Some Properties of Liquids and Solids

Several gas laws have been introduced in this chapter, but no explanation as to why those laws apply to all gases has been proposed. This section introduces the kinetic molecular theory of gases, which explains the gas laws and when extended, also explains some properties of liquids and solids. Five postulates explain why gases behave as they do ... 

List some properties of liquids and solids that reflect the difference in e degree of order in the two states. 

Rheometers can be divided into two broad types viscometers, used to measure the rheological properties of liquids, and solids rheometers, used to measure the rheological properties of solids. Viscometers and solids rheometers are not mutually exclusive in application some viscometer geometries can be used for testing solids, while some solids rheometer geometries can be used for testing (viscous) liquids. 

Some of the most important properties of liquids and solids are concerned with changes of physical state. Therefore, we begin with this topic. In later sections, we will look specifically at the liquid and solid states and see how their properties depend on the forces of attraction between particles. 

Surface Protection. The surface properties of fluorosihcones have been studied over a number of years. The CF group has the lowest known intermolecular force of polymer substituents. A study (6) of liquid and solid forms of fluorosihcones has included a comparison to fluorocarbon polymers. The low surface tensions for poly(3,3,3-trifluoropropyl)methylsiloxane and poly(3,3,4,4,5,5,6,6,6-nonafluorohexyl)methylsiloxane both resemble some of the lowest tensions for fluorocarbon polymers, eg, polytetrafluoroethylene. 

In this section, you have pieced together the main components that determine the structure and polarity of molecules. Why is the polarity of a molecule important Polar molecules attract one another more than nonpolar molecules do. Because of this attraction, many physical properties of substances are affected hy the polarity of their molecules. In the next section, you will consider some of these physical properties for liquid and solid substances, and learn about other forces that have a significant effect on the interactions within and among molecules. 

Surface and Interfacial Tension. Some properties of liquid surfaces are suggestive of a skin that exercises a contracting force or tension parallel to the surface. Mathematical models based on this effect have been used in explanation of surface phenomena, such as capillary rise. The terms surface tension (gas—liquid or gas—solid interface) and interfacial tension (liquid—liquid or liquid—solid) relate to these models which do not reflect the actual behavior of molecules and ions at interfaces. Surface tension is the force per unit length required to create a new unit area of gas—liquid surface (mN/m (= dyn/cm)). It is numerically equal to the free-surface energy. Similady, interfacial tension is the force per unit length required to create a new unit area of liquid—liquid interface and is numerically equal to the interfacial free energy. 

The expansion on heating is most marked in the case of gases, and was noticed first in them as early as 100 b.c., we find this property of gases made use of by Hero of Alexandria in some ingenious experiments. In the case of liquids and solids the expansion is much less noticeable in the latter case it is even somewhat difficult to determine. Gases and liquids are most suitable for the measurement of temperature, and for the construction of temperature measuring instruments, or thermometers, as they are called. 

When a small-scale chemical of the TD type, irrespective of liquid and solid, including every small-scale gas-permeable oxidatively-heating substance, the diameter of which is of the order of, say, 10 mm, is charged, or confined, in some one of the open-cup, the draft or the closed cell, in accordance with the self-heating property of the chemical, and subjected to either of the above-mentioned two kinds of adiabatic tests, the spatially uniform distribution of temperature is effected necessarily in the chemical so it is done, while the selfheating process is, in particular, in the early stages. 

Very few of the elements listed on the Periodic Table of Elements exist as liquids at standard temperature and pressure. On the other hand, approximately three-fourths of our planet is covered with the liquid known as water, so you should be very familiar with the properties of liquids. Unlike solids, liquids do not have definite shape. If you pour a liquid from a cylindrical bottle into a square container, it changes shape to match the container. This is possible because the motion of the individual particles within the liquid is much less restricted than in a solid. The particles are not locked into fixed positions, and they push past each other, allowing the liquid sample to flow. Some liquids, such as water, flow readily, whereas other liquids, such as molasses, are said to be viscous and flow slowly. The viscosity of a liquid is its relative resistance to flow. Regardless of how fluid a liquid is, the space that a liquid occupies is more fixed, and it will not expand to occupy an entire vessel the way a gas will. 

An understanding of the properties of liquids and solutions at interfaces is very important for many practical reasons. Some reactions only take place at an interface, for example, at membranes, and at the electrodes of an electrochemical cell. The structural description of these systems at a molecular level can be used to control reactions at interfaces. This subject entails the important field of heterogeneous catalysis. In the discussion which follows in this chapter the terms surface and interface are used interchangeably. There is a tendency to use the term surface more often when one phase is in contact with a gas, for example, in the case of solid I gas and liquid gas systems. On the other hand, the term interface is used more often when condensed phases are involved, for example, for liquid liquid and solid liquid systems. The term interphase is used to describe the region near the interface where the structure and composition of the two phases can be different from that in the bulk. The thickness of the interphase is generally not known without microscopic information but it certainly extends distances corresponding to a few molecular diameters into each phase. 

GASES and in solids, THE ATOMS AND MOLECULES ARE PACKED EVEN MORE TIGHTLY TOGETHER. In FACT, IN A SOLID THEY ARE HELD IN WELL-DEFINED POSITIONS AND ARE CAPABLE OF LITTLE FREE MOTION RELATIVE TO ONE ANOTHER. In THIS CHAPTER WE WILL EXAMINE THE STRUCTURE OF LIQUIDS AND SOLIDS AND DISCUSS SOME OF THE FUNDAMENTAL PROPERTIES OF THESE TWO STATES OF MATTER. 

Molecular motion is more restricted in liquids than in gases and in solids, the atoms and molecules are packed even more tightly together. In fact, in a solid they are held in well-defined positions and are capable of little free motion relative to one another. In Chapter 6, we examine the structure of liquids and solids and discuss some of the fundamental properties of these two states of matter. 

Some models assign to the liquid either the properties of a compressed gas or those of a perturbed crystal by virtue of common properties of liquids and gases or liquids and solids. The gas-liquid continuity may be shown by experiments proving that close to the critical point, a continuous... 

As water is a unique liquid, so is amorphous silica a unique solid. They are much alike, both consisting mainly of oxygen atoms with the smaller hydrogen or silicon atoms in the interstices. As pointed out by Weyl and Marboe (2), Some properties of water and silica are so similar that the transition between hydrated silicic acids and the aqueous matrix is a gradual one. Washburn (3) noted that water and amorphous silica both have a temperature of minimum. volume. Ephraim (4) observed another similarity between silica and water in that water is much less dense than e, pected from close packing of the constituent atoms and from X-ray diffraction studies. Bernal and Fowler (5a) concluded that water molecules are arranged in a rather open structure like quartz, and undcrcooled water has a still ifiore open structure, like tridymite. Another model has been proposed by Weres and Rice (5b). 

Perhaps the most important property that differentiates solids and liquids is flow. Liquids flow and take the shape of the container, whereas solids do not flow and tend to retain their shape. The optical properties of liquids and some solids can also be quite different. For example, some solids change the polarisation of light whereas liquids do not. With these ideas in mind, it is not surprising that early investigators who found substances which did not neatly fall into these categories noted their findings and began to ask questions. 

Some selected basic properties of water are given in Table 15.1. The unique characteristics of water, although not obvious, will become apparent as we learn more about it. The three-dimensional phase diagram for water, up to the pressure of 10,000 atm and for the temperature range of —50°C to - - 50°C, is shown in Fig. 15.1. The density of ice (ice I) is less than that of water up to about 2,200 atm. Above this pressure, ice exists in various different crystalline modifications. The two-dimensional phase diagram for water is shown in Fig. 15.2, where the triple point, 0.0100°C, consists of solid ice, water, and water vapor at 4.579 torr in equilibrium. Also shown is the critical point above which liquid water cannot exist in the liquid state. The negative slope of the P-T line is due to the difference in molar volumes of liquid and solid, that is, (V, — VJ < 0, and because... 

We examine in some detail the important diffusion coefficient, which, when viewed mathematically, is the proportionality constant in Pick s law. It is also a material property that depends on the nature of the diffusion species, the matrix through which diffusion takes place, as well as on temperature, and, in the case of gases, on pressure. We consider in some detail the diffusivities of gases within gases, within liquids, and within solids, and the diffusivities associated with the interdiffusion of liquids and solids. These coefficients are of considerable practical importance in various engineering disciplines, in materials processing, and in the biological and environmental... 

The model of free volume going back to the classical papers of Frenkel and Firing [48, 80, 144-147] has been widespread in the physics of liquid and solid states of matter. Some concepts allowing improvement in the nature of fluctuation free volume have been offered in the last 15 years [148-150]. Nevertheless, there is one more aspect of the problem, which has not been mentioned earlier. As a rule, the application of free volume theory for the description of the properties of amorphous bodies is based on a notion that the free volume characterises the structure of the indicated bodies. This postulate is due to a considerable extent to the absence of a quantitative model of the structure of the amorphous condensed state, including the structure of amorphous state polymers. Strictly speaking, one should understand that by structure we mean distribution of body elements in space [151]. It is evident that free volume microvoids cannot be structural elements and at best only mirror the structural state of the studied object. Taking the introduction of some structural elements (relaxators, see for example, [148]) into consideration has practically no influence on the structural representation of free volume. 

This property of solids and, in some circumstances, of liquids and gases, to suffer deformation under applied forces and then to recover completely their original shape when the forces are removed is called elasticity. The behaviour of real materials can be extrapolated to prescribe an ideal elastic material, which is one that ... 

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