precipitation

precipitation

[pri-sip-i-tey-shuhn]
precipitation, in chemistry, a process in which a solid is separated from a suspension, sol, or solution. In a suspension such as sand in water the solid spontaneously precipitates (settles out) on standing. In a sol the particles are precipitated by coagulation. A solute (dissolved substance) may be precipitated from a solution by several means. A solution of salt may be concentrated by evaporation until the salt crystallizes. When a saturated solution of sugar is cooled, sugar crystals form. The addition of a solution of silver nitrate to a solution containing chloride ions results in the formation of insoluble silver chloride: AgNO3+Cl-→NO3-+AgCl↓. In each case the precipitate formed may settle out spontaneously or may be collected by filtration or centrifugation. It is often difficult to obtain a pure substance by a single precipitation, and a substance may be further purified by reprecipitation after it has been redissolved. The term precipitation is also applied to the separation of particles of a solid or liquid suspended in a gas.
precipitation, in meteorology, condensed moisture that falls to the surface of the earth in the form of rain, sleet, snow, hail, frost, or dew.

All liquid and solid water particles that fall from clouds and reach the ground, including drizzle, rain, snow, ice crystals, and hail. The essential difference between a precipitation particle and a cloud particle is size; an average raindrop has a mass equivalent to that of about one million cloud droplets. Precipitation elements (ice crystals or droplets that form around soluble particles such as salt) form directly from the vapour state and get larger through collision and coalescence. Eventually they become large enough to respond to gravity, and they fall to the ground.

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