What happens when water heats up. Properties of water in liquid state. Properties of various states of matter
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Japanese physicist Masakazu Matsumoto has put forward a theory that explains why water contracts instead of expanding when heated from 0 to 4°C. According to his model, water contains microformations - “vitrites”, which are convex hollow polyhedra, the vertices of which contain water molecules, and the edges are hydrogen bonds. As the temperature rises, two phenomena compete with each other: the elongation of hydrogen bonds between water molecules and the deformation of vitrites, leading to a decrease in their cavities. In the temperature range from 0 to 3.98°C, the latter phenomenon dominates the effect of elongation of hydrogen bonds, which ultimately gives the observed compression of water. There is no experimental confirmation of Matsumoto’s model yet, as well as other theories explaining the compression of water.
Unlike the vast majority of substances, water can reduce its volume when heated (Fig. 1), that is, it has a negative coefficient of thermal expansion. However, we are not talking about the entire temperature range where water exists in a liquid state, but only about a narrow section - from 0°C to approximately 4°C. With b O At higher temperatures, water, like other substances, expands.
By the way, water is not the only substance that has the property of contracting when temperature increases (or expanding when cooling). Bismuth, gallium, silicon and antimony can also boast of similar behavior. However, due to its more complex internal structure, as well as its prevalence and importance in various processes, it is water that attracts the attention of scientists (see The study of the structure of water continues, “Elements”, 10/09/2006).
Some time ago, the generally accepted theory answering the question of why water increases its volume as the temperature decreases (Fig. 1) was the model of a mixture of two components - “normal” and “ice-like”. This theory was first proposed in the 19th century by Harold Whiting and was later developed and improved by many scientists. Relatively recently, within the framework of the discovered polymorphism of water, Wieting’s theory was rethought. It is now believed that there are two types of ice-like nanodomains in supercooled water: high-density and low-density amorphous ice-like regions. Heating supercooled water leads to the melting of these nanostructures and the appearance of two types of water: with higher and lower density. The cunning temperature competition between the two “grades” of the resulting water gives rise to a non-monotonic dependence of density on temperature. However, this theory has not yet been confirmed experimentally.
You need to be careful with this explanation. It is no coincidence that we are talking here only about structures that resemble amorphous ice. The fact is that nanoscopic regions of amorphous ice and its macroscopic analogues have different physical parameters.
Japanese physicist Masakazu Matsumoto decided to find an explanation for the effect discussed here “from scratch,” discarding the theory of a two-component mixture. Using computer simulations, he examined the physical properties of water over a wide temperature range - from 200 to 360 K at zero pressure - to understand on a molecular scale the true reasons for the expansion of water when it cools. His article in the magazine Physical Review Letters it's called: Why Does Water Expand When It Cools? (“Why does water expand when it cools?”).
Initially, the author of the article asked the question: what affects the coefficient of thermal expansion of water? Matsumoto believes that for this it is enough to find out the influence of only three factors: 1) changes in the length of hydrogen bonds between water molecules, 2) topological index - the number of bonds per water molecule, and 3) deviation of the angle between bonds from the equilibrium value (angular distortion).
Before we talk about the results obtained by the Japanese physicist, we will make important comments and clarifications regarding the above three factors. First of all, the usual chemical formula of water, H 2 O, corresponds only to its vapor state. In liquid form, water molecules are combined into groups (H 2 O) through hydrogen bonding. x, Where x- number of molecules. The most energetically favorable combination of five water molecules ( x= 5) with four hydrogen bonds, in which the bonds form equilibrium, so-called tetrahedral angle, equal to 109.47 degrees (see Fig. 2).
Having analyzed the dependence of the length of the hydrogen bond between water molecules on temperature, Matsumoto came to the expected conclusion: an increase in temperature gives rise to a linear elongation of hydrogen bonds. And this, in turn, leads to an increase in the volume of water, that is, to its expansion. This fact contradicts the observed results, so he further examined the influence of the second factor. How does the coefficient of thermal expansion depend on the topological index?
Computer modeling gave the following result. At low temperatures, the largest volume of water in percentage terms is occupied by water clusters, which have 4 hydrogen bonds per molecule (topological index is 4). An increase in temperature causes a decrease in the number of associates with index 4, but at the same time the number of clusters with indices 3 and 5 begins to increase. Having carried out numerical calculations, Matsumoto discovered that the local volume of clusters with topological index 4 practically does not change with increasing temperature, and the change in the total volume of associates with indices 3 and 5 at any temperature mutually compensate each other. Consequently, a change in temperature does not change the total volume of water, and therefore the topological index does not have any effect on the compression of water when it is heated.
It remains to be clarified the effect of angular distortion of hydrogen bonds. And this is where the most interesting and important begins. As mentioned above, water molecules tend to unite so that the angle between the hydrogen bonds is tetrahedral. However, thermal vibrations of water molecules and interactions with other molecules not included in the cluster prevent them from doing this, deviating the hydrogen bond angle from the equilibrium value of 109.47 degrees. To somehow quantitatively characterize this process of angular deformation, Matsumoto and colleagues, building on their previous work Topological building blocks of hydrogen bond networks in water, published in 2007 in Journal of Chemical Physics, hypothesized the existence of three-dimensional microstructures in water that resemble convex hollow polyhedra. Later, in subsequent publications, they called such microstructures showcases(Fig. 3). In them, the vertices are water molecules, the role of edges is played by hydrogen bonds, and the angle between hydrogen bonds is the angle between the edges in vitrite.
According to Matsumoto's theory, there is a huge variety of forms of vitritis, which, like mosaic elements, make up the majority of the structure of water and which at the same time evenly fill its entire volume.
Water molecules tend to create tetrahedral angles in vitrites, since vitrites must have the lowest possible energy. However, due to thermal motions and local interactions with other vitrites, some microstructures do not exhibit geometries with tetrahedral angles (or angles close to this value). They accept such structurally nonequilibrium configurations (which are not the most favorable for them from an energetic point of view), which allow the entire “family” of vitrites as a whole to obtain the lowest energy value among possible ones. Such vitritis, that is, vitritis that seem to sacrifice themselves to “common energy interests,” are called frustrated. If in unfrustrated vitritis the volume of the cavity is maximum at a given temperature, then frustrated vitritis, on the contrary, have the minimum possible volume.
Computer modeling conducted by Matsumoto showed that the average volume of vitrite cavities decreases linearly with increasing temperature. In this case, frustrated vitritis significantly reduces its volume, while the volume of the cavity of unfrustrated vitritis remains almost unchanged.
So, the compression of water with increasing temperature is caused by two competing effects - the elongation of hydrogen bonds, which leads to an increase in the volume of water, and a decrease in the volume of the cavities of frustrated vitrites. In the temperature range from 0 to 4°C, the last phenomenon, as calculations have shown, prevails, which ultimately leads to the observed compression of water with increasing temperature.
It remains to wait for experimental confirmation of the existence of vitrites and their behavior. But this, alas, is a very difficult task.
In water heating systems, water is used to transfer heat from its generator to the consumer.
The most important properties of water are:
heat capacity;
change in volume during heating and cooling;
boiling characteristics when changing external pressure;
cavitation.
Let's consider these physical properties of water.
Specific heat
An important property of any coolant is its heat capacity. If we express it through the mass and temperature difference of the coolant, we get the specific heat capacity. It is denoted by the letter c and has dimension kJ/(kg K)
Specific heat- this is the amount of heat that must be transferred to 1 kg of a substance (for example, water) to heat it by 1 °C. Conversely, a substance releases the same amount of energy when cooled. The average specific heat capacity of water between 0 °C and 100 °C is:
c = 4.19 kJ/(kg K) or c = 1.16 Wh/(kg K)
Amount of heat absorbed or released Q, expressed in J or kJ, depends on mass m, expressed in kg, specific heat capacity c and temperature difference, expressed in K.
Increasing and decreasing volume
All natural materials expand when heated and contract when cooled. The only exception to this rule is water. This unique property is called water anomaly. Water has its highest density at +4 °C, at which 1 dm3 = 1 liter has a mass of 1 kg.
If water is heated or cooled relative to this point, its volume increases, which means its density decreases, i.e., the water becomes lighter. This can be clearly seen in the example of a tank with an overflow point. The tank contains exactly 1000 cm3 of water with a temperature of +4 °C. As the water heats up, some will flow out of the reservoir into the measuring cup. If you heat water to 90 °C, exactly 35.95 cm3 will pour into the measuring container, which corresponds to 34.7 g. Water also expands when it is cooled below +4 °C.
Thanks to this anomaly of water near rivers and lakes, it is the top layer that freezes in winter. For the same reason, ice floats on the surface and the spring sun can melt it. This would not happen if the ice were heavier than water and sank to the bottom.
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Reservoir with overflow point
However, this ability to expand can be dangerous. For example, car engines and water pumps can burst if the water in them freezes. To avoid this, additives are added to the water to prevent it from freezing. Glycols are often used in heating systems; Refer to manufacturer's specifications for water to glycol ratio.
Boiling characteristics of water
If water is heated in an open container, it will boil at a temperature of 100 °C. If you measure the temperature of boiling water, it will remain at 100 °C until the last drop evaporates. Thus, constant heat consumption is used to completely evaporate water, i.e. change its state of aggregation.
This energy is also called latent (latent) heat. If the heat supply continues, the temperature of the resulting steam will begin to rise again.
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The described process is given at an air pressure of 101.3 kPa at the water surface. At any other air pressure, the boiling point of water shifts from 100 °C.
If we were to repeat the experiment described above at an altitude of 3000 m - for example, on the Zugspitze, the highest peak in Germany - we would find that water there already boils at 90 °C. The reason for this behavior is the decrease in atmospheric pressure with altitude.
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The lower the pressure at the surface of the water, the lower the boiling point will be. Conversely, the boiling point will be higher as the pressure at the surface of the water increases. This property is used, for example, in pressure cookers.
The graph shows the dependence of the boiling point of water on pressure. The pressure in heating systems is intentionally increased. This helps prevent gas bubbles from forming during critical operating conditions and also prevents outside air from entering the system.
Expansion of water when heated and protection against overpressure
Water heating systems operate at water temperatures up to 90 °C. Typically the system is filled with water at 15°C, which then expands when heated. This increase in volume must not be allowed to lead to excess pressure and fluid overflow.
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When the heating is turned off in the summer, the water volume returns to its original value. Thus, to ensure unhindered expansion of water, it is necessary to install a sufficiently large tank.
Old heating systems had open expansion tanks. They were always located above the highest section of the pipeline. As the temperature in the system increased, causing the water to expand, the level in the tank also increased. As the temperature decreased, it decreased accordingly.
Modern heating systems use membrane expansion tanks (MEVs). When the pressure in the system increases, the pressure in pipelines and other elements of the system must not be allowed to increase above the limit value.
Therefore, a prerequisite for every heating system is the presence of a safety valve.
When the pressure rises above normal, the safety valve must open and release the excess volume of water that the expansion tank cannot accommodate. However, in a carefully designed and maintained system such a critical condition should never occur.
All these considerations do not take into account the fact that the circulation pump further increases the pressure in the system. The relationship between the maximum water temperature, the selected pump, the size of the expansion tank and the response pressure of the safety valve must be established with the greatest care. Random selection of system elements - even based on their cost - is unacceptable in this case.
The membrane expansion tank is supplied filled with nitrogen. The initial pressure in the expansion diaphragm tank must be adjusted depending on the heating system. Expanding water from the heating system enters the tank and compresses the gas chamber through a diaphragm. Gases can be compressed, but liquids cannot.
Pressure
Pressure determination
Pressure is the static pressure of liquids and gases, measured in vessels and pipelines relative to atmospheric pressure (Pa, mbar, bar).
Static pressure
Static pressure is the pressure of a stationary fluid.
Static pressure = level above the corresponding measuring point + initial pressure in the expansion tank.
Dynamic pressure
Dynamic pressure is the pressure of a moving fluid stream. Pump Discharge Pressure This is the pressure at the outlet of a centrifugal pump during operation.
Pressure drop
The pressure developed by a centrifugal pump to overcome the total resistance of the system. It is measured between the inlet and outlet of a centrifugal pump.
Operating pressure
The pressure available in the system when the pump is running. Allowable operating pressure The maximum value of operating pressure allowed under the conditions of safe operation of the pump and system.
Cavitation
Cavitation- this is the formation of gas bubbles as a result of the appearance of local pressure below the vaporization pressure of the pumped liquid at the inlet of the impeller. This leads to a decrease in performance (pressure) and efficiency and causes noise and destruction of the material of the internal parts of the pump. By collapsing air bubbles in higher pressure areas (such as the impeller outlet), microscopic explosions cause pressure surges that can damage or destroy a hydraulic system. The first sign of this is noise in the impeller and its erosion.
An important parameter of a centrifugal pump is NPSH (the height of the liquid column above the pump suction pipe). It defines the minimum pump inlet pressure required by a given type of pump to operate without cavitation, i.e. the additional pressure required to prevent bubbles. The NPSH value is affected by the impeller type and pump speed. External factors influencing this parameter are liquid temperature and atmospheric pressure.
Preventing Cavitation
To avoid cavitation, the liquid must enter the inlet of the centrifugal pump at a certain minimum suction height, which depends on temperature and atmospheric pressure.
Other ways to prevent cavitation are:
Increasing static pressure
Reducing liquid temperature (reducing vaporization pressure PD)
Selecting a pump with a lower constant hydrostatic head (minimum suction lift, NPSH)
Agrovodcom specialists will be happy to help you decide on the optimal choice of pump. Contact us!
Alexander
2013-10-22 09:38:26
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Topic: Inanimate nature
Lesson: Properties of liquid water
In its pure form, water has no taste, smell or color, but it is almost never like that, because it actively dissolves most substances in itself and combines with their particles. Water can also penetrate into various bodies (scientists have found water even in stones).
Chlorine has a weak point: it can react to form chloramines and chlorinated hydrocarbons, which are dangerous carcinogens. A by-product of this reaction is chlorite. Toxicology studies have shown that the disinfection byproduct of chlorine dioxide, chlorite, does not pose a significant risk to human health. Feel free to contact us if you have any other questions.
Our children see the world differently. Nothing can escape their attention, and their curiosity knows no bounds. They constantly ask questions and want to answer that question. But problems with children often hinder us. We'll be sharing the most frequently asked questions and their answers with you so you can be prepared for next time.
If you fill a glass with tap water, it will appear clean. But in fact, it is a solution of many substances, among which there are gases (oxygen, argon, nitrogen, carbon dioxide), various impurities contained in the air, dissolved salts from the soil, iron from water pipes, tiny undissolved dust particles, etc.
When water heats up, its molecules begin to move. As this movement increases, the distance between the molecules becomes greater. Finally, there comes a time when the relationships between molecules become too weak. The molecules disperse and become water vapor. This process is called "evaporation".
What keeps planes in the air? What keeps the huge air in the air? The force of work here is called "lifting". Lift occurs when air passes above and below the wing plane at the same time. Because the air moves faster than the tip of the wing, it exerts less pressure. At the same time, the dense air under the wings pushes the plane upward. The higher the speed of the aircraft, the higher the rise.
If you pipette drops of tap water onto clean glass and let it evaporate, barely visible spots will remain.
The water of rivers and streams, and most lakes contains various impurities, for example, dissolved salts. But there are few of them, because this water is fresh.
When viewed individually, each snowflake is colorless and transparent. The answer is that when snowflakes form a large mass, they reflect sunlight. Reflected light is white because the sun is also white. Why can't human hair be natural?
Human hair contains pigments that make it black, brown, blond or red. Our hair also contains small air bubbles. Combinations of pigments and the number of air bubbles in the hair determine the color. The pigments that are found in our hair cannot result in blue or green when combined.
Water flows on the ground and underground, fills streams, lakes, rivers, seas and oceans, creating underground palaces.
Making its way through easily soluble substances, water penetrates deep underground, taking them with it, and through slits and cracks in rocks, forming underground caves, dripping from their roofs, creating bizarre sculptures. Billions of water droplets evaporate over hundreds of years, and substances dissolved in water (salts, limestones) settle on the cave arches, forming stone icicles called stalactites.
Why do astronauts travel in space? Contrary to what many people think, astronauts aboard the International Space Station are not free from gravity. The severity of the Earth affects all objects in orbit. But the high altitude at which the station is located makes this a permanent fall. It's as if the orbital object is still not touching the surface of our planet and is instead flying above the Earth. Imagine an elevator car falling from the top floor of a skyscraper. The person inside this cabin will experience temporary weightlessness.
Astronauts in orbit experience the same thing, but constantly. As the sun's rays enter the planet's atmosphere, they are scattered and broken. Initially, white sunlight is divided into 7 colors of the rainbow. Because blue diffuses more than other colors, it is dominant. But the sky is never completely blue due to the presence of other colors in the spectrum.
Similar formations on the floor of a cave are called stalagmites.
And when a stalactite and stalagmite grow together to form a stone column, it is called a stalagnate.
Fog consists of thousands of tiny drops of water or ice crystals hanging in the air just above the ground. It forms when the air is cold and the ground is warm or vice versa. In both cases, a thick cloud of water vapor or ice particles appears and spreads across the surface.
Water is formed by a chemical reaction in which hydrogen is oxidized by oxygen and heat is released. Since it has already retreated, the water cannot burn under natural conditions. Why do clocks rotate clockwise? Before making mechanical watches, people use sun watches to get an idea of how long it takes. Sundials appear for the first time in the Northern Hemisphere, where the movement of the sun causes shadows to move from left to right. Later in the history of mechanical watches, they inherited this movement from the sun.
Observing ice drift on a river, we see water in a solid (ice and snow), liquid (flowing underneath) and gaseous state (tiny particles of water rising into the air, which are also called water vapor).
The round shape is ideal for rolling on flat surfaces. Because all points on the wheel are equidistant from their axle, the axle remains at the same height above the ground and the vehicle does not move up and down as it travels down the road. Apart from ensuring what our underwear provides, it also protects our private parts from infections and injuries. Hygiene is the main reason we wear underwear. Previously, clothes were very expensive, and people often could not change them.
This attempt takes a little longer, so plan it over two sessions and gradually “grow” decorative, edible and inedible crystals. You can create a crystal display, crystals to name yourself, create crystal images, look forward to your ideas and photos.
Water can be in all three states at the same time: there is always water vapor in the air and clouds, which consist of water droplets and ice crystals.
Water vapor is invisible, but it can be easily detected if you leave a glass of water chilled in the refrigerator for an hour in a warm room, droplets of water will immediately appear on the walls of the glass. Upon contact with the cold walls of the glass, the water vapor contained in the air is converted into water droplets and settles on the surface of the glass.
Edible and inedible crystals You can open and download the entire text or. Topic: Crystallization, saturated solutions. Solids are divided into amorphous and crystalline substances. The arrangement of particles of amorphous substances is random, and their structure resembles that of liquids. Particles of crystalline substances are located in a crystal lattice. The basis of this grid is a unit cell that is constantly repeated.
Crystallization or crystallization is a phenomenon in which solid regular crystals are formed by a liquid due to the environment. Crystals can form from solutions, melts or vapors, where changes in pressure, temperature or concentration of a substance can lead to crystallization. For a smooth process, at least one of the following conditions is required: A decrease in the temperature of the source liquid. Increase in crystallizer concentration due to solvent evaporation. Acidification of the starting material with a crystallizer.
Rice. 11. Condensation on the walls of a cold glass ()
For the same reason, the inside of the window glass fogs up during the cold season. Cold air cannot contain as much water vapor as warm air, so some of it condenses - turns into water droplets.
Crystallization from solution occurs when the crystallization substance is dissolved until the solution is saturated at a given temperature. After heating, the solution becomes unsaturated again, but upon cooling or evaporation of the solvent, the solution becomes oversaturated and crystallization occurs. Natural crystallization occurs after the formation of nucleation nuclei. Crystallization can also be artificially caused by so-called inoculation - by introducing a foreign body into a solution, and this method is used, for example, in the production of sugar.
The white trail behind a plane flying in the sky is also the result of water condensation.
If you bring a mirror to your lips and exhale, tiny droplets of water will remain on its surface, this proves that when breathing a person inhales water vapor with the air.
The name comes from the Arabic beetroot - white. Further use in the chemical and food industries, glass, paper, agriculture as fertilizer and for forge welding. For these purposes, it is also prepared artificially. Tools: Borax, kettle, water, clear glass, twirl or straw, thread or wire, pipe cleaner, food coloring, spoon.
Design: We form any shape from the pipe cleaner. We attach this shape to a thread or wire. We hang the stick on a spoon or straw. We pour water into the kettle and pour it into a glass. Mix borax in water until a saturated solution is obtained. If residual borax remains in the container, reconstitute the solution into a clean glass. Using a kebab, hang our hairy wire body in the glass so that it is completely submerged in the saturated borax solution we have created and that it does not touch the walls or bottom of the glass at any point in time.
When water is heated, it “expands.” This can be proven by a simple experiment: a glass tube was lowered into a flask of water and the water level in it was measured; then the flask was lowered into a vessel with warm water and, after heating the water, the level in the tube was re-measured, which rose noticeably, since water increases in volume when heated.
The entire system is left in solution overnight so that the borax can crystallize. Explanation: The fluffy wire is where the crystallization nuclei are very well formed, to which the borax crystals gradually pack and the crystal grows. Crystallization is accelerated by using hot water to form a saturated solution and cooling and evaporation to make excess solution.
Time: preparation of the experiment and preparation of all aids 5 minutes. Experiment test5 min. Crystal growth 24 hours. Designation of crystals. Estimate 10 minutes. Test 5 minutes. After 25 minutes and 24 hours. Further discussion of the experiment and its modification is possible.
Rice. 14. A flask with a tube, the number 1 and a line indicates the initial water level
Rice. 15. A flask with a tube, the number 2 and a line indicates the water level when heated
It expresses how the internal energy changes, i.e. the sum of the energy of motion and position of the particles of a body when that body cools or increases its temperature. Heat is equal to the energy that a warm enclosure provides during heat exchange. Heat transfer Flows through radiation.
In all states, molecules are in constant disordered motion. Each particle has its own place vibrating around it. When particles heat up, they vibrate faster. When the temperature increases enough, the particles will break free from their fixed position and begin to move freely. At this point, the solid will begin to turn into a liquid. We call this melting occurring, and we say that the tissue is melting.
When water cools, it “compresses.” This can be proven by a similar experiment: in this case, a flask with a tube was lowered into a vessel with ice; after cooling, the water level in the tube decreased relative to the original mark, because the water decreased in volume.
Solidification When a liquid is cooled, it begins to solidify at a certain temperature and changes into tissue. Particles that move freely move more slowly as the temperature decreases until they converge and settle in a specific position, around which they then vibrate. The liquid becomes solid. We call this solidification, and we say that the substance will harden.
Boiling occurs when a liquid is heated to its boiling point. The boiling point differs for different liquids. The boiling point also depends on the pressure above the liquid. This also affects boiling in vessels of considerable height. Liquid turns into gas only from the surface. The evaporating liquid removes heat from the environment. Evaporation occurs at any liquid temperature.
Rice. 16. A flask with a tube, the number 3 and a line indicates the water level during cooling
This happens because water particles, molecules, move faster when heated, collide with each other, are repelled from the walls of the vessel, the distance between the molecules increases, and therefore the liquid occupies a larger volume. When water cools, the movement of its particles slows down, the distance between molecules decreases, and the liquid requires less volume.
Government Affairs Lesson Plans, Student Activities and Graphic Organizers
The higher the temperature, the faster the evaporation, surface to surface dimensions, faster evaporation, properties of the liquid, gas flow over the liquid, gas vapor pressure over the liquid. Matter can be described as something that occupies space in our universe. The type of particles and the way the particles are arranged determine what the question will look like and what it can do. A good understanding of the state of matter is key to describing the universe around us.
Properties of various states of matter
Type of individual or group assignment.Rice. 17. Water molecules at normal temperature
Rice. 18. Water molecules when heated
Rice. 19. Water molecules during cooling
Not only water, but also other liquids (alcohol, mercury, gasoline, kerosene) have such properties.
Knowledge of this property of liquids led to the invention of a thermometer (thermometer) that uses alcohol or mercury.
When water freezes, it expands. This can be proven if a container filled to the brim with water is loosely covered with a lid and placed in the freezer; after a while we will see that the formed ice will lift the lid, going beyond the container.
This property is taken into account when laying water pipes, which must be insulated so that when freezing, the ice formed from the water does not rupture the pipes.
In nature, freezing water can destroy mountains: if water accumulates in rock cracks in the fall, it freezes in winter, and under the pressure of ice, which occupies a larger volume than the water from which it was formed, rocks crack and collapse.
Water freezing in the cracks of roads leads to the destruction of asphalt pavement.
Long ridges resembling folds on tree trunks are wounds from wood ruptures under the pressure of tree sap freezing in it. Therefore, in cold winters you can hear the crackling of trees in a park or forest.
- Vakhrushev A.A., Danilov D.D. The world around us 3. M.: Ballas.
- Dmitrieva N.Ya., Kazakov A.N. The world around us 3. M.: Fedorov Publishing House.
- Pleshakov A.A. The world around us 3. M.: Education.
- Festival of Pedagogical Ideas ().
- Science and education ().
- Public class ().
- Make a short test (4 questions with three answer options) on the topic “Water around us.”
- Conduct a small experiment: place a glass of very cold water on a table in a warm room. Describe what will happen, explain why.
- *Draw the movement of water molecules in a heated, normal and cooled state. If necessary, write captions on your drawing.
Water is the most common substance on the planet, and has a feature that distinguishes it from other liquids: when heated from its melting point up to 40 ° C, its compressibility increases and then decreases.
Unique properties of water
There is no substance on Earth more important for humans than water. Oceans and seas occupy ¾ of the planet's surface, another 20% of the land surface is covered with snow and ice - solid water. If it were not for water, which directly affects climate, the Earth would turn into a lifeless stone flying through space.
Humanity consumes at least 1 billion tons of water per day, while the total amount of resource on the planet remains the same. Millions of years ago there was as much water on the surface of the Earth as there is now.
Living organisms inhabiting the planet have learned to adapt to unfavorable conditions. But no creature can exist without water - this substance is found in all animals and plants. The human body consists of ¾ water.
Water content in the human bodyBasic properties of water:
Has no color;
Transparent;
Odorless and tasteless;
Capable of being in three states of aggregation;
Capable of transitioning from one state of aggregation to another;
Experiment demonstrating the properties of water during heating and cooling
To conduct the experiment at home, you will need two containers and two laboratory flasks with a gas outlet tube, as well as substances: ice, hot water and water at room temperature.
Pour water at room temperature into two identical flasks, mark the water level with a mark and lower it into two containers - with hot water and ice. What is the result of the experiment? Water in a flask, immersed in hot water, rises above the mark. The water in the flask, placed in ice, drops below the mark.
Conclusion: as a result of heating, water expands, and when cooled, it contracts.
Experience demonstrating the properties of water when stored under different conditions
The experiment is carried out at home in the evening. Fill three identical containers (glasses will do) with 100 ml of water. We place one glass on the windowsill, the second on the table, the third near the radiator.
In the morning we compare the results: in the glass left on the windowsill, the water has evaporated by 1/3, in the glass on the table the water has evaporated by half, the glass near the radiator turned out to be empty and dry: the water has evaporated from it. Conclusion: water evaporation depends on the ambient temperature, and the higher it is, the faster the water evaporates.
Conversion of water vapor into water
To conduct the experiment, we prepare special equipment:
Alcohol lamp;
Metal plate;
A flask with a gas outlet tube.
Pour water into the flask and heat it on an alcohol lamp until it boils. We hold a cold metal plate near the gas outlet tube - steam settles on it in the form of water droplets. The transformation of gaseous water into liquid is called condensation. Conclusion: when heated strongly, water turns into steam and returns to a liquid state when it comes into contact with a cold surface.
Condensation on glass surface
Heating water to a boiling point
Water that reaches the boiling point has characteristic features: the liquid boils, bubbles appear inside, and thick steam rises. This happens because water molecules, when heated, receive additional energy from the heat source and move faster. When heated for a long time, the liquid reaches its boiling point: bubbles appear on the walls of the container.
Heated water
If the boiling is not stopped, the process continues until all the water turns into gas. As the temperature increases, the pressure increases, water molecules move faster and overcome the intermolecular forces that bind them. Atmospheric pressure opposes vapor pressure. Water boils when the steam pressure exceeds or reaches the external pressure.
One of the most common substances on Earth: water. It, like air, is necessary for us, but sometimes we don’t notice it at all. She just is. But it turns out
One of the most common substances on Earth: water. It, like air, is necessary for us, but sometimes we don’t notice it at all. She just is. But it turns out that ordinary water can change its volume and weigh either more or less. When water evaporates, heats and cools, truly amazing things happen, which we will learn about today.
Muriel Mandell, in her entertaining book “Phycisc Experiments for Children,” outlines interesting thoughts about the properties of water, on the basis of which not only young physicists can learn a lot of new things, but also adults will refresh their knowledge, which has not had to be used for a long time, so it turned out to be slightly forgotten.Today we will talk about the volume and weight of water. It turns out that the same volume of water does not always weigh the same. And if you pour water into a glass and it does not spill over the edge, this does not mean that it will fit in it under any circumstances.
1. When water is heated, it expands in volume
Place the jar filled with water in a pan filled with about five centimeters of boiling water. water and maintain a simmer over low heat. The water from the jar will begin to overflow. This happens because when water heats up, like other liquids, it begins to take up more space. The molecules repel each other with greater intensity and this leads to an increase in the volume of water.2. When water cools, it contracts
Allow the water in the jar to cool at room temperature, or add new water and place it in the refrigerator. After a while, you will discover that the previously full jar is no longer full. When cooled to 3.89 degrees Celsius, water decreases in volume as the temperature decreases. The reason for this was a decrease in the speed of movement of molecules and their approach to each other under the influence of cooling.It would seem that everything is very simple: the colder the water, the less volume it occupies, but...
3. ...the volume of water increases again when it freezes
Fill the jar with water to the brim and cover with a piece of cardboard. Place it in the freezer and wait until it freezes. You will find that the cardboard “lid” has been pushed out. At temperatures between 3.89 and 0 degrees Celsius, that is, when approaching its freezing point, water begins to expand again. It is one of the few known substances with this property.If you use a tight lid, the ice will simply smash the jar. Have you ever heard that even water pipes can be broken by ice?4. Ice is lighter than water
Place a couple of ice cubes in a glass of water. Ice will float on the surface. When water freezes, it increases in volume. And, as a result, ice is lighter than water: its volume is about 91% of the corresponding volume of water.This property of water exists in nature for a reason. It has a very specific purpose. They say that in winter the rivers freeze. But in reality this is not entirely true. Usually only a small top layer freezes. This ice sheet does not sink because it is lighter than liquid water. It slows down the freezing of water at the depth of the river and serves as a kind of blanket, protecting fish and other river and lake life from severe winter frosts. Studying physics, you begin to understand that a lot of things in nature are arranged expediently.
5. Tap water contains minerals
Pour 5 tablespoons of regular tap water into a small glass bowl. When the water evaporates, a white border will remain on the bowl. This rim is formed by minerals that were dissolved in the water as it passed through the layers of soil.Look inside your kettle and you will see mineral deposits. The same coating forms on the drainage hole in the bathtub.Try evaporating rainwater to test for yourself whether it contains minerals.Is it expanding or contracting? The answer is: with the arrival of winter, water begins its expansion process. Why is this happening? This property sets water apart from all other liquids and gases, which, on the contrary, compress when cooled. What is the reason for this behavior of this unusual liquid?
Physics 3rd grade: does water expand or contract when it freezes?
Most substances and materials increase in volume when heated and decrease in volume when cooled. Gases show this effect more noticeably, but various liquids and solid metals exhibit the same properties.
One of the most striking examples of gas expansion and contraction is air in a balloon. When we take a balloon outside in sub-zero weather, the balloon immediately decreases in size. If we bring a ball into a heated room, it immediately increases. But if we bring the balloon into the bathhouse, it will burst.
Water molecules require more space
The reason that these processes of expansion and contraction of various substances occur are molecules. Those that receive more energy (this happens in a warm room) move much faster than molecules in a cold room. Particles that have more energy collide much more actively and more often; they need more space to move. To contain the pressure exerted by the molecules, the material begins to increase in size. Moreover, this happens quite quickly. So, does water expand or contract when it freezes? Why is this happening?
Water does not obey these rules. If we start to cool water to four degrees Celsius, then it reduces its volume. But if the temperature continues to drop, then the water suddenly begins to expand! There is such a property as an anomaly in water density. This property occurs at a temperature of four degrees Celsius.
Now that we have established whether water expands or contracts when it freezes, let's find out how this anomaly occurs in the first place. The reason lies in the particles of which it is composed. The water molecule is created from two hydrogen atoms and one oxygen atom. Everyone knows the formula for water since elementary school. The atoms in this molecule attract electrons in different ways. Hydrogen creates a positive center of gravity, while oxygen, on the contrary, creates a negative center of gravity. When water molecules collide with each other, the hydrogen atoms of one molecule are transferred to the oxygen atom of a completely different molecule. This phenomenon is called hydrogen bonding.
Water needs more space when it cools
At the moment when the process of forming hydrogen bonds begins, places begin to appear in the water where the molecules are in the same order as in an ice crystal. These blanks are called clusters. They are not durable, like in a solid water crystal. As the temperature rises, they collapse and change their location.
During the process, the number of clusters in the liquid begins to rapidly increase. They require more space to spread, as a result of which the water increases in size after reaching its anomalous density.
When the thermometer drops below zero, the clusters begin to turn into tiny ice crystals. They start to rise up. As a result of all this, water turns into ice. This is a very unusual ability of water. This phenomenon is necessary for a very large number of processes in nature. We all know, and if we don’t know, then we remember that the density of ice is slightly less than the density of cool or cold water. Thanks to this, ice floats on the surface of the water. All bodies of water begin to freeze from top to bottom, which allows the aquatic inhabitants at the bottom to exist calmly and not freeze. So now we know in detail whether water expands or contracts when it freezes.
Hot water freezes faster than cold water. If we take two identical glasses and pour hot water into one and the same amount of cold water into the other, we will notice that hot water will freeze faster than cold water. This is not logical, do you agree? Hot water needs to cool down before it begins to freeze, but cold water does not need to. How to explain this fact? Scientists to this day cannot explain this mystery. This phenomenon is called the “Mpemba Effect”. It was discovered in 1963 by a scientist from Tanzania under an unusual set of circumstances. A student wanted to make himself ice cream and noticed that hot water freezes faster. He shared this with his physics teacher, who at first did not believe him.