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A Watch Glass is a circular, slightly convex-concave piece of glass used as a surface in labs for evaporating a liquid, holding solids while being weighed, or covering a beaker. The name, watch glass, has been given due to its similarity to the front of old-fashioned watch or wall clock. When used as an evaporation surface, a watch glass allows closer observation of precipitates. It can also be placed on a surface of contrasting color to improve the overall visibility.
Watch glasses can also be used as lids for flasks and beakers. However, it is not very much effective as a cover because exchange of gases could still take place; nevertheless, it prevents dust, or finer particles from entering beakers or flasks.