What is a watch glass?

A watch glass is a small, circular piece of glass that is typically used in chemistry laboratories for various purposes. It has a flat bottom and a curved top, resembling a watch crystal, which is where it gets its name from.

Watch glasses are often used to cover beakers or flasks to prevent evaporation or contamination of the contents. They can also be used as a surface to evaporate solutions to concentrate them or as a lid for a crucible during heating.

In addition to their practical uses, watch glasses can also be used in qualitative analysis to observe the appearance of a reaction or to watch the progress of a reaction over time.

Watch glasses are usually made of borosilicate glass, making them resistant to thermal shock and chemical reactions. They come in various sizes ranging from a few centimeters to several inches in diameter.