A watch glass is used in the chemistry laboratory to weigh solids and to hold small amounts of liquids. It is a concave dish and is designed to be lightweight. Watch glasses don't cost very much money to acquire.
A watch glass can be used to dry small amounts of solids. It's also used for covering containers with liquids. A glass jar containing a gas can be covered with a watch glass. It's also a handy tool for placing small amounts of a substance beneath the eyepiece of a compound microscope. Additionally, watch glasses are also used for evaporating small amounts of a liquid.