PHYSICAL PROPERTIES OF SUBSTANCES

In this paragraph only two physical properties are discussed viz. the boiling point and solubility. These properties are affected by the attractive forces which the particles of the pure substance or in a mixture exert on each other. Particles attract when they are charged, so when they are ions, when they possess a dipole or when they are able to form hydrogen bridges. 

In general substances which consist of positive as well as negative ions have high boiling points because a lot of energy is required to loosen the particles from the crystal lattice. These substances mostly dissolve well in a solvent whose molecules possess a dipole. These dipoles surround the ions, due to which the latter are no longer able to unit to form a crystal. This surrounding by molecules of the solvent is called solvation, or, in the case of water, hydration. In Fig. 3.11 you can see what happens to the ions when sodium chloride (NaCl) is dissolved in water. 

The degree in which substances are soluble in water varies from very well (hundreds of grammes per litre) like in the case of sodium chloride, to extremely bad ( about ten milligrammes per litre) like for instance calcium carbonate (just think of marl). 

When particles possess dipoles, they attract. The attraction between the - end of one molecule and the + end of another results in the formation of a network. This attraction is third in strength, after the ion attraction and the H-bridge formation. In general, the boiling points of these substances are consequently lower than those of substances with nearly equal particle masses with ion bonds or H-bridge formations. 

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The small differences in physical properties of substances containing elements with isotopes are manifested through mea.surement of isotope ratios. When water evaporates, the vapor is richer in its lighter isotopes ( Hj O) than the heavier one ( Hj O). Such differences in vapor pressures vary with temperature and have been used, for example, to estimate sea temperatures of 10,000 years ago (see Chapter 47). 

THE EFFECT OF THE HYDROGEN BOND ON THE PHYSICAL PROPERTIES OF SUBSTANCES... 

Richet C. On the relationship between the toxicity and the physical properties of substances. Compt Rend Soc Biol 1893 (9)5 775-6. 

The physical properties of substances do not involve chemical changes. Color (see Textbox 17) and crystal structure (see Textbox 21), for example, are physical properties that are characteristic of a substance that serve to identify most substances. Other physical properties, such as density, hardness (see Table 3), refractive index (see Table 19), and heat capacity (see Table 101), are also useful for characterizing and identifying substances as well as distinguishing between different substances. 

The utility of the van der Waals a parameter should not be underestimated when physical properties of substances are being correlated and interpreted. Neither for that matter should the b parameter, because it is related to effective molecular dimensions, but that is not our concern in this chapter. 

In this section, you have used Lewis structures to represent bonding in ionic and covalent compounds, and have applied the quantum mechanical theory of the atom to enhance your understanding of bonding. All chemical bonds—whether their predominant character is ionic, covalent, or between the two—result from the atomic structure and properties of the bonding atoms. In the next section, you will learn how the positions of atoms in a compound, and the arrangement of the bonding and lone pairs of electrons, produce molecules with characteristic shapes. These shapes, and the forces that arise from them, are intimately linked to the physical properties of substances, as you will see in the final section of the chapter. 

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. 

Forces that influence the physical properties of substances are called intermolecular forces. These are forces of attraction and repulsion that act between molecules or ions. 

As a result of these dipole-dipole forces of attraction, polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would. The energy required to separate polar molecules from one another is therefore greater than that needed to separate non-polar molecules of similar molar mass. This is indicated hy the extreme difference in melting and boiling points of these two types of molecular substances. (Recall that melting and boiling points are physical properties of substances.)... 

Dipole-dipole, ion-dipole, and dispersion forces are collectively known as van der Waals forces. Johaimes Diederick van der Waals (1837-1923) received the 1910 Nobel Prize in physics for his work on fluids. We have seen how hydrogen bonding and van der Waals forces affect the physical properties of substances, and more is said about these forces as we examine the different states of matter. 

Most of the chemical and physical properties of substances are determined by their structure. 

The physical properties of substances these depend on the properties of individual molecules and on how the molecules interact in the bulk material. For example, the strength and melting point of a polymer (e.g. a plastic) depend on how well the molecules fit together and on how strong the forces between them are. People who investigate things like this work in the field of materials science. 

Dewar s later work involved investigating the chemical and physical properties of substances at low temperatures, including low-temperature calorimetry. With the outbreak of the Great War (or World War I, 1914-1918), the laboratory at the Royal Institution lost most of its staff and Dewar turned his attention to soap bubbles. By the end of the war Dewar, now in his late seventies, did not have the energy to restart the laboratory, nor would he retire. He died on March 27, 1923, and his funeral service was held in the director s flat at the Royal Institution, see also Kekule, Friedrich August. 

Physical properties of substances with different bonds... 

Influence of Desiccation upon the Physical Properties of Substances.—Liquids that have been thoroughly desiccated by exposure to phosphorus pentoxide for several years manifest a decided elevation in boiling-point, ranging in extent from 80° to 60° C, This is well illustrated by the following data 1... 

The elucidation of the dependence of various chemical and physical properties of substances on molecular structure can be considered as one of the main goals of theoretical chemistry. Although an immense knowledge has accumulated in this field, a fairly limited number of direct, causal and quantitative (or at least semiquantitative) structure-property relations have been discovered so far. The main reason for this is the enormous complexity of the quantum-chemical calculations, by means of which the contemporary theoretical chemists try to describe and predict the behaviour of molecules. During such calculations the insight into the actual connection between the input (e.g. molecular structure) and output (e.g. certain molecular properties) is usually completely lost. 

As we previously learned, mixtures are not pure substances. In order to obtain one of the components in a mixture, we need to separate them. The separation of mixtures can only be possible when we use the physical properties of substances. 

Physical properties of substances the formation of crystal faces, cleavage, density, solubility, electrical conductivity, thermal conductivity, malleability, ductility, hardness, color. 

Detection of solutes is an essential link in the separation chain. It has to reveal the solutes separation by detecting them in the column effluent and, in some cases, it could permit their characterization. Toward these objectives, it is based on the various physical properties of substances. 

Compare the physical properties of substances that have different bond types, and relate bond types to electronegativity differences. 

The fact that substances can change form, as in the example of water changing to steam, is another important concept in chemistry. If you review what you just learned about physical properties of substances, you can see that because the particular form of a substance is a physical property, changing the form introduces or adds another physical property to its list of characteristics. In fact, resources that provide tables of physical and chemical properties of substances, such as the CRC Handbook of Chemistry and Physics, generally include the physical properties of substances in all of the states in which they can exist. 

Skill 14.2 Demonstrating knowledge of methods and equipment used for measuring the physical properties of substances... 

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