Fluid is one of the three states in which matter exists. Most physicists also distinguish the fourth state of matter - plasma. A liquid (along with gas) is a fluid state of matter, that is, when exposed to small external forces, it easily changes its shape, but its volume remains constant at the same pressure and temperature. The answer to the question of what explains the ability of liquids to maintain their volume lies in the features of their structure.
The difference between liquids and solids and gases
Molecules that make up liquids and gases are not in constant positions. They can freely move throughout the volume, which distinguishes these two types of matter from solids, in which each atom or molecule occupies a strictly defined place. Due to the free movement of molecules, liquid and gas take the form of the vessel in which they are placed. Therefore, when answering the question of what properties liquids have, their ability to change their shape should be called first.
However, gases and liquids have a significant difference between themselves. For example, gases do not retain volume and are able to fill it completely in the space that is provided to them. Fluids, in turn, retain volume. What explains the ability of liquids to maintain their volume?
The whole difference between the three states of matter lies in the magnitude of the forces that bind molecules and atoms. In solids, these forces are large, so each particle of a solid substance retains its position. In liquids, these forces are weaker, and therefore they allow molecules to move freely throughout their entire volume. However, these forces are much greater than those for gases, in which in some cases their existence can be neglected.
What properties do liquids have?
A liquid, like gas, refers to fluid bodies, that is, to such a state of matter, the action of a minimum external force on which causes a spatial displacement of its individual molecules and entire layers relative to each other. If we talk only about the structure and properties of liquids, then we need to name the following basic physical characteristics:
- viscosity;
- surface tension;
- cohesion and adhesion;
- capillarity;
- density;
- hydrostatic pressure.
The concept of viscosity
Viscosity is a physical quantity that determines the ability of a fluid to resist any displacement in its volume. Despite the fact that the liquid is a fluid matter, under the influence of an external force that tends to displace the liquid molecules, internal forces appear in it that prevent such a displacement. These internal forces are called cohesive. They are responsible not only for the existence of viscosity, but also are the answer to the question posed. This explains the ability of liquids to maintain their volume.
If the fluid did not have viscosity, then it could flow through any pipe under the action of its own inertia, and it would not be necessary to apply the pressure difference at the ends of this pipe to set the fluid in motion.
The viscosity of such substances depends on temperature inversely, that is, with its increase, the viscosity decreases. This dependence is associated with a weakening of intermolecular forces with increasing temperature of liquids. For example, water at a temperature of 0 ΒΊC has a viscosity of 0.0018 Pa * s, and at 60 ΒΊC it is already 0.00065 Pa * s.
Surface tension
Speaking about the surface properties of a liquid, surface tension should be called first. The essence of this physical quantity is that the free surface of a substance exhibits the properties of a thin elastic membrane. A similar phenomenon is associated with the presence of attractive forces between liquid molecules.
The forces of attraction that act on each molecule in the bulk of the substance are directed in all directions, so they cancel each other out. The resulting attractive forces acting on the surface molecules of the fluid are directed inside the fluid. In other words, perpendicular to its surface. It is this force that is responsible for the formation of surface tension.
Due to the forces of surface tension, the liquid always tends to take the form corresponding to the smallest surface area with such a volume. This form is a ball. That is why water droplets in zero gravity have the shape of a ball. Surface tension decreases with increasing temperature due to a decrease in cohesive forces between liquid molecules.
Cohesion, adhesion and capillarity
Cohesion is the property of molecules of the same liquid attracted to each other. The forces acting as a result of cohesion are called cohesive. Thanks to these forces, liquid molecules tend to come together. The phenomenon of cohesion answers the question of what explains the ability of liquids to maintain their volume. For gases, cohesive forces can be considered equal to zero, so they occupy any volume provided to them.
Adhesion - the ability of a liquid and a solid to attract each other. Such situations often arise when liquid is poured into a vessel or poured on the surface. She comes in contact with a solid. The amount of adhesive force depends on the properties of a particular fluid and solid.
Cohesion and adhesion are competing properties in fluid physics. So, if the cohesive force is greater than the adhesive force, then such a substance does not wet the surface of the solid, and vice versa, if the adhesion is stronger than cohesion, then we talk about the wettability phenomenon.
The phenomenon of adhesion and cohesion is also associated with such a property of liquids as capillarity, that is, the ability of liquids to rise or fall in thin tubes to a height different from the general liquid level. The thickness of the tubes should be about 1 mm, so that the phenomenon of capillarity is noticeable.
Fluid density
This property is characteristic of any state of matter and is defined as the amount of this matter or its mass contained in a unit volume. Given that with increasing temperature, many bodies expand, their density decreases. However, for some substances, this rule may not apply in certain temperature ranges. So, water at 0 ΒΊC has a lower density than at 4 ΒΊC.
The density, according to its definition, is equal to: Ο = m / V, from here we can determine what the volume of the liquid is or what its mass is, if you know two other quantities. For example, water at 4 ΒΊC weighing 1 kg occupies a volume of V = m / Ο = 0.001 m 3 = 1 l.
Hydrostatic pressure
Hydrostatic pressure is understood to mean the pressure that a liquid exerts on a body immersed in it due to the presence of its own weight in the liquid. This pressure increases linearly with depth. Hydrostatic pressure acts in all directions equally. In addition to depth, it also depends on the density of the liquid. The presence of this property determines the existence of a buoyancy force discovered by Archimedes.
Note that the liquid molecules are located quite close to each other, in addition, with a slight convergence of them, large repulsive forces arise, therefore, an increase in external pressure over a wide range practically does not change the density of the liquid. This fact is the answer to the question of what explains the ability of liquids to maintain their volume and not to compress in a significant range of external pressures.