Watch glass

A watch glass is a circular concave piece of glass used in chemistry as a surface to evaporate a liquid, to hold solids while being weighed, for heating a small amount of substance, and as a cover for a beaker. When used to cover beakers, the purpose is generally to prevent dust or other particles from entering the beaker; the watch glass does not completely seal the beaker, so gas exchanges still occur. When used as an evaporation surface, a watch glass allows closer observation of precipitates or crystallization. It can be placed on a surface of contrasting colors to improve the visibility overall. Watch glasses are also sometimes used to cover a glass of whisky, to concentrate the aromas in the glass, and to prevent spills when the whisky is swirled.^[1] Watch glasses are named so because they are similar to the glass used for the front of old-fashioned pocket watches. These large watch glasses are occasionally known as clock glasses.