Watch glass (laboratory equipment)

Design and characteristics

Typically circular and slightly concave, a watch glass is thin and lightweight. Materials are usually soda-lime glass or borosilicate glass for greater thermal resistance; plastic variants exist for disposable or non-reactive uses. Sizes vary from a few centimeters to larger diameters used as lids. Because they are open and shallow, evaporation and visual inspection are easy, but they provide minimal protection from contamination.

Common uses

• Evaporating small quantities of solvent to obtain a residue or concentrate a solution.
• Holding solids prior to weighing or transfer on an analytical balance.
• Acting as a loose cover for beakers to reduce contamination or splashing.
• Serving as a surface for recrystallization trials, observation of precipitates, or temporary mixing of reagents.

Care, safety and distinctions

Watch glasses are fragile; handle with care and avoid subjecting ordinary glass to direct high flames. Borosilicate varieties tolerate moderate heating better. Cleaning is by rinsing and standard laboratory detergents; persistent residues may require solvents or acid cleaning in proper facilities. A watch glass differs from an evaporating dish (deeper, intended for more substantial heating) and from a Petri dish (often used for biological cultures and may have a lid designed to reduce contamination).

Notable facts: the piece is prized for its simplicity and versatility—many small tasks in routine laboratory work, from observation to minor drying steps, are conveniently accomplished with a watch glass.