The International Bureau of Weights and Measures (BIPM) was established by Article 1 of the Metre Convention, which was signed on May 20, 1875. BIPM is charged with providing the basis for a single, coherent system of measurements to be used throughout the world and operates under the authority of the International Committee of Weights and Measures (CIPM). In 1960, the 11th General Conference on Weights and Measures (CGPM) formally defined and established the International System of Units (SI). Since then, the SI has been periodically updated to take into account advances in science and the need for measurements in new domains.
In 2018, the 26th CGPM (2019) decided that the SI would be based on the fixed numerical values of a set of seven defining constants from which the definitions of the seven base units of the SI would be deduced. This major change was made to allow anchoring of the SI to specific experimental realizations of measurable quantities and to remove the base units from dependence on physical artifacts. The change has been taking place over a number of years, and its completion was made possible by realization of the base unit kilogram separate from its previous physical artifact as stored at BIPM in Sevres, France. This discussion of the the newly constituted SI is based on more complete documentation in References 1 and 2. Because of the importance of the SI in science, much of the discussion below is taken verbatim from these references.
The core of the SI is the seven base units for physical quantities as shown in Table 1.
As of May 20, 2019, the SI is the system of units in which the base units are now defined by the seven fundamental constants given in Table 2. More complete discussions of these constants and their experimental realizations can be found in References 1 and 2.
The hertz, joule, coulomb, lumen, and watt are related to the units second, meter, kilogram, ampere, kelvin, mole, and candela as follows in Table 3. (Note sr is steradian.)
Table 4 provides the definitions of the base quantities. The defintions replace older definitions as discussed in detail in References 1 and 2. These definitions specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit. By fixing the exact numerical value, the unit becomes defined because the product of the numerical value and the unit has to equal the value of the constant, which is invariant. The defining constants have been chosen such that, when taken together, their units cover all of the units of the SI. In general, there is no one-to-one correspondence between the defining constants and the SI base units, except for the cesium frequency ΔνCs and the Avogadro constant N_{A}. Any SI unit is a product of powers of these seven constants and a dimensionless factor.
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{-1} and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particles, or specified group of particles.
Derived units are units that may be expressed in terms of base units by means of the mathematical symbols of multiplication and division (and, in the case of °C, subtraction). Certain derived units have been given special names and symbols, and these special names and symbols may themselves be used in combination with those for base and other derived units to express the units of other quantities. Table 5 lists some examples of derived units expressed directly in terms of base units.
For convenience, certain derived units, which are listed in Table 6, have been given special names and symbols. These names and symbols may themselves be used to express other derived units. The special names and symbols are a compact form for the expression of units that are used frequently. The final column shows how the SI units concerned may be expressed in terms of SI base units. In this column, factors such as m^{0}, kg^{0} …, which are all equal to 1, are not shown explicitly.
The CRC Handbook of Chemistry and Physics (HBCP) contains over 700 tables in over 450 documents which may be divided into several pages, all categorised into 17 major subject areas. The search on this page works by searching the content of each page individually, much like any web search. This provides a challenge if you want to search for multiple terms and those terms exist on different pages, or if you use a synonym/abbreviation that does not exist in the document.
We use metadata to avoid some of these issues by including certain keywords invisibly behind each table. Whilst this approach works well in many situations, like any web search it relies in the terms you have entered existing in the document with the same spelling, abbreviation etc.
Since chemical compounds and their properties are immutable, a single centralised database has been created from all chemical compounds throughout HBCP. This database contains every chemical compound and over 20 of the most common physical properties collated from each of the >700 tables. What's more, the properties can be searched numerically, including range searching, and you can even search by drawing a chemical structure. A complete list of every document table in which the compound occurs is listed, and are hyperlinked to the relevant document table.
The 'Search Chemicals' page can be found by clicking the flask icon in the navigation bar at the top of this page. For more detailed information on how to use the chemical search, including adding properties, saving searches, exporting search results and more, click the help icon in to top right of this page, next to the welcome login message.
Below is an example of a chemical entry, showing its structure, physical properties and document tables in which it appears.
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