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Concentration is a ratio of how much of one ingredient is present in a mixture, compared to the whole mixture or compared to the main ingredient, often the solvent. The amounts of each substance can be expressed in mass or volume units, and many different units can be used. The components of the mixture can be gases, liquids, or solids.

Earth's atmosphere, for example, is a mixture of gases, and 78% of the total volume is nitrogen gas. (Percentages are the number of parts of a certain substance per hundred parts of the mixture.) Different types of steel are mixtures of iron with other elements. For example, stainless steel has close to 20%, by weight, of chromium. Sometimes a combination of mass and volume measurements are used; vinegar can be said to be a 5% solution of acetic acid, meaning 5 g of aceticacid per 100 mL of solution. Because it is not usually practical to analyze the whole substance in question, (Earth's atmosphere, for example) only samples are taken. Getting a true representation of the whole is crucial, or the concentration determined will not be accurate. For example, the concentration of nitrogen in the atmosphere changes slightly with altitude.

Many commonly used mixtures are liquid solutions. For chemical reactions, molarity is a useful unit of concentration. It is the number of moles of solute per liter of solution. Concentrated hydrochloric acid is 12 M, meaning that there are 12 moles of hydrogen chloride per liter of water solution. Other useful units of concentrations are molality, or the number of moles of solute per kilogram of solvent; mole fraction, which is the ratio of the numbers of moles of solute and solution; and normality, which is the number of chemical equivalents per liter of solution.

When very small amounts of a substance are present, parts per million or parts per billion may be used. A sample of tap water may contain 35 parts per million of dissolved solids. The concentration of a radioactive gas such as radon in air can be reported in picocuries per liter, or the amount of radioactivity per unit volume of air. For homes that are tested for the presence of radon, the safe limit is about 4 picocuries per liter. Exposure levels of dust or vapors in air may be given in units of mass of substance per volume of air. The maximum acceptable level for human exposure to ammonia vapor, for example, is 27 mg per cubic meter of air for short term exposure, that is, during a 15-minute period. Because modern analytical instruments require only small samples, results are often reported in milligrams per milliliter or nanograms per milliliter. Clinical laboratory reports of substances in blood or urine may be reported as milligrams per deciliter.

Concentration also refers to the process of removing solvent from a solution to increase the proportion of solute. The Dead Sea becomes more concentrated in salts as water evaporates from the surface. Ores are produced by the concentration of valuable minerals, such as those containing gold or silver, in small region of Earth's crust.

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