When evaporation occurs, the watchglass allows crystals or precipitates that form to be viewed clearly, especially if it is placed onto a contrastingly coloured surface. Watchglasses are sometimes used in fractional distillation where each fraction of the oil is dripped onto a watchglass so its viscosity can easily be seen.
Large watchglasses may be referred to as clock glasses.
They are also sometimes used to cover beakers, making sure any dust or other particles cannot enter the beaker. If used as such, the beaker is not fully sealed, allowing gas exchanges to occur.