A watch glass is a circular concave piece of glass commonly used in chemistry laboratories for various purposes:

1. Evaporation Surface: It serves as a surface to evaporate liquids. When you need to observe precipitates or crystallization closely, a watch glass allows for better visibility.

2. Holding Solids: A watch glass can hold solids while they are being weighed. Before weighing a substance, the watch glass is placed on the scale, and then the scale is zeroed to obtain the weight of the sample substance.

3. Cover for Beakers: When placed as a cover on beakers, the watch glass helps prevent dust or other particles from entering the beaker. Although it doesn’t completely seal the beaker, it still allows gas exchanges.

4. Drying Substances: If you need to further dry a substance, a watch glass is useful. Spread the solid on the watch glass, and place a folded piece of filter paper above it to prevent contamination. For faster drying, you can place the watch glass inside a fume hood or pass a gentle stream of dry air over it using an inverted funnel.

5. Types of Watch Glass:

   • Glass Watch Glasses: These can be reused after sterilization in an autoclave or laboratory oven. They offer high resistance to thermal shock, chemical exposure, and mechanical durability.