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A watch glass is a circular, slightly piece of glass used in Chemistry as a surface to Evaporate a liquid, to hold solids while being weighed, or as a cover for a Beaker . The latter use is generally applied to prevent dust or other particles entering the beaker; the watch glass does not completely seal the beaker, and so gas exchanges still occur. When used as an evaporation surface, a watch glass allows closer observation of Precipitate s or Crystallisation , and can be placed on a surface of contrasting colour to improve the visibility overall. Watch glasses are so named because they are identical to the glass used for the front of old-fashioned pocket watches. In reference to this, large watch glasses are occasionally known as clock glasses. |
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