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In chemistry, the term "concentration" refers to the amount of one component in a substance, relative to the total amount of all components in that substance. There are a number of ways to enumerate concentration, the most common of these being some form of "percent concentration." For example, on a bag of solid fertilizer the percentages of nitrogen, phosphorous and potassium are generally listed in terms of weight-per-weight (w/w), or "weight percent." Alcohol in wine is listed volume-per-volume (v/v) or "volume percent." Saline solution — salt dissolved in water — is listed as weight-per-volume (w/v) percent concentration.
If the percent concentration is of the w/w or the v/v varieties, there are generally no associated units of measure. For example, sea salt contains about 98 percent sodium chloride (NaCl) plus traces of other minerals. Every 10 ounces (oz) (283.5 g) of sea salt therefore contains about 9.8 oz (277.83 g) NaCl. The weight percent needs no units, since the sodium chloride in the numerator is in ounces and the total weight in the denominator is also in ounces — ounces cancels ounces. If one is instead measuring 100 pounds (45.36 kg) of sea salt, it contains 98 pounds (44.45 kg) of NaCl, and the units again disappear — pounds cancels pounds.
Likewise, a 10 percent v/v solution of glycerin in water requires no units, whether the volume is one dram (dr) or one liter (l). The units in the numerator are, again, the same as those in the denominator; they cancel each other. On the other hand, a 15 percent w/v sugar solution has a unit of weight in the numerator, but a unit of volume in the denominator. It is defined as a percentage, somewhat artificially; a 1 percent solution contains 1 gram per 100 milliliters (ml) of solution. This form of percent concentration is used in the biological sciences.
Sometimes, as in the case of an active ingredient contained in a medication — whether tablet, salve or syrup — concentration is compared to strength. It is similar for alcoholic beverages, although the concentration or strength of the "spirits" is generally given as "proof." That term very closely relates to percent concentration; in the US, 80 proof is precisely equal to 40 volume-percent. The term, however, was first defined in the UK as 7/4, and not 1/2, the percent concentration of alcohol. This is because the desirability of the spirits were proven by the ignition of an admixture with gunpowder, which first occurs successfully at a concentration of just over 57 volume-percent — approximately a 7/4-ratio of the desired alcohol to water.
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