The SI System of Units

Units. The SI system of units and conversion factors (qv) has been formally adopted worldwide, with the exception of Bmnei, Burma, Yemen, and the United States. The participation of the United States in the metrication movement is evident by the passage of the Metric Acts of 1866 and 1975 and the subsequent estabUshment of the American National Metric Council (private) and the U.S. Metric Board (pubHc) to plan, coordinate, monitor, and encourage the conversion process. 

Density is defined as the mass of a substance contained in a unit volume. In the SI system of units, the ratio of the density of a substance to the density of water at I5°C is known as its relative density, while the older term specific gravity is the ratio relative to water at 60°F. Various units of density, such as kg/m, Ib-mass/fF, and g/cm, are commonly used. In addition, molar densities, or the density divided by the molecular weight, is often specified. This section briefly discusses methods of correlation of density as a function of temperature and presents the most common accurate methods for prediction of vapor, liquid, and solid density. 

Prior to the now almost universal adoption of the SI system of units, the unit of heat was defined as the quantity of heat required to raise the temperature of unit mass of water by one degree. This heat quantity is designated the calorie in the cgs system and the kilocalorie in the mks system, and in both cases temperature is expressed in degrees Celsius (Centigrade). As the specific heat capacity is a function of temperature, it has been necessary to set a datum temperature which is chosen as 298 K or 25°C. 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

The SI system of units is generally used throughout this book. It should be noted, however, that according to present practice, there are some exceptions to this, for example, wavenumber (cm ) and ionization energy (eV). 

In the SI system of units the distance is measured in meters, mass in kilograms, and the force in Newtons. 

Mass, length and time are commonly used primary units, other units being derived from them. Their dimensions are written as M, L and T respectively. Sometimes force is used as a primary unit. In the Systeme International d Unites, commonly known as the SI system of units, the primary units are the kilogramme kg, the metre m, and the second s. A number of derived units are listed in Table 1.1. 

In producing a third edition, we have taken the opportunity, not only of updating the material but also of expressing the values of all the physical properties and characteristics of the systems in the SI System of units, as has already been done in Volumes 1 and 3. The SI system, which is described in detail in Volume 1, is widely adopted in Europe and is now gaining support elsewhere in the world. However, because some readers will still be more familiar with the British system, based on the foot, pound and second, the old units have been retained as alternatives wherever this can be done without causing confusion. 

The mole was adopted as the seventh SI base unit in 1971. An important factor of the SI system of units is coherence, by which is meant that derived units are defined by the multiplication and/or division of the base units, without the need for any numerical factors. 

Table 3.3 gives some relationships between commonly used energy units. Today the SI system of units is in general use, although much of the data in the literature is in the older units. Thus we use both types of units for energy, that is calories or kilocalories and joules or kilojoules, where 1 cal = 4.184 J. 

As this book was going to press the International Uraon of Pure and Applied Chemistry recommended that (he word proton be used only when the H isotope was intended, and that ihc more general Irydron be used everywhere dsc, as in hydron donor. See Appendix t. Section 8. We have not attempted a) the last minute to change ail of these "protons to hydrons Like the SI system of units, (his change, if accepted by the world s chemsts. wtl take some lime, and the term "proton donor wil not soon disappear. 

It should be noted that polarizabilities of various orders can be defined in an alternative way in the SI system of units to that discussed previously. A quantity having the dimension of volume a = a/47re0 can be considered to be an SI analogue of the cgs polarizability. Analogously, y = -y/47re0 (or y = y/eo) can be used as the third-order hyperpolarizability in the SI system, with y having the units of m5 V 3. The presence or absence of the factor of An in the definition of the hyperpolarizability is, unfortunately, not always obvious in literature data. 

The good news is that the SI system of units is consistent so that substitution of quantities with SI units into an equation will give a result in SI units. 

Most chemical experiments involve enormous numbers of atoms or molecules. For this reason, the SI system of units defines the mole, abbreviated mol, as the amount of a substance that contains the same number of atoms as 12 g of 12C. This number is called Avogadro s number, NA = 6.0221 1023 mol-1. The mole thus is a... 

The heat transfer rate, q, is taken as positive in the direction wall-to-fluid so that it will have the same sign as(Tw -T/) and h will always, therefore, be positive. A number of names have been applied to h including convective heat transfer coefficient , heat transfer coefficient , film coefficient , film conductance , and unit thermal convective conductance . The heat transfer coefficient, h, has the units W/m2-K or, since its definition only involves temperature differences, W/m2oC, in the SI system of units. In the imperial system of units, h has the units Btu/ft2-hr-°F. 

This term was first proposed in 1853 by the Scottish engineer William Rankine (1820-1872). In the SI system of units with mass in kg, elevation in m, and the acceleration of gravity in m s-2, potential energy has the units of kg m2 s-2. This is the newton-meter or joule, the unit of work, in agreement with Eq. (1.6). 

This equation is the mathematical expression of the first law for a steady-state-flow process. All the terms are expressions for energy per unit mass of fluid in the SI system of units, energy is expressed in joules or in some multiple of the joule. For the English engineering system of units, this equation must be reexpressed to include the dimensional constant gc in the kinetic- and potential-energy terms ... 

In all of the equations written here, the energy unit is presumed to be the joule, in accord with the SI system of units. For the English system of units, the kinetic- and potential-energy terms, wherever they appear, require division by the dimensional constant gc (see Secs. 1.3 and 1.8). However, in many applications, the kinetic- and potential-energy terms are omitted, because they are negligible compared with oth r terms. Exceptions are applications to nozzles, metering devices, wind tunnels, and hydroelectric power stations. 

Now, substitute numerical values into Equation 5.48. In the SI system of units gc, is not needed. Because one bar equals IxlO Pascals (N/ffl ), we have, after multiplying and dividing the first term in Equation 5.48 by kg,... 

In view of equations (5) and (6) it would appear an unwarranted extravagance to use four variables when two would suffice. As we shall see presently, the distinction between D and E in free space, which stems from the SI system of units, will prove of considerable benefit when we come to setting up the equations for the potential gradient in polarizable matter. 

The expression stoichiometric quantities of reactants means molar amounts of the reactants numerically equal to their stoichiometric coefficients. For the calcium carbide reaction, stoichiometric quantities of the reactants in the SI system of units would be I mol of CaC (s) and 2 mol of H20(1). If stoichiometric quantities of the reactants are fed and the reaction proceeds to completion, both reactants would be completely consumed and stoichiometric quantities of the products would be formed. (Convince yourself.)... 

Confusion can often arise in the units that are used to express compression force or pressure and the strength of the tablet. Table 7 gives examples of units that have been used recently for the axes of tablet strength profiles. Comparison between different tablet formulations would be greatly facilitated if authors used and journal editors insisted on the use of the SI system of units. The SI unit of length is the meter, that of force the Newton and that of pressure the Pascal. The unit for physical strength is the Newton and that of tensile strength calculated from Eq. (1) is the Pascal. 

A wide range of units is commonly used to express solution concentration, and confusion often arises in the interconversion of one set of units to another. Wherever possible throughout this book we have used the SI system of units. Although this is the currently recommended system of units in Great Britain, other more traditional systems are still widely used and these will be also described in this section. 

Appendices A, B, and C are important pedagogically. Appendix A discusses experimental error and scientific notation. Appendix B introduces the SI system of units used throughout the book and describes the methods used for converting units. Appendix B also provides a brief review of some fundamental principles in physics,... 

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