What is a watch glass primarily used for in a laboratory?

A watch glass is primarily used in a laboratory setting as a cover for beakers and test tubes. Its concave shape allows it to rest on the rim of a container, preventing contaminants from falling into the liquid while also minimizing evaporation. This makes it an essential tool for safely containing substances during experiments or reactions.

While other laboratory items, such as graduated cylinders and pipettes, are specifically designed for measuring liquid volumes or holding samples, the unique design and purpose of the watch glass align it with covering containers. It can also be used to observe small quantities of substances; however, its primary function remains as a protective cover rather than an observational tool.