Water is not only one of the most necessary, but also the most amazing phenomena on our planet.
It is known that virtually all substances of natural or artificial origin are capable of being in different states of aggregation and change them depending on conditions environment. And although scientists know more than a dozen phase states, some of which can only be obtained within the laboratory, in nature only three such states are most often found: liquid, solid and gaseous. Water can exist in all three of these states, changing from one to another under natural conditions.
The water in liquid state has weak bound molecules who are in constant movement and try to group themselves into a structure, but are unable to do so due to the heat. In this form, water can take on absolutely any shape, but is not able to hold it on its own. When heated, the molecules begin to move much faster, they move away from each other, and when water gradually turns into a gaseous state, that is, turns into water vapor, the bonds between the molecules are finally broken. When water is exposed to low temperatures, the movement of molecules slows down greatly, molecular bonds become very strong and the molecules, no longer hampered by heat, are ordered into a crystalline 
structure having a hexagonal shape. We have all seen similar hexagons falling to the ground in the form of snowflakes. The process of turning water into ice is called crystallization or freezing. In a solid state, water can retain any form it takes for a long time.
The process of water crystallization begins at a temperature of 0 degrees on the Celsius scale, which has 100 units. This measuring system used in many European countries and the CIS. In America, temperature is measured using the Fahrenheit scale, which has 180 divisions. Along it, water passes from liquid to solid at 32 degrees.
However, water does not always freeze at these temperatures; very clean water can be supercooled to a temperature of -40 °C and it will not freeze. The point is that in very clean water there are no impurities that serve as the basis for the construction crystal structure. The impurities to which the molecules are attached can be dust particles, dissolved salts, etc.
A special feature of water is the fact that while other substances shrink when frozen, it, having transformed into ice, on the contrary, expands. This happens because when water changes from liquid to solid, the distance between its molecules increases slightly. And since ice has a lower density than water, it floats on its surface.
Speaking about the freezing of water, one cannot fail to mention the interesting fact that hot water freezes faster than cold water, no matter how paradoxical it may sound. This phenomenon was known back in the time of Aristotle, but neither famous philosopher, nor his followers managed to solve this mystery and the phenomenon was forgotten for a long time. long years. People started talking about it again only in 1963, when a schoolboy from Tanzania, Erasto Mpemba, noticed that when preparing ice cream, a delicacy made from heated milk hardens faster. The boy told his physics teacher about this, but he laughed at him. Only in 1969, having met physics professor Dennis Osborne, the young man was able to find confirmation of his guess after joint experiments. Since then, many hypotheses have been put forward regarding this phenomenon, for example that hot water freezes faster due to its rapid evaporation, which leads to a decrease in the volume of water and, as a result, faster freezing. But none of them could explain the nature of this phenomenon.
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Alexandra
24.08.2017 12:05
11.03.2015 21:11
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The crystallization temperature of water during this period of time corresponds to the pp pressure. There's a shift happening phase equilibrium and some of the unfrozen water turns into ice. The system again temporarily acquires a state that is stable at a given temperature (section 5 - 6), then the process is repeated t (section 6 - 7), but the pressure value p decreases to the value p and remains constant. This indicates the cessation of the relaxation process.
When determining the crystallization temperature of water and salt solution, it is necessary to avoid overcooling.
At negative pressures The crystallization temperature of water should increase, but for the achieved stretching the temperature shift is insignificant. In addition, for other substances, the derivative dTldps, related to liquid-crystal equilibrium, has opposite sign. IN pure substances There are no special pre-crystallization phenomena, so a decrease in strength can be caused by the influence of impurities, for example, the release of gas bubbles.
In this work, the highest temperature is the crystallization temperature of water. To set up, place the thermometer in a porcelain glass with a mixture of crushed ice and distilled water. The mercury level in the capillary should be within the range of 3 5 - 4 5 degrees.
Combating freezing of cargo through the use of preventive agents in the form of additives or coating with them internal surfaces carriages can be achieved by searching for substances that reduce the temperature of crystallization of water, selecting surfactant hydrophobic substances and reducing the degree of dispersion of materials subject to freezing. When choosing preventive measures, the main condition is that the reagents used must have a greater chemical affinity with the particles of the material compared to the chemical affinity of water for it. Hydrophobic substances include substances that do not have a chemical affinity for water.
The pipe (pipe) with welded plugs is filled with water and cooled under natural conditions or in refrigeration units. At the crystallization temperature of water, the volume increases and, as a result, stresses arise in the pipe wall. The voltage value is regulated by the amount of water poured. Such pipe tests do not require special equipment and are practically safe. A crack in such a pipe propagates in a constant stress field mainly due to the energy of elastic deformation of the wall. It is these destructions that are characteristic of avalanche destruction of real gas pipelines. In addition, it becomes possible to create AC voltage along the length of the pipe due to the use of a special conical insert installed inside the pipe. This allows you to set the values of critical stresses in the wall to initiate the initiation of a crack and stop a propagating crack in any obviously fixed zones of the welded joint of pipes.
Atmospheric conditions play the main role in the occurrence of carburetor icing, and the gasoline used affects the degree of cooling of the fuel-air mixture. However, anti-icing additives can be added to gasoline, which either reduce the crystallization temperature of water or, having surface-active properties, prevent ice from settling on carburetor parts.
Record thermometer readings for 4 minutes every 20 seconds with occasional stirring. Mark on the graph the temperature at which the first ice crystals appear - this is the crystallization temperature of water. Sometimes the water becomes supercooled, and when ice appears, its temperature rises slightly and then begins to fall again.
In this case, when ice appears, the water temperature rises. The crystallization temperature of water should be considered the temperature that will be established after such a jump. To do this, remove the test tube from the device and heat it with your hand until the ice crystals dissolve.
The 0B curve expresses the equilibrium in the two-phase water-ice system. At atmospheric pressure this equilibrium is established, as is known, at a temperature of 0 C. Since water increases in volume during crystallization, an increase in pressure in accordance with Le Chatelier’s principle promotes the melting of ice and lowers the crystallization temperature of water. At point O, all three curves that characterize equilibria in the indicated two-phase systems intersect. This point corresponds to the equilibrium between all three phases: ice - water - steam and is called the triple point.
Physical methods- thermal (boiling), distillation and freezing. Temporary hardness salts are removed thermally. Distilled water, which does not contain salts, is obtained by distillation in special distillation units. Freezing is based on the difference in the crystallization temperatures of water and impurities.
The transition of a substance from a solid crystalline state to a liquid is called melting. To melt a solid crystalline body, it must be heated to a certain temperature, that is, heat must be supplied.The temperature at which a substance melts is calledmelting point of the substance.
The reverse process—the transition from a liquid to a solid state—occurs when the temperature decreases, i.e., heat is removed. The transition of a substance from a liquid to a solid state is calledhardening , or crystallization . The temperature at which a substance crystallizes is calledcrystal temperaturetions .
Experience shows that any substance crystallizes and melts at the same temperature.
The figure shows a graph of temperature dependence crystalline body(ice) from heating time (from point A to the point D) and cooling time (from point D to the point K). It shows time along the horizontal axis, and temperature along the vertical axis.
The graph shows that observation of the process began from the moment when the ice temperature was -40 °C, or, as they say, the temperature was starting moment time tbeginning= -40 °C (point A on the graph). With further heating, the temperature of the ice increases (on the graph this is the section AB). The temperature increases to 0 °C - the melting temperature of ice. At 0°C, ice begins to melt and its temperature stops rising. During the entire melting time (i.e. until all the ice is melted), the temperature of the ice does not change, although the burner continues to burn and heat is, therefore, supplied. The melting process corresponds to the horizontal section of the graph Sun . Only after all the ice has melted and turned into water does the temperature begin to rise again (section CD). After the water temperature reaches +40 °C, the burner is extinguished and the water begins to cool, i.e., heat is removed (to do this, you can place a vessel with water in another, larger vessel with ice). The water temperature begins to decrease (section DE). When the temperature reaches 0 °C, the water temperature stops decreasing, despite the fact that heat is still removed. This is the process of water crystallization - ice formation (horizontal section E.F.). Until all the water turns into ice, the temperature will not change. Only after this does the ice temperature begin to decrease (section FK).
The type of graph considered is explained in the following way. Location on AB Due to the heat supplied, the average kinetic energy of ice molecules increases, and its temperature rises. Location on Sun all the energy received by the contents of the flask is spent on the destruction of the ice crystal lattice: the ordered spatial arrangement of its molecules is replaced by a disordered one, the distance between the molecules changes, i.e. The molecules are rearranged in such a way that the substance becomes liquid. The average kinetic energy of the molecules does not change, so the temperature remains unchanged. Further increase in the temperature of molten ice-water (in the area CD) means an increase in the kinetic energy of water molecules due to the heat supplied by the burner.
When cooling water (section DE) part of the energy is taken away from it, water molecules move at lower speeds, their average kinetic energy drops - the temperature decreases, the water cools. At 0°C (horizontal section E.F.) molecules begin to line up in in a certain order, forming crystal lattice. Until this process is completed, the temperature of the substance will not change, despite the heat being removed, which means that when solidifying, the liquid (water) releases energy. This is exactly the energy that the ice absorbed, turning into liquid (section Sun). The internal energy of a liquid is greater than that of solid. Upon melting (and crystallization) internal energy bodies change abruptly.
Metals that melt at temperatures above 1650 ºС are called refractory(titanium, chromium, molybdenum, etc.). Tungsten has the highest melting point among them - about 3400 ° C. Refractory metals and their compounds are used as heat-resistant materials in aircraft manufacturing, rocketry and space technology, nuclear energy.
Let us emphasize once again that when melting, a substance absorbs energy. During crystallization, on the contrary, it releases it into the environment. Receiving a certain amount of heat released during crystallization, the medium heats up. This is well known to many birds. No wonder they can be seen in winter in frosty weather sitting on the ice that covers rivers and lakes. Due to the release of energy when ice forms, the air above it is several degrees warmer than in the trees in the forest, and birds take advantage of this.
Melting of amorphous substances.
Availability of a certain melting points- This important sign crystalline substances. It is by this feature that they can be easily distinguished from amorphous bodies, which are also classified as solids. These include, in particular, glass, very viscous resins, and plastics.
Amorphous substances(unlike crystalline ones) do not have a specific melting point - they do not melt, but soften. When heated, a piece of glass, for example, first becomes soft from hard, it can easily be bent or stretched; with more high temperature the piece begins to change its shape under the influence of its own gravity. As it heats up, the thick viscous mass takes the shape of the vessel in which it lies. This mass is first thick, like honey, then like sour cream, and finally becomes almost the same low-viscosity liquid as water. However, indicate a certain temperature the transition of a solid into a liquid is impossible here, since it does not exist.
The reasons for this lie in the fundamental difference in the structure of amorphous bodies from the structure of crystalline ones. Atoms in amorphous bodies are arranged randomly. Amorphous bodies their structure resembles liquids. Already in solid glass, the atoms are arranged randomly. This means that increasing the temperature of glass only increases the range of vibrations of its molecules, giving them gradually greater and greater freedom of movement. Therefore, the glass softens gradually and does not exhibit a sharp “solid-liquid” transition, characteristic of the transition from the arrangement of molecules in in strict order to the disorderly.
Heat of fusion.
Heat of Melting- this is the amount of heat that must be imparted to a substance at constant pressure and constant temperature, equal temperature melting to completely transform it from a solid crystalline state to a liquid. The heat of fusion is equal to the amount of heat that is released during the crystallization of a substance from the liquid state. During melting, all the heat supplied to a substance goes to increase the potential energy of its molecules. The kinetic energy does not change since melting occurs at a constant temperature.
Experientially studying melting various substances of the same mass, you can see that to turn them into liquid it takes different quantities warmth. For example, in order to melt one kilogram of ice, you need to expend 332 J of energy, and in order to melt 1 kg of lead - 25 kJ.
The amount of heat released by the body is considered negative. Therefore, when calculating the amount of heat released during the crystallization of a substance with a mass m, you should use the same formula, but with a minus sign:
Heat of combustion.
Heat of combustion(or calorific value , calorie content) is the amount of heat released during complete combustion of fuel.
To heat bodies, the energy released during the combustion of fuel is often used. Conventional fuel (coal, oil, gasoline) contains carbon. During combustion, carbon atoms combine with oxygen atoms in the air to form carbon dioxide molecules. The kinetic energy of these molecules turns out to be greater than that of the original particles. Increase kinetic energy molecules during combustion are called energy release. The energy released during complete combustion of fuel is the heat of combustion of this fuel.
The heat of combustion of fuel depends on the type of fuel and its mass. The greater the mass of the fuel, the greater the amount of heat released during its complete combustion.
Physical quantity showing how much heat is released during complete combustion of fuel weighing 1 kg is called specific heat of combustion of fuel.The specific heat of combustion is designated by the letterqand is measured in joules per kilogram (J/kg).
Quantity of heat Q released during combustion m kg of fuel is determined by the formula:
To find the amount of heat released during complete combustion of fuel of an arbitrary mass, you need specific heat combustion of this fuel multiplied by its mass.
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Introduction
Water is not only one of the most necessary, but also the most amazing phenomena on our planet. The role of water in the emergence and maintenance of life on Earth is extremely important, in chemical structure living organisms, in the formation of climate and weather. Water is the most important substance for all living beings on Earth.
Introduction
Most of the Earth's surface is covered with water (oceans, seas, lakes, rivers, ice). On Earth, approximately 96.5% of water comes from the oceans, 1.7% of the world's reserves are groundwater, another 1.7% are glaciers and ice caps of Antarctica and Greenland, a small part is in rivers, lakes and swamps, and 0.001% in clouds, which are formed from airborne particles of ice and liquid water.
Water at normal conditions is in a liquid state, but at a temperature of 0 ° C it turns into solid state– ice and boils (turns into water vapor) at a temperature of 100 °C.
The values of 0 °C and 100 °C were chosen to correspond to the melting temperatures of ice and the boiling temperatures of water when creating the “Celsius” temperature scale.
Ice is found in nature in the form of ice itself (continental, floating, underground), as well as in the form of snow, frost, and frost. Under the influence of its own weight, ice acquires plastic properties and fluidity.
Natural ice usually much purer than water, since when water crystallizes, water molecules are the first to form into the lattice.
The total ice reserves on Earth are about 30 million km³. The main ice reserves are concentrated in the polar caps (mainly in Antarctica, where the thickness of the ice layer reaches 4 km).
In the world's oceans, the water is salty and this prevents the formation of ice, so ice forms only in polar and subpolar latitudes, where winters are long and very cold. Some shallow seas located in the temperate zone freeze over.
In addition, there is evidence of the presence of ice on planets solar system(for example, on Mars), their satellites, on dwarf planets and in comet nuclei.
Research into the properties of water is necessary for humanity.
At the same time, the process of water crystallization can be studied at home, as well as in classes in high school.
The relevance of the work is to be used in physics lessons to familiarize students with the properties of water during crystallization.
The object of study is the crystallization of water.
The subject of the research is to study the properties of water during crystallization.
The purpose of the work is to conduct experiments on water crystallization and prepare proposals for their implementation.
The main task is to study the properties of water during crystallization.
For solutions main task necessary:
The theoretical significance of the work lies in the systematization of the basic properties of water and the significance of water crystallization for the flora and fauna of the Earth.
The practical significance of the work is the study of the process of water crystallization during experiments, as well as the preparation of proposals for conducting experiments in lessons in high school.
1. Preparation for the study
1.1 Analysis of the basic properties of water
Water is one of the most amazing substances On the Earth. You can find water almost everywhere under natural conditions, both on the surface of the planet and in its depths in three possible ways: physical conditions for substances: liquid, solid, gaseous (i.e. water, ice, water vapor).
Of course, there are substances that can be obtained in the form of a liquid, solid or gas. However, there is no such thing chemical substance, which occurs naturally in the above three physical states.
Properties of water:
- Water is a substance that has no color, no smell, no taste.
- Water is the only thing on planet Earth known to science substance found in natural conditions in three physical states: solid, liquid, gas.
- Water is a universal solvent, having the ability to dissolve large quantity salts, as well as other substances than any other substances.
- Water is very difficult to oxidize. Water is quite chemically stable, that is, decomposing it into its component parts or burning it is quite problematic.
- Almost all natural metals can be oxidized by water; especially hard rocks are also destroyed under its influence.
- Water like physical substance, is characterized by great affinity with itself. This affinity for water is the greatest among all liquids. As a result, water is placed on the surface in the form of spherical drops, since the sphere has the smallest surface area for a given volume.
- Freezing of water does not occur under temperature conditions; highest density(at 4 degrees Celsius), and at zero degrees Celsius. These are the properties of fresh water. However, freezing sea water occurs at more low temperatures: minus 1.9 degrees Celsius, with a salinity of 35%.
- Water has a very high heat capacity, but heats up relatively little. Water also has a fairly high latent heat of fusion (about 80 cal/g), as well as evaporation (about 540 cal/g). Water can absorb significant amounts of additional heat. The temperature during freezing or boiling remains unchanged.
- Distilled water practically does not conduct electricity, however, the presence of even a small amount of salts in water significantly increases its conductive properties.
Properties of snow:
- When mixing salt with snow, two processes are observed: the destruction of the crystalline structure of the salt, which occurs with the absorption of heat, and the hydration of ions. The last process occurs with the release of heat into the environment. For table salt and calcium chloride, the first process prevails over the second. Therefore, when snow is mixed with these salts, heat is actively removed from the environment. Another feature salt solutions is that their freezing point is below 0 degrees. To make the snow on the sidewalks melt at temperatures below 0 degrees, these salts are sprinkled on it.
- Snow has amazing property- memory. It keeps traces. You can, for example, study physics using the tracks. The larger the animal, the deeper its footprint, therefore, the more pressure it exerts on the snow. A dog's tracks are deeper than those of her puppies. Mice and caresses leave shallow lines. Nature has provided ungulates with the ability to spread their hooves and increase the area of support. This helps them in winter, when moving through snowy forests and fields, not to sink so deeply into the snow.
1.2 The importance of water crystallization for flora and fauna
We love snow not only because it gives us magnificent winter landscapes. Our love for snow has many rational reasons. “Snow in the fields - bread in the bins”, “Winter without snow - summer without bread,” - old Russian proverbs rightly say. Snow cover is a huge supply of moisture, much needed by the fields, and at the same time it is a kind of giant blanket that protects the surface of the earth from cold winds. Academician B.I. Vernadsky emphasized that snow cover is “not just a warm cover for winter crops, it is a life-giving cover”; in the spring it provides melt water saturated with oxygen. It is known that the amount of nitrogen compounds in the soil in summer is proportional to the height of the melted snow cover. It is not for nothing that snow reclamation is considered today as one of the most important conditions obtaining high and sustainable yields.
Snow reserves significantly affect water levels in rivers and determine climate changes over large areas.
Besides, the snow is good building material for various buildings in the north - from igloos (Eskimo dwellings) to large warehouses. There is the world's largest hotel, made entirely of ice and snow, located in Swedish Lapland, 200 kilometers from the Arctic Circle.
It serves as the basis winter roads and even airfields.
Thanks to snow, every year we admire fabulous winter landscapes, play snowballs, build snow towns, fortresses, skiing, sledding, and a wonderful New Year's holiday comes to us in the snow.
The importance of ice is difficult to underestimate. Ice has a great influence on the living conditions and life of plants and animals, different types economic activity person. Covering the water from above, ice plays in nature the role of a kind of floating screen, protecting rivers and reservoirs from further freezing and preserving life. underwater world. If the density of water increased when it froze, the ice would be heavier than water and begin to sink, which would lead to the death of all living creatures in rivers, lakes and oceans, which would freeze completely, turning into blocks of ice, and the Earth would become an ice desert, which is inevitable would lead to the death of all living things.
Ice can cause a number of natural Disasters with harmful and devastating consequences– icing aircraft, ships, structures, road surfaces and soil, hail, blizzards and snow drifts, river congestion with floods, ice avalanches, etc. Natural ice is used for storage and cooling food products, biological and medical preparations, for which it is specially produced and prepared.
1.3 Selection and justification of experiments for research
To conduct experiments with water, it is necessary to select those that most fully characterize and confirm the properties of water.
The analysis showed that this will be best achieved by performing the following experiments:
- Freezing of salt water.
- Expansion of water when freezing.
- Freezing of liquid at external influence snow.
- Freezing soap bubbles.
- Icicle fusion.
- The creaking of dry snow.
- Freezing to the surface.
2. Conducting research
2.1 Preparation of the material part
The following were taken for the experiments:
- objects - a saucepan, a glass bottle, a plastic bottle, disposable glasses, thin copper wire, a straw;
- substances - snow, icicle, salt, water, soap solution, juice.
2.2 Conducting experiments with a description of the main results
1. Freezing of salt water.
Pour water into two molds - clean and very salty. Take the molds out into the cold or put them in the freezer. You will notice that clean fresh water has turned into ice, and salt water will freeze in very severe frost.
Water freezes not at the temperature conditions of its highest density (at 4 degrees Celsius), but at zero degrees Celsius. These are the properties of fresh water.
Wherein, sea ice differs from freshwater in a number of respects. Salt water has a lower freezing point as salinity increases. In the salinity range from 30 to 35 ppm, the freezing point varies from -1.6 to -1.9 degrees. Education sea ice can be considered as the freezing of fresh water with the displacement of salts into seawater cells inside the ice column. When the temperature reaches freezing, ice crystals form and “surround” the unfrozen water.
2. Expansion of water when freezing.
Fill a plastic cup, a plastic bottle and a glass bottle with water. Leave them out in the cold. When water freezes, it increases in volume, “crawls out” of the glass, and breaks the glass bottle even when it is half filled. Plastic bottle remains without visible changes.
When water freezes, it has unique properties extensions. Thanks to these properties, ice floats on water, which is in the form of a liquid.
In winter, due to this property of water, accidents occur in water pipelines. IN very coldy The main cause of such accidents is freezing of flowing water. It expands, so that the resulting ice easily breaks the pipes, since the density of ice is 917 kg/m3, and the density of water is 1000 kg/m3, that is, the volume increases by 1.1 times, which is quite significant.
3. Freezing of liquid when exposed to external snow.
Pour juice into a plastic glass (test tube) and place it in a pan with salted snow. The juice will freeze, and very soon you will be enjoying popsicles.
When mixing salt with snow, the crystalline structure of the salt is destroyed, which occurs with the absorption of heat. Therefore, when snow is mixed with salt, heat is actively removed from the juice and the juice turns into ice.
4. Freezing of soap bubbles.
Prepare a soap solution. In cold weather, keep the solution in a mitten so that it does not freeze. Blow bubbles with a juice straw. Due to the temperature difference between inside and outside the bubble, a large lifting force arises, instantly carrying the bubbles upward. A thin film of soap freezes quickly in the cold, turning bubbles into ice balls.
Thus, the thinnest film The soap bubble freezes in a matter of seconds.
5. Icicle fusion.
Take the icicle. Throw a thin wire over it, the ends of which are weighted with weights. Watch how the wire melts the ice and penetrates deeper into the icicle. The water above the icicle freezes again.
This confirms the heat absorption property greater mass ice.
Ice grows from below, immediately above the wire, as the melt water flowing down freezes upon contact with the cold walls of the icicle.
6. The creaking of dry snow.
Pour granulated sugar into a plate and start crushing it with a spoon. You will hear a characteristic creak. Wet the sand and rub again. The creaking disappeared. On frosty days, sound travels long distances.
Snow creaks only in cold weather (below -5°C), and the sound of the creaking changes depending on the air temperature - the stronger the frost, the higher the pitch of the creaking. With enough experience, you can estimate the air temperature by the sound that creaking snow makes. The creaking is formed due to the fact that the smallest snow crystals are destroyed under pressure. Moreover, each of them individually is very small to produce a sound accessible to the human ear, but together they break quite loudly. Increasing frosts make ice crystals harder and more fragile. With every step, the ice needles break. When the air temperature is below -50°C, the creaking of snow becomes so strong that it can be heard through triple glass (this is also facilitated by high density frosty air).
7. Freezing to the surface.
Add to the pan with snow table salt in a ratio of approximately 1 to 6. Stir the mixture thoroughly. If you now want to move the pan, you will have to lift it along with the stool.
This also confirms the absorption of heat from the environment.
When salt is mixed with snow, a solution is formed, accompanied by strong cooling due to the large absorption of heat by the ice when it melts and by the salt when it dissolves. For example, the temperature of a mixture of 29 grams of salt and 100 grams of ice drops to -21°C. And if you take 143 grams of salt and 100 grams of ice, then the temperature can be lowered to -55°C.
2.3 Proposals for conducting experiments
It is advisable to conduct experiments to study the properties of water in classes in secondary and primary schools.
For students high school It is possible to conduct experiments No. 3, 6 and 7 in lessons under the guidance of a teacher, and experiments No. 4 and 5 - optionally or independently at home.
Conclusion
Thus, research into the properties of water is necessary for humanity.
The process of water crystallization can be studied at home, as well as in classes in secondary and primary schools.
In the work we managed to solve the following problems:
- Analyze the basic properties of water.
- Study the significance of water crystallization for the flora and fauna of the Earth.
- Determine the main experiments for conducting the study.
- Conduct experiments and describe the main results.
- Prepare proposals for conducting experiments in high school lessons.
The significance of the work on systematizing the basic properties of water and the significance of water crystallization for the flora and fauna of the Earth was confirmed.
The process of transformation, or more precisely, the transition of a substance from a liquid substance to a solid state is called crystallization. Most a shining example similar chemical reaction is ice. The result of the process is called a crystal.
To start the process, it is necessary to create a state of supersaturation in the solution on which the experiment is performed. Phase transition liquid flows as follows:
- The temperature level of the liquid changes.
- Part of the solvent is removed.
- A combination of the two previous actions occurs.
- From the resulting melts, the process of crystallization occurs.
Crystallization and methods for obtaining crystals from liquids
There are two methods of crystallization: isothermal and polythermal.
In the first method, the solution is subjected to intense cooling, during which crystals begin to separate, but the amount of solvent liquid remains the same.
In isothermal crystallization, the appearance of crystals occurs by evaporation. The process is named because the entire reaction occurs at a constant temperature, which is the boiling point of the solution. In practice, both methods are used together. In this case, part of the solvent evaporates by boiling, while at the same time the liquid is cooled.
There is another option for crystallization, in which substances are added to the solution that have a good ability to absorb water and reduce the susceptibility of the salt contained in the liquid to dissolution. A variant of this development of events is called salting out. In this case, drugs are used that can “bind water” (in this way sodium sulfate is crystallized, during which ammonia or alcohol is added), or they have the same ion as the salt used. An example is a chemical reaction aimed at crystallization copper sulfate or sodium chloride.
To grow a crystal, start with fine particle, called the "embryo". In other words, this is a kind of center around which, during a chemical reaction, a crystal begins to form. In this case, the process in which the formation of nuclei occurs and the crystallization process itself occurs at the same time. If this is not the case, for example, nuclei form faster, many too small crystals appear, but in the opposite case there are few of them, but bigger size.
Thanks to this property, it is possible to control the amount and speed at which crystallization occurs. This is accomplished using the following factors:
- The solution must cool quickly.
- Liquids cannot be left at rest.
- Need higher temperature.
- The molecular weight of the crystals should be low.
All of the above nuances contribute to the resulting products of small caliber; to obtain larger crystals, the following is required:
- Slow cooling.
- Liquid at rest.
- Significantly lower temperature.
- High molecular weight.
To facilitate the moment when embryos begin to form, elements are added to the solution crystalline substance, in the form of finely ground powder. In this case, the crystallization process itself occurs due to the subsequent introduction of particles of the same element. The amount of the introduced substance depends on the size of the desired crystal, for example, for a larger one, it is not used a large number of seed material.
The size of the crystals matters during their further processing; for example, large crystals are capable of releasing large amounts of moisture during washing and filtration. They dry faster, settle, and are easier to filter.
Since the main purpose of crystallization is to obtain a final substance that is ideally pure and without impurities, the resulting crystals are usually subjected to recrystallization processes, with the removal of excess impurities and repeated washing and drying.