Description Watch Glasses – 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. Glass watch glasses – This can be reused after it is sterilized in an autoclave or a laboratory oven. The glasses made provide high resistance to thermal shock, chemical resistance and withstand mechanical durability. Uses of watch glasses One of the generic uses of a watch glass as mentioned previously includes as a lid for beakers. In this case a watch glass is placed above the container, which makes it easier to control and alter vapour saturation conditions. A watch glass is often used to house solids being weighed on the scale. Prior to weighing desired amount of solid, a watch glass is placed on the scale, followed by taring or zeroing the scale so that only the weight of the sample substance is obtained. Watch glass can also be used for drying solids. When further drying is required, a watch glass is often used in cases where a particular type of solid needs to be separated from its comparatively volatile solvent. The solid is spread on a watch glass and, often time, a folded filter paper is placed above to keep out airborne particles from contaminating the product. To maximize the drying rate, a watch glass can be placed inside a fume hood to provide good air circulation for an adequate amount of time. Another technique used in chemistry laboratories to increase the drying rate is passing a gentle stream of dry air or nitrogen gas over the watch glass from an inverted funnel clamped above it.
