Watch glass

Caesium fluoride sample on a 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, as a cover for a beaker, and for heating a small amount of substance. The latter use is generally applied to prevent dust or other particles 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, and can be placed on a surface of contrasting color 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.

Watch glasses are named so because they are similar 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.

Uses

Crystal solids on a watch glass with folded paper above

Drying crystal solids using a watch glass and passing a stream of dry air from an inverted funnel

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. Moreover, a watch glass is often used to house solids being weigh 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.

Not only can a watch glass be used for observing precipitation patterns and crystals, it 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 a 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.