Interaction forces between molecules. Structure of gaseous, liquid and solid bodies
Structure of gases, liquids and solids.
Basic principles of molecular kinetic theory:
All substances are made up of molecules, and molecules are made up of atoms,
atoms and molecules are in constant motion,
There are forces of attraction and repulsion between molecules.
IN gases molecules move chaotically, the distances between molecules are large, molecular forces are small, the gas occupies the entire volume provided to it.
IN liquids molecules are arranged in an orderly manner only at short distances, and at large distances the order (symmetry) of the arrangement is violated - “short-range order”. The forces of molecular attraction keep molecules close together. The movement of molecules is “jumping” from one stable position to another (usually within one layer. This movement explains the fluidity of a liquid. A liquid has no shape, but has volume.
Solids are substances that retain their shape, divided into crystalline and amorphous. Crystalline solids bodies have a crystal lattice, in the nodes of which there may be ions, molecules or atoms. They oscillate relative to stable equilibrium positions.. Crystal lattices have a regular structure throughout the entire volume - “long-range order” of arrangement.
Amorphous bodies retain their shape, but do not have a crystal lattice and, as a result, do not have a pronounced melting point. They are called frozen liquids, since they, like liquids, have a “short-range” order of molecular arrangement.
Molecular interaction forces
All molecules of a substance interact with each other through forces of attraction and repulsion. Evidence of the interaction of molecules: the phenomenon of wetting, resistance to compression and tension, low compressibility of solids and gases, etc. The reason for the interaction of molecules is the electromagnetic interactions of charged particles in a substance. How to explain this? An atom consists of a positively charged nucleus and a negatively charged electron shell. The charge of the nucleus is equal to the total charge of all the electrons, so the atom as a whole is electrically neutral. A molecule consisting of one or more atoms is also electrically neutral. Let's consider the interaction between molecules using the example of two stationary molecules. Gravitational and electromagnetic forces can exist between bodies in nature. Since the masses of molecules are extremely small, negligible forces of gravitational interaction between molecules can be ignored. At very large distances there is also no electromagnetic interaction between molecules. But, as the distance between molecules decreases, the molecules begin to orient themselves in such a way that their sides facing each other will have charges of different signs (in general, the molecules remain neutral), and attractive forces arise between the molecules. With an even greater decrease in the distance between molecules, repulsive forces arise as a result of the interaction of negatively charged electron shells of the atoms of the molecules. As a result, the molecule is acted upon by the sum of the forces of attraction and repulsion. At large distances, the force of attraction predominates (at a distance of 2-3 diameters of the molecule, attraction is maximum), at short distances the force of repulsion prevails. There is a distance between molecules at which the attractive forces become equal to the repulsive forces. This position of the molecules is called the position of stable equilibrium. Molecules located at a distance from each other and connected by electromagnetic forces have potential energy. In a stable equilibrium position, the potential energy of the molecules is minimal. In a substance, each molecule interacts simultaneously with many neighboring molecules, which also affects the value of the minimum potential energy of the molecules. In addition, all molecules of a substance are in continuous motion, i.e. have kinetic energy. Thus, the structure of a substance and its properties (solid, liquid and gaseous bodies) are determined by the relationship between the minimum potential energy of interaction of molecules and the reserve of kinetic energy of thermal motion of molecules.
Structure and properties of solid, liquid and gaseous bodies
The structure of bodies is explained by the interaction of particles of the body and the nature of their thermal movement.
Solid
Solids have a constant shape and volume and are practically incompressible. The minimum potential energy of interaction of molecules is greater than the kinetic energy of molecules. Strong particle interaction. The thermal motion of molecules in a solid is expressed only by vibrations of particles (atoms, molecules) around a stable equilibrium position.
Due to the large forces of attraction, molecules practically cannot change their position in matter, this explains the invariability of the volume and shape of solids. Most solids have a spatially ordered arrangement of particles that form a regular crystal lattice. Particles of matter (atoms, molecules, ions) are located at the vertices - nodes of the crystal lattice. The nodes of the crystal lattice coincide with the position of stable equilibrium of the particles. Such solids are called crystalline.
Liquid
Liquids have a certain volume, but do not have their own shape; they take the shape of the vessel in which they are located. The minimum potential energy of interaction between molecules is comparable to the kinetic energy of molecules. Weak particle interaction. The thermal motion of molecules in a liquid is expressed by vibrations around a stable equilibrium position within the volume provided to the molecule by its neighbors. Molecules cannot move freely throughout the entire volume of a substance, but transitions of molecules to neighboring places are possible. This explains the fluidity of the liquid and the ability to change its shape.
In liquids, molecules are quite firmly bound to each other by forces of attraction, which explains the invariance of the volume of the liquid. In a liquid, the distance between molecules is approximately equal to the diameter of the molecule. When the distance between molecules decreases (compression of the liquid), the repulsive forces increase sharply, so liquids are incompressible. In terms of their structure and the nature of thermal movement, liquids occupy an intermediate position between solids and gases. Although the difference between a liquid and a gas is much greater than between a liquid and a solid. For example, during melting or crystallization, the volume of a body changes many times less than during evaporation or condensation.
Gases do not have a constant volume and occupy the entire volume of the vessel in which they are located. The minimum potential energy of interaction between molecules is less than the kinetic energy of molecules. Particles of matter practically do not interact. Gases are characterized by complete disorder in the arrangement and movement of molecules.
The distance between gas molecules is many times greater than the size of the molecules. Small attractive forces cannot keep molecules close to each other, so gases can expand without limit. Gases are easily compressed under the influence of external pressure, because the distances between molecules are large, and the interaction forces are negligible. The gas pressure on the walls of the container is created by the impacts of moving gas molecules.
All nonliving matter is made up of particles that may behave differently. The structure of gaseous, liquid and solid bodies has its own characteristics. The particles in solids are held together by being very close together, which makes them very strong. In addition, they can maintain a certain shape, since their smallest particles practically do not move, but only vibrate. Molecules in liquids are quite close to each other, but they can move freely, so they do not have their own shape. Particles in gases move very quickly and there is usually a lot of space around them, which means they can be easily compressed.
Properties and structure of solids
What is the structure and structural features of solids? They consist of particles that are located very close to each other. They cannot move and therefore their shape remains fixed. What are the properties of a solid? It does not compress, but if it is heated, its volume will increase with increasing temperature. This happens because the particles begin to vibrate and move, causing the density to decrease.
One of the characteristics of solids is that they have a constant shape. When a solid heats up, the movement of the particles increases. Faster moving particles collide more violently, causing each particle to push its neighbors. Therefore, an increase in temperature usually results in an increase in body strength.
Crystal structure of solids
The intermolecular forces of interaction between neighboring molecules of a solid are strong enough to keep them in a fixed position. If these smallest particles are in a highly ordered configuration, then such structures are usually called crystalline. Questions of the internal order of particles (atoms, ions, molecules) of an element or compound are dealt with by a special science - crystallography.
Solids are also of particular interest. By studying the behavior of particles and how they are structured, chemists can explain and predict how certain types of materials will behave under certain conditions. The smallest particles of a solid are arranged in a lattice. This is the so-called regular arrangement of particles, where various chemical bonds between them play an important role.
The band theory of the structure of a solid body considers it as a collection of atoms, each of which, in turn, consists of a nucleus and electrons. In the crystalline structure, the nuclei of atoms are located in the nodes of the crystal lattice, which is characterized by a certain spatial periodicity.
What is the structure of a liquid?
The structure of solids and liquids is similar in that the particles of which they are composed are located at close range. The difference is that the molecules move freely, since the force of attraction between them is much weaker than in a solid body.
What properties does the liquid have? The first is fluidity, and the second is that the liquid will take the shape of the container in which it is placed. If you heat it up, the volume will increase. Due to the close proximity of the particles to each other, the liquid cannot be compressed.
What is the structure and structure of gaseous bodies?
The gas particles are arranged randomly, they are so far from each other that no attractive force can arise between them. What properties does gas have and what is the structure of gaseous bodies? As a rule, the gas evenly fills the entire space in which it was placed. It compresses easily. The speed of particles of a gaseous body increases with increasing temperature. At the same time, pressure also increases.
The structure of gaseous, liquid and solid bodies is characterized by different distances between the smallest particles of these substances. Gas particles are much further apart than solid or liquid particles. In air, for example, the average distance between particles is about ten times the diameter of each particle. Thus, the volume of molecules occupies only about 0.1% of the total volume. The remaining 99.9% is empty space. In contrast, liquid particles fill about 70% of the total liquid volume.
Each gas particle moves freely along a straight path until it collides with another particle (gas, liquid or solid). The particles usually move quite quickly, and after two of them collide, they bounce off each other and continue on their way alone. These collisions change direction and speed. These properties of gas particles allow gases to expand to fill any shape or volume.
State change
The structure of gaseous, liquid and solid bodies can change if they are exposed to a certain external influence. They can even transform into each other's states under certain conditions, such as during heating or cooling.
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- Evaporation. The structure and properties of liquid bodies allow them, under certain conditions, to transform into a completely different physical state. For example, if you accidentally spill gasoline while refueling your car, you can quickly notice its pungent odor. How does this happen? Particles move throughout the liquid, eventually reaching the surface. Their directed motion can carry these molecules beyond the surface into the space above the liquid, but gravity will pull them back. On the other hand, if a particle moves very quickly, it can become separated from others by a considerable distance. Thus, with an increase in the speed of particles, which usually occurs when heated, the process of evaporation occurs, that is, the conversion of liquid into gas.
Behavior of bodies in different physical states
The structure of gases, liquids, and solids is mainly due to the fact that all these substances consist of atoms, molecules or ions, but the behavior of these particles can be completely different. Gas particles are randomly spaced from each other, liquid molecules are close to each other, but they are not as rigidly structured as in a solid. Gas particles vibrate and move at high speeds. The atoms and molecules of a liquid vibrate, move, and slide past each other. Particles of a solid body can also vibrate, but movement as such is not characteristic of them.
Features of the internal structure
In order to understand the behavior of matter, you must first study the features of its internal structure. What are the internal differences between granite, olive oil and helium in a balloon? A simple model of the structure of matter will help answer this question.
A model is a simplified version of a real object or substance. For example, before actual construction begins, architects first construct a model of the construction project. Such a simplified model does not necessarily imply an exact description, but at the same time it can give an approximate idea of what a particular structure will be like.
Simplified models
In science, however, models are not always physical bodies. The last century has seen a significant increase in human understanding about the physical world. However, much of the accumulated knowledge and experience is based on extremely complex concepts, such as mathematical, chemical and physical formulas.
In order to understand all this, you need to be quite well versed in these exact and complex sciences. Scientists have developed simplified models to visualize, explain, and predict physical phenomena. All this greatly simplifies the understanding of why some bodies have a constant shape and volume at a certain temperature, while others can change them, and so on.
All matter is made up of tiny particles. These particles are in constant motion. The amount of movement is related to temperature. An increased temperature indicates an increase in movement speed. The structure of gaseous, liquid and solid bodies is distinguished by the freedom of movement of their particles, as well as by how strongly the particles are attracted to each other. Physical depend on his physical condition. Water vapor, liquid water and ice have the same chemical properties, but their physical properties are significantly different.
Gases. In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. For example, at atmospheric pressure the volume of a vessel is tens of thousands of times greater than the volume of the molecules in it.
Gases are easily compressed, and the average distance between molecules decreases, but the molecules do not compress each other.
Molecules move at enormous speeds - hundreds of meters per second - in space. When they collide, they bounce off each other in different directions like billiard balls. The weak attractive forces of gas molecules are not able to hold them near each other. Therefore, gases can expand without limit. They retain neither shape nor volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.
Liquids. Liquid molecules are located almost close to each other, so a liquid molecule behaves differently than a gas molecule. In liquids, there is so-called short-range order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters. The molecule vibrates around its position, colliding with neighboring molecules. Only from time to time she makes another “jump”, getting into a new equilibrium position. In this equilibrium position, the repulsive force is equal to the attractive force, i.e. the total interaction force of the molecule is zero. The time of settled life of a water molecule, i.e. the time of its oscillations around one specific equilibrium position at room temperature, is on average 10-11 s. The time of one oscillation is much less (10-12-10-13 s). With increasing temperature, the residence time of molecules decreases. The nature of molecular motion in liquids, first established by the Soviet physicist Ya.I. Frenkel, allows you to understand the basic properties of liquids. Liquid molecules are located directly next to each other. As the volume decreases, the repulsive forces become very large. This explains the low compressibility of liquids. As you know, liquids are fluid, that is, they do not retain their shape. This can be explained this way. The external force does not noticeably change the number of molecular jumps per second. But jumps of molecules from one stationary position to another occur predominantly in the direction of the external force (Fig. 8.8). This is why liquid flows and takes the shape of the container.
Solids.
Atoms or molecules of solids vibrate around certain equilibrium positions, therefore solids retain not only volume, but also shape
If you connect the center of equilibrium of atoms or ions of a solid, you get a regular spatial lattice, called a crystalline lattice
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Crystalline bodies.
Crystals are solid bodies whose atoms or molecules occupy a certain, orderly position in space. Therefore, the crystals have flat edges. For example, a grain of ordinary table salt has flat edges that form right angles with each other.
Anisotropy of crystals.
The correct external shape is not the only or even the most important consequence of the ordered structure of the crystal. The main thing is the dependence of physical properties on the direction chosen in the crystal. For example, a piece of mica easily delaminates in one direction into thin plates, but it is much more difficult to tear it in the direction perpendicular to the plates. Many crystals conduct heat and electrical current differently in different directions. The optical properties of crystals also depend on the direction. Thus, a quartz crystal refracts light differently depending on the direction of the rays incident on it. The dependence of physical properties on the direction inside the crystal is called anisotropy. All crystalline bodies are anisotropic.
Single crystals and polycrystals.
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Metals have a crystalline structure. If you take a large piece of metal, then at first glance its crystalline structure does not appear in any way either in the appearance of the piece or in its physical properties
Typically, a metal consists of a huge number of small crystals fused together. The properties of each crystal depend on the direction, but the crystals are randomly oriented relative to each other. As a result, in a volume significantly larger than the volume of individual crystals, all directions within metals are equal and the properties of metals are the same in all directions.
A solid consisting of a large number of small crystals is called polycrystalline. Single crystals are called single crystals.
Gases Gas (gaseous state) (from Dutch gas) is a state of aggregation of a substance, characterized by very weak bonds between its constituent particles (molecules, atoms or ions), as well as their high mobility. Gas particles move almost freely and chaotically in the intervals between collisions, during which a sharp change in the nature of their movement occurs. The gaseous state of a substance under conditions where the existence of a stable liquid or solid phase of the same substance is possible is usually called vapor. Like liquids, gases have fluidity and resist deformation. Unlike liquids, gases do not have a fixed volume and do not form a free surface, but tend to fill the entire available volume (for example, a vessel).
The gaseous state is the most common state of matter in the Universe (interstellar matter, nebulae, stars, planetary atmospheres, etc.). The chemical properties of gases and their mixtures are very diverse - from low-active inert gases to explosive gas mixtures. Gases sometimes include not only systems of atoms and molecules, but also systems of other particles - photons, electrons, Brownian particles, as well as plasma
Gases can expand indefinitely. They do not retain their shape or volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.
LIQUID Liquid is one of the aggregate states of matter. The main property of a liquid, which distinguishes it from other states of aggregation, is the ability to unlimitedly change its shape under the influence of tangential mechanical stresses, even arbitrarily small, while practically maintaining its volume.
Liquid is a physical body that has two properties: It has fluidity, due to which it has no shape and takes the shape of the vessel in which it is located. It changes shape and volume little with changes in pressure and temperature, in which it is similar to a solid body.
The liquid state is usually considered intermediate between a solid and a gas: a gas retains neither volume nor shape, but a solid retains both. The shape of liquid bodies can be determined entirely or partly by the fact that their surface behaves like an elastic membrane. So, water can collect in drops. But a liquid is capable of flowing even under its stationary surface, and this also means unpreserved forms (of the internal parts of a liquid body). Liquid molecules do not have a definite position, but at the same time they do not have complete freedom of movement. There is an attraction between them, strong enough to keep them close. A substance in a liquid state exists in a certain temperature range, below which it turns into a solid state (crystallization occurs or transformation into a solid-state amorphous state - glass), above which it turns into a gaseous state (evaporation occurs). The boundaries of this interval depend on pressure. As a rule, a substance in the liquid state has only one modification. (The most important exceptions are quantum liquids and liquid crystals.) Therefore, in most cases, a liquid is not only a state of aggregation, but also a thermodynamic phase (liquid phase). All liquids are usually divided into pure liquids and mixtures. Some mixtures of liquids are of great importance for life: blood, sea water, etc. Liquids can act as solvents.
Formation of a free surface and surface tension Due to the conservation of volume, a liquid is capable of forming a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other there is a gaseous phase (steam), and, possibly, other gases, for example, air. If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the interface area - surface tension forces. The interface behaves like an elastic membrane that tends to contract. Surface tension can be explained by the attraction between liquid molecules. Each molecule attracts other molecules, strives to “surround” itself with them, and therefore leave the surface. Accordingly, the surface tends to decrease. Therefore, soap bubbles and bubbles tend to take a spherical shape when boiling: for a given volume, a sphere has the minimum surface area. If only surface tension forces act on a liquid, it will necessarily take a spherical shape - for example, water drops in zero gravity. Small objects with a density greater than that of the liquid are able to “float” on the surface of the liquid, since the force of gravity is less than the force that prevents the increase in surface area.
The transition of liquids from one state to another Evaporation is the gradual transition of a substance from a liquid to a gaseous phase (steam). During thermal movement, some molecules leave the liquid through its surface and become vapor. At the same time, some molecules pass back from vapor to liquid. If more molecules leave a liquid than enter, then evaporation occurs. Condensation is a reverse process, the transition of a substance from a gaseous state to a liquid one. In this case, more molecules pass into the liquid from the vapor than into the vapor from the liquid. Boiling is the process of vaporization inside a liquid. At a sufficiently high temperature, the vapor pressure becomes higher than the pressure inside the liquid, and vapor bubbles begin to form there, which (under the conditions of gravity) float to the top. Wetting is a surface phenomenon that occurs when a liquid comes into contact with a solid surface in the presence of steam, that is, at the interfaces of three phases. Miscibility is the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible liquids: water and liquid oil.
A solid is one of the four states of aggregation of matter, which differs from other states of aggregation (liquids, gases, plasma) in the stability of its shape and the nature of the thermal motion of atoms that perform small oscillations around equilibrium positions.
Molecular kinetic theory makes it possible to understand why a substance can be in different states of aggregation: gaseous, liquid and solid.
The external distinguishing features of these states are compressibility (volume change) and fluidity (shape retention).
From the point of view of molecular kinetic theory, states of aggregation differ in the value of the average distance between molecules and the nature of the movement of molecules relative to each other.
By increasing the temperature of a gas at a fixed pressure, a partially and then fully ionized plasma can be obtained, often considered the fourth state of matter. With increasing pressure, matter can go into the fifth - neutron - state, which is realized in nature in the form of neutron stars.
Based on MCT, we will consider the differences and similarities of the thermal motion of particles of gases, liquids and solids.
Gases are bodies in which molecules move almost freely chaotically in the intervals between collisions, during which the nature of their movement changes dramatically. According to MCT, gas molecules are located from each other at distances exceeding the size of the molecules themselves by several times. In this case, the attractive forces are already small, therefore, participating in chaotic movement, gas molecules can move away to any distance. Gas occupies the volume of a vessel of any size. It can be significantly compressed under the influence of external forces.
For example, the volume of a vessel can be tens of thousands of times greater than the volume of the molecules in it.
Gases are easily compressed if the average distance between the molecules decreases, but the shape of the molecule does not change. Molecules, moving in space at enormous speeds - hundreds of meters per second, collide, then bounce off each other in different directions like billiard balls. The weak attractive forces of gas molecules are not able to hold them near each other.
Therefore, gases can expand without limit. They retain neither shape nor volume.
Numerous impacts of molecules on the walls of the vessel create gas pressure. An example of this would be a balloon. It cannot be inflated on one side. The gas or air in the ball spreads throughout the entire volume.
How can you judge the concentration of molecules inside the ball? The more gas inside the ball, the more densely it is inflated, i.e. becomes more elastic.
Liquids are bodies formed by substances in a state in which the shape of the body is not maintained under the influence of gravity or a small load. However, liquid is difficult to compress even under significant forces.
Liquid molecules do not form a constant spatial structure; located from each other at distances comparable to the size of the molecules themselves, almost close to each other, so a liquid molecule behaves differently than a gas molecule. The nature of the movement of these molecules is a set of oscillations relative to the equilibrium position, as a result of collisions with neighboring molecules, i.e. temporary sedentary position, alternating with jumps to a new sedentary position.
In liquids, there is so-called short-range order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters. The molecule oscillates around its equilibrium position: here the repulsive force is equal to the attractive force, i.e., the total interaction force of the molecule is zero. The time of sedentary life of a water molecule: the time of its oscillations around one specific equilibrium position at room temperature is on average 10-11 s. The time of one oscillation is much less than 10-12-10-13 s. With increasing temperature, the residence time of molecules decreases.
The nature of molecular motion in liquids was first established by the Soviet physicist Yakov Ilyich Frenkel. The results of his work allow us to understand the basic properties of liquids.
Liquid molecules are located directly next to each other. As the volume decreases, the repulsive forces become very large. This explains the low compressibility of liquids.
Liquids are fluid, that is, they do not retain their shape, since the external force does not noticeably change the number of molecular jumps per second. But jumps of molecules from one stationary position to another occur predominantly in the direction of the action of the external force. This is why liquid flows and takes the shape of the container.
A solid is a state of aggregation of a substance, characterized by stability of shape under significant loads (comparable to the effects of gravity) and thermal movement of atoms in the form of small vibrations around equilibrium positions (hence the occurrence of deformations only under large external forces). In addition, the distance between the molecules is comparable to the size of the molecules themselves, and when compressed, repulsive forces arise between them (hence the incompressibility of solids).
Atoms or molecules of solids, unlike atoms and molecules of liquids, vibrate around certain equilibrium positions. For this reason, solids retain not only volume, but also shape. The potential energy of interaction between solid molecules is significantly greater than their kinetic energy.
There is another important difference between liquids and solids. A liquid can be compared to a crowd of people, where individuals are restlessly jostling in place, and a solid body is like the same crowd of people who, although they do not stand at attention, maintain on average certain distances between themselves. If you connect the centers of equilibrium positions of atoms or ions of a solid, you get a regular spatial lattice, called a crystalline lattice.
The drawings depict crystal lattices of table salt and diamond. The internal order in the arrangement of atoms in crystals leads to regular external geometric shapes.
There are crystalline and amorphous solids.
In amorphous bodies, atoms vibrate around randomly located points, the order of which is observed only at distances comparable to interatomic ones.
In crystals, periodicity in the location of these points is observed for arbitrarily distant atoms.
From the point of view of MCT, these properties are explained by the ordered arrangement of atoms (molecules) in the body. This arrangement does not change for a long time.
A crystal is a solid that has a three-dimensional periodic atomic or molecular structure. Usually such a body has the shape of a regular symmetrical polyhedron. Large single crystals are called single crystals. Single crystals of various sizes are found in nature: from very large quartz crystals (up to several hundred kilograms) to small ones (scattering diamond crystals). A distinctive feature of crystalline bodies is:
1) anisotropy of single crystals (dependence of properties on direction); for example, if you put a glass jar down, you can easily crush it by standing on it. However, if you put the jar down, it will easily support your weight;
2) the presence of a fixed melting temperature.
An amorphous body does not have an ordered (crystalline) structure of molecules; it retains its shape only due to the difficulty of moving molecules relative to each other.
When heated, the amorphous body softens gradually. Mechanical, thermal and other properties are the same along all directions of such a body.
The amorphous state is characteristic of molecules that have a large length compared to the transverse size of the molecules themselves (organic polymers, glasses). With prolonged exposure to low forces, amorphous bodies, like liquids, exhibit fluidity.