Properties apparent

Macromechanics — The study of composite material behavior wherein the material is assumed homogeneous and the effects of the constituent materials are detected only as averaged apparent properties of the composite material. 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Those familiar with processing can detect and correct visible problems or readily measure factors such as color, surface condition, and dimension. However, less-apparent property changes are another matter. These may not show up until the products are in service, unless extensive testing and quality control are used. 

From the nature of apparent properties, we note that the apparent molar enthalpy assigns all of the enthalpy change in forming a mixture to the solute. The result, as shown in equation (7.79), is that all we need to do to calculate AH for a solution process is find the difference in oL between the products and reactants. Thus, to solve Example 7.2 using apparent molar enthalpies, we would write... 

This classification can be loosely linked to the purposes of testing. For quality control, fundamental properties are not needed, apparent properties will usually be acceptable, although functional properties would certainly be desirable. For predicting service performance, the most suitable properties would be functional ones. For design data, fundamental properties are really needed, although considerable help can often be got from functional properties. For investigating failures, the most useful test will depend on the individual circumstances but it is unlikely that fundamental methods would be necessary. 

Blend morphology commonly depends on the weight fraction and viscoelastic properties of each component, the interfacial tension between components, the shape and sizes of the discontinuous phase, and the fabrication conditions and setup. Most rheological experiments applied to homogeneous melts can also be similarly applied to these immiscible blends [55,63,88,89]. The viscoelastic properties arising from these studies should be labeled with a subscript apparent since the equations used to translate rheometer transducer responses to properties incorrectly assume that the material is homogeneous. Nevertheless, these apparent properties are often found to be excellent metrics of fabrication performance. 

Experiment 120. — Put three or four crystals of potassium bromide in a test tube, and add a few drops of concentrated sulphuric acid. The white product is hydrobromic acid. Observe its most apparent properties — cautiously. Test the gas with both kinds of litmus paper. Does this product resemble hydrochloric acid Are they similar enough to be closely related Give any evidence of a secondary reaction. 

Thus to convert traditional to apparent properties of formation or vice versa, one need only look up the properties G — )/T and (Gy — )/298 (or )... 

Tests relating to fundamental properties Tests relating to apparent properties Tests relating to functional properties... 

Apparent properties are closely related to fundamental properties but are not so tightly defined or controlled. An example is tear strength, where the standard methods yield results strongly dependent on test piece geometry. 

In establishing design data, it is mostly fundamental properties that are needed, but these arc in short supply. Many thermal and chemical tests are fundamental in nature, but most mechanical tests give apparent properties. In the absence of established and verified procedures for extrapolating results to other conditions, multipoint data have to be produced at defined levels of all the parameters likely to influence the test result. Consequently, reliable tables of properties for designers are difficult and expensive to establish. 

Standard test methods giving apparent properties are best suited to quality control, and only in relatively few cases are they ideal for design data. Quality control tests are the most easily established, and many existing methods fulfil this need. In seeking an improve-tneiit in test procedure it is not always a more accurate test that is required. Depending on the purpose for which testing is undertaken, it m iy be quicker or cheaper tests that are required, or most important of all, tests that are relevant to service performance. 

Environmental contaminants are often encountered as mixtures and the behaviour of a chemical in a mixture may not correspond to that predicted from data on the pure compound. Interactions of components in a mixture can cause complex and substantial changes in the apparent properties of its constituents. The cosolutes in a mixture may induce either increased (synergistic) or decreased (antagonistic) effects as compared to the ideal behaviour (independently additive effects). No satisfactory approaches are yet available to account systematically for mutual interactions with regard to different properties and endpoints. 

Mutual interactions are hence likely to influence the apparent properties of chemicals substantially, and assessments based on data from pure compounds, either from experimental or QSAR sources, may not realistically reflect the hazard posed by environmental contaminants. However, because no systematic approaches are yet available to account quantitatively for interferences caused by multichemical exposures, the additivity concept remains a practical option for the time being. 

We mentioned in Chapter 3 the two conventions used to define apparent properties, which are used as a form of formation from the element properties at temperatures above 298.15 K. The Benson-Helgeson convention was described there. The Berman-Brown convention, although the same for AH°, is different for AG°. 

Gibbs energy has a completely different functional relationship to concentration than do the other properties. They all use the apparent properties, which we defined in 10.2.3, whereas with Gibbs energy we use Equations (8.30). 

And, as with the other partial molar properties (except fi), it is convenient to use apparent properties, so from Equation (10.9),... 

The aggregation numbers obtained by Eqn. 2 should be treated as quencher-averaged aggregation numbers, q [23-25]. The q is an apparent property, dependent on... 

An experimentally determined value is referred to as an apparent property value if it depends on system parameters, for instance, the rate at which the experiment is performed. An example of a rate dependent property is viscosity. By definition, the intrinsic value of a rate dependent property is the extrapolated value in regards of an infinite time period over which the property is obtained. There are properties that are combinations of truly independent properties, e.g., the material density as the mass per unit volume. The properties of foremost interest are intensive properties, i.e., properties that are independent of the size of a system. 

In MO theory, the covalent—ionic resonance energy is embedded into the total energy and is not an immediately apparent property. In addition, the reference state for MO theory is the Hartree-Fock (HF) wave function, which at the same time lacks electron correlation and also does not provide any bonding for F-F. It is therefore challenging to demonstrate that CSB emerges also from MO theory. 

---