged hydrogen atoms are attracted to the negatively charged oxygen atoms of neighboring water molecules.

When the water is warm, the molecules are very active, move around a lot, and quickly form and break bonds with other water molecules. They have the energy to get closer to each other and move quickly. So why doesn't ice sink in water? Chemistry hides the answer.

Physico-chemistry of ice

As the water temperature drops below 4°C, the kinetic energy of the liquid decreases, so the molecules no longer move. They do not have the energy to move and break and form bonds as easily as at high temperatures. Instead, they form more hydrogen bonds with other water molecules to form hexagonal lattice structures.

They form these structures to keep the negatively charged oxygen molecules away from each other. In the middle of the hexagons formed as a result of the activity of molecules, there is a lot of emptiness.

Ice sinks in water - reasons

Ice is actually 9% less dense than liquid water. Therefore, ice takes up more space than water. Practically, this makes sense because ice expands. This is why it is not recommended to freeze a glass bottle of water - frozen water can create large cracks even in concrete. If you have a liter bottle of ice and a liter bottle of water, then the ice water bottle will be lighter. The molecules are further apart at this point than when the substance is in a liquid state. This is why ice does not sink in water.

As ice melts, the stable crystalline structure breaks down and becomes denser. When water warms up to 4°C, it gains energy and the molecules move faster and further. This is why hot water takes up more space than cold water and floats on top of cold water - it is less dense. Remember, when you are on a lake, while swimming, the top layer of water is always pleasant and warm, but when you put your feet deeper, you feel the cold of the lower layer.

The importance of the freezing process of water in the functioning of the planet

Despite the fact that the question “Why doesn’t ice sink in water?” for grade 3, it is very important to understand why this process occurs and what it means for the planet. Thus, the buoyancy of ice has important consequences for life on Earth. Lakes freeze in cold places during the winter, allowing fish and other aquatic animals to survive under a blanket of ice. If the bottom were frozen, there is a high probability that the entire lake could be frozen.

Under such conditions, not a single organism would remain alive.

If the density of ice were higher than the density of water, then the ice in the oceans would sink, and the ice caps, which in this case would be at the bottom, would not allow anyone to live there. The bottom of the ocean would be full of ice - and what would it all turn into? Among other things, polar ice is important because it reflects light and protects planet Earth from overheating.

Ice and water.
It is known that a piece of ice placed in a glass of water does not sink. This happens because a buoyant force acts on the ice from the water.

Rice. 4.1. Ice in the water.

As can be seen from Fig. 4.1, the buoyant force is the resultant of water pressure forces acting on the surface of the submerged part of the ice (shaded area in Fig. 4.1). Ice floats on water because the force of gravity pulling it to the bottom is balanced by the buoyant force.
Let's imagine that there is no ice in the glass, and the shaded area in the figure is filled with water. Here there will be no interface between water located within this area and outside it. However, in this case, the buoyant force and the force of gravity acting on the water contained in the shaded area balance each other. Since in both cases discussed above the buoyancy force remains unchanged, this means that the force of gravity acting on a piece of ice and on water within the above region is the same. In other words, they have equal weight. It is also true that the mass of ice is equal to the mass of water in the shaded area.
Having melted, the ice will turn into water of the same mass and fill a volume equal to the volume of the shaded area. Therefore, the water level in a glass with water and a piece of ice will not change after the ice melts.
Liquid and solid states.
Now we know that the volume of a piece of ice is greater than the volume occupied by water of equal mass. The ratio of the mass of a substance to the volume it occupies is called the density of the substance. Therefore, the density of ice is less than the density of water. Their numerical values, measured at 0 °C, are: for water - 0.9998, for ice - 0.917 g/cm3. When heated, not only ice, but also other solids reach a certain temperature at which their transition to the liquid state begins. If a pure substance melts, its temperature will not begin to increase when heated until its entire mass has passed into a liquid state. This temperature is called the melting point of a given substance. Once melting is complete, heating will cause the temperature of the liquid to rise further. If a liquid is cooled, lowering the temperature to the melting point, it will begin to transform into a solid state.
For most substances, unlike the case with ice and water, the density in the solid state is higher than in the liquid state. For example, argon, usually in a gaseous state, solidifies at a temperature of -189.2 °C; the density of solid argon is 1.809 g/cm3 (in the liquid state the density of argon is 1.38 g/cm3). So, if we compare the density of a substance in the solid state at a temperature close to the melting point with its density in the liquid state, it turns out that in the case of argon it decreases by 14.4%, and in the case of sodium - by 2.5%.
The change in the density of a substance upon passing through the melting point for metals is usually small, with the exception of aluminum and gold (0 and 5.3%, respectively). For all these substances, unlike water, the solidification process begins not on the surface, but on the bottom.
There are, however, metals whose density decreases upon transition to the solid state. These include antimony, bismuth, gallium, for which this decrease is, respectively, 0.95, 3.35 and 3.2%. Gallium, whose melting point is -29.8 °C, together with mercury and cesium belongs to the class of fusible metals.
Difference between solid and liquid states of matter.
In the solid state, unlike the liquid state, the molecules that make up the substance are arranged in an orderly manner.

Rice. 4.2. Difference between liquid and solid states of matter

In Fig. Figure 4.2 (right) shows an example of a dense packing of molecules (conventionally depicted in circles), characteristic of a substance in the solid state. Next to it is a disordered structure characteristic of a liquid. In a liquid state, molecules are located at greater distances from each other, have greater freedom of movement, and, as a result, a substance in a liquid state easily changes its shape, that is, it has the property of fluidity.
Fluid substances, as noted above, are characterized by a random arrangement of molecules, but not all substances with such a structure are capable of flow. An example is glass, the molecules of which are arranged randomly, but it does not have fluidity.
Crystalline substances are substances whose molecules are arranged in an orderly manner. In nature, there are substances whose crystals have a characteristic appearance. These include quartz and ice. Hard metals such as iron and lead do not occur in nature in the form of large crystals. However, by studying their surface under a microscope, it is possible to distinguish clusters of small crystals, as can be seen in the photograph (Fig. 4.3).

Rice. 4.3. Microphotograph of the surface of iron.

There are special methods that make it possible to obtain large crystals of metallic substances.
Whatever the size of the crystals, what they all have in common is an ordered arrangement of molecules. They are also characterized by the existence of a completely definite melting point. This means that the temperature of a melting body does not increase when heated until it completely melts. Glass, unlike crystalline substances, does not have a specific melting point: when heated, it gradually softens and turns into an ordinary liquid. Thus, the melting point corresponds to the temperature at which the ordered arrangement of molecules is destroyed and the crystal structure becomes disordered. In conclusion, we note another interesting property of glass, explained by its lack of a crystalline structure: by applying a long-term tensile force to it, for example, for a period of 10 years, we will be convinced that the glass flows like an ordinary liquid.
Packaging of molecules.
Using X-rays and electron beams, it is possible to study how molecules are arranged in a crystal. X-rays have a much shorter wavelength than visible light, so they can be diffracted by a geometrically regular crystalline structure of atoms or molecules. By recording a diffraction pattern on a photographic plate (Fig. 4.4), it is possible to establish the arrangement of atoms in the crystal. Using the same method for liquids, you can make sure that the molecules in them are arranged in a disorderly manner.

Rice. 4.4. X-ray diffraction by a periodic structure.
Rice. 4.5. Two ways to tightly pack balls.

The molecules of a solid in a crystalline state are arranged in a rather complex manner relative to each other. The structure of substances consisting of atoms or molecules of the same type looks relatively simple, such as the argon crystal shown in Fig. 4.5 (left), where atoms are conventionally designated by balls. You can densely fill a certain amount of space with balls in various ways. Such dense packing is possible due to the presence of intermolecular attractive forces, which tend to arrange the molecules so that the volume they occupy is minimal. However, in reality the structure in Fig. 4.5 (right) does not occur; It is not easy to explain this fact.
Since it is quite difficult to imagine different ways of placing balls in space, let us consider how coins can be tightly arranged on a plane.

Rice. 4.6. Orderly arrangement of coins on a plane.

In Fig. 4.6 shows two such methods: in the first, each molecule is in contact with four neighboring ones, the centers of which are the vertices of a square with side d, where d is the diameter of the coin; with the second, each coin comes into contact with six neighboring ones. The dotted lines in the figure indicate the area occupied by one coin. In the first case
it is equal to d 2, and again this area is smaller and equal to √3d 2 /2.
The second method of placing coins significantly reduces the gap between them.
Molecule inside a crystal. The purpose of studying crystals is to determine how the molecules are arranged in them. Crystals of metals such as gold, silver, and copper are structured similarly to argon crystals. In the case of metals, we should talk about the ordered arrangement of ions, not molecules. A copper atom, for example, loses one electron and becomes a negatively charged copper ion. Electrons move freely between ions. If the ions are conventionally represented as spheres, we obtain a structure characterized by close packing. Crystals of metals such as sodium and potassium are somewhat different in structure from copper. Molecules of CO 2 and organic compounds, consisting of different atoms, cannot be represented in the form of balls. When they turn into a solid state, they form an extremely complex crystalline structure.

Rice. 4.7. Dry ice crystal (large large balls - carbon atoms)

In Fig. Figure 4.7 shows crystals of solid CO2, called dry ice. Diamond, which is not a chemical compound, also has a special structure, since chemical bonds are formed between carbon atoms.
Liquid density. Upon transition to the liquid state, the molecular structure of the substance becomes disordered. This process can be accompanied by both a decrease and an increase in the volume occupied by a given substance in space.


Rice. 4.8. Brick models corresponding to the structure of water and solids.

As an illustration, consider what is shown in Fig. 4.8 brick building. Let each brick correspond to one molecule. A brick building destroyed by an earthquake turns into a pile of bricks, the dimensions of which are smaller than the size of the building. However, if all the bricks are neatly stacked one to one, the amount of space they occupy will become even smaller. A similar relationship exists between the density of a substance in the solid and liquid states. Crystals of copper and argon can be matched to the dense packing of bricks shown. The liquid state in them corresponds to a pile of bricks. The transition from solid to liquid under these conditions is accompanied by a decrease in density.
At the same time, the transition from a crystalline structure with large intermolecular distances (which corresponds to a brick building) to a liquid state is accompanied by an increase in density. However, in reality, many crystals retain large intermolecular distances during the transition to the liquid state.
Antimony, bismuth, gallium and other metals, unlike sodium and copper, are not characterized by dense packing. Due to the large interatomic distances during the transition to the liquid phase, their density increases.

Ice structure.
A water molecule consists of an oxygen atom and two hydrogen atoms located on opposite sides of it. Unlike a carbon dioxide molecule, in which a carbon atom and two oxygen atoms are located along one straight line, in a water molecule the lines connecting the oxygen atom to each of the hydrogen atoms form an angle of 104.5° with each other. Therefore, there are interaction forces between water molecules that are electrical in nature. In addition, due to the special properties of the hydrogen atom, when water crystallizes, it forms a structure in which each molecule is connected to four neighboring ones. This structure is presented in a simplified manner in Fig. 4.9. Large balls represent oxygen atoms, small black balls represent hydrogen atoms.

Rice. 4.9. Crystal structure of ice.

In this structure, large intermolecular distances are realized. Therefore, when ice melts and the structure collapses, the volume per molecule decreases. This leads to the fact that the density of water is higher than the density of ice and ice can float on water.

Study 1
WHY IS THE DENSITY OF WATER HIGHEST AT 4 °C?

Hydrogen bonding and thermal expansion. When ice melts, it turns into water, which has a higher density than ice. With a further increase in water temperature, its density increases until the temperature reaches 4 °C. If at 0°C the density of water is 0.99984 g/cm3, then at 4°C it is 0.99997 g/cm3. A further increase in temperature causes a decrease in density and at 8°C it will again have the same value as at 0°C.

Rice. 4.10. Crystal structure of ice (large balls are oxygen atoms).

This phenomenon is due to the presence of a crystalline structure in ice. It is shown in Fig. 1 with all the details. 4.10, where for clarity, atoms are depicted as balls, and chemical bonds are indicated by solid lines. A feature of the structure is that the hydrogen atom is always located between two oxygen atoms, being located closer to one of them. Thus, the hydrogen atom promotes the adhesion force between two neighboring water molecules. This adhesive force is called hydrogen bonding. Since hydrogen bonds occur only in certain directions, the arrangement of water molecules in a piece of ice is close to tetrahedral. When ice melts and turns into water, a significant part of the hydrogen bonds are not destroyed, due to which a structure close to tetrahedral with its characteristic large intermolecular distances is preserved. With increasing temperature, the speed of translational and rotational movement of molecules increases, as a result of which hydrogen bonds are broken, the intermolecular distance decreases and the density of water increases.
However, parallel to this process, as the temperature increases, thermal expansion of water occurs, which causes a decrease in its density. The influence of these two factors leads to the fact that the maximum density of water is achieved at 4 °C. At temperatures above 4°C, the factor associated with thermal expansion begins to dominate and the density decreases again.

Study 2
ICE AT LOW TEMPERATURES OR HIGH PRESSURES

Varieties of ice. Since the intermolecular distances increase during water crystallization, the density of ice is less than the density of water. If a piece of ice is subjected to high pressure, one can expect that the intermolecular distance will decrease. Indeed, by exposing ice at 0°C to a pressure of 14 kbar (1 kbar = 987 atm), we obtain ice with a different crystal structure, the density of which is 1.38 g/cm3. If water under such pressure is cooled at a certain temperature, it will begin to
crystallize. Since the density of such ice is higher than that of water, the crystals cannot stay on its surface and sink to the bottom. Thus, the water in the vessel crystallizes, starting from the bottom. This type of ice is called ice VI; regular ice - ice I.
At a pressure of 25 kbar and a temperature of 100 ° C, water solidifies, turning into ice VII with a density of 1.57 g/cm3.

Rice. 4.11. State diagram of water.

By changing temperature and pressure, you can get 13 varieties of ice. The areas of parameter change are shown in the state diagram (Fig. 4.11). From this diagram you can determine which type of ice corresponds to a given temperature and pressure. Solid lines correspond to temperatures and pressures at which two different ice structures coexist. Ice VIII has the highest density of 1.83 g/cm3 among all types of ice.
At a relatively low pressure, 3 kbar, there is ice II, the density of which is also higher than that of water, and is 1.15 g/cm3. It is interesting to note that at a temperature of -120 °C the crystalline structure disappears and the ice turns into a glassy state.
As for water and ice I, the diagram shows that as pressure increases, the melting point decreases. Since the density of water is higher than that of ice, the ice-water transition is accompanied by a decrease in volume, and externally applied pressure only accelerates this process. For ice III, whose density is higher than that of water, the situation is exactly the opposite - its melting point increases as the pressure increases.

No one doubts that ice floats on water; everyone has seen this hundreds of times both on the pond and on the river.

But how many people have thought about this question: do all solids behave the same way as ice, that is, float in the liquids formed when they melt?

Melt paraffin or wax in a jar and throw another piece of the same solid substance into this liquid, it will immediately sink. The same will happen with lead, and with tin, and with many other substances. It turns out that, as a rule, solids always sink in liquids that are formed when they melt.

Handling water most often, we are so accustomed to the opposite phenomenon that we often forget this property, characteristic of all other substances. It must be remembered that water is a rare exception in this regard. Only the metal bismuth and cast iron behave in the same way as water.

If ice were heavier than water and did not stay on its surface, but sank, then even in deep reservoirs the water would freeze completely in winter. In fact, ice falling to the bottom of the pond would displace the lower layers of water upward, and this would happen until all the water turned into ice.

However, when water freezes, the opposite occurs. The moment water turns to ice, its volume suddenly increases by about 10 percent, making ice less dense than water. That is why it floats in water, just as any body floats in a liquid of high density: an iron nail in mercury, a cork in oil, etc. If we assume the density of water to be equal to unity, then the density of ice will be only 0.91. This figure allows us to find out the thickness of the ice floe floating on the water. If the height of the ice floe above the water is, for example, 2 centimeters, then we can conclude that the underwater layer of the ice floe is 9 times thicker, that is, equal to 18 centimeters, and the entire ice floe is 20 centimeters thick.

In the seas and oceans there are sometimes huge ice mountains - icebergs (Fig. 4). These are glaciers that have slid down from the polar mountains and been carried by the current and wind into the open sea. Their height can reach 200 meters, and their volume can reach several million cubic meters. Nine-tenths of the iceberg's total mass is hidden under water. Therefore, meeting him is very dangerous. If the ship does not notice the moving ice giant in time, it may suffer serious damage or even die in a collision.

The sudden increase in volume during the transition of liquid water into ice is an important feature of water. This feature often has to be taken into account in practical life. If you leave a barrel of water in the cold, the water will freeze and burst the barrel. For the same reason, you should not leave water in the radiator of a car parked in a cold garage. In severe frosts, you need to be wary of the slightest interruption in the supply of warm water through water heating pipes: the water that has stopped in the outer pipe can quickly freeze, and then the pipe will burst.

Freezing in rock cracks, water often causes mountain collapses.

Let us now consider one experiment that is directly related to the expansion of water when heated. Staging this experiment requires special equipment, and it is unlikely that any reader can reproduce it at home. Yes, this is not a necessity; The experience is easy to imagine, and we will try to confirm its results using examples that are familiar to everyone.

Let’s take a very strong metal, preferably a steel cylinder (Fig. 5), pour some shot into the bottom, fill it with water, secure the lid with bolts and begin turning the screw. Since water compresses very little, you won’t have to turn the screw for a long time. After just a few revolutions, the pressure inside the cylinder rises to hundreds of atmospheres. If you now cool the cylinder even 2-3 degrees below zero, the water in it will not freeze. But how can you be sure of this? If we open the cylinder, then at this temperature and atmospheric pressure the water will instantly turn into ice, and we will not know whether it was liquid or solid when it was under pressure. The sprinkled pellets will help us here. When the cylinder has cooled, turn it upside down. If the water is frozen, the shot will lie at the bottom; if it is not frozen, the shot will collect at the lid. Let's unscrew the screw. The pressure will drop and the water will definitely freeze. After removing the lid, we make sure that all the shot has collected near the lid. This means that water under pressure did not freeze at temperatures below zero.

Experience shows that the freezing point of water decreases with increasing pressure by approximately one degree for every 130 atmospheres.

If we began to base our reasoning on the basis of observations of many other substances, we would have to come to the opposite conclusion. Pressure usually helps liquids solidify: under pressure, liquids freeze at a higher temperature, and this is not surprising if you remember that most substances decrease in volume when they solidify. Pressure causes a decrease in volume and this facilitates the transition of liquid to solid state. When water hardens, as we already know, it does not decrease in volume, but, on the contrary, expands. Therefore, pressure, preventing the expansion of water, lowers its freezing point.

It is known that in the oceans at great depths the water temperature is below zero degrees, and yet the water at these depths does not freeze. This is explained by the pressure created by the upper layers of water. A layer of water one kilometer thick presses with a force of about one hundred atmospheres.

If water were a normal liquid, we would hardly experience the pleasure of skating on ice. It would be the same as rolling on perfectly smooth glass. Skates do not slip on glass. It's a completely different matter on ice. Skating on ice is very easy. Why? Under the weight of our body, the thin blade of the skate produces quite strong pressure on the ice, and the ice under the skate melts; a thin film of water is formed, which serves as an excellent lubricant.