What Is a Watch Glass?
A watch glass is a circular, slightly concave piece of glass used in a laboratory. Its name comes from its resemblance to the front glass of an old-fashioned pocket watch. It is a versatile piece of glassware used for various non-heating purposes.
Section 2: Primary Functions of a Watch Glass
The primary functions of a watch glass include holding a small sample of a solid substance for weighing or observation, serving as a lid for a beaker to prevent dust or other particles from entering, and providing a surface for the slow evaporation of a liquid to yield a solid precipitate or crystals.
Section 3: A Practical Example
A common use for a watch glass is in a simple crystallization experiment. After dissolving a solid in a solvent, a small amount of the solution can be placed on a watch glass. As the solvent slowly evaporates at room temperature, the dissolved solid will be left behind in the form of crystals, which can then be observed under a microscope.
Section 4: Why It's an Essential Lab Tool
The watch glass is an essential tool in chemistry labs due to its simplicity and versatility. It allows for safe observation of reactions, prevents contamination when used as a beaker cover, and facilitates controlled evaporation without direct heating, which is crucial for delicate substances. Its smooth, non-porous surface is also easy to clean.