Evaporating dishes (or crystallizing dishes) are shallow bowls with a flat bottom and pour spout that may be used as reaction vessels, or for the separation of the solute from a solution through crystallization. They are made from a range of thermally stable materials, but the ones at RHS are almost always porcelain.
The shape of the evaporating dish – shallow, broad, and wider at the top than bottom – facilitates rapid vaporization of liquids from the dish. Vapors will not collect in the shallow dish as they would in a beaker and the rounded shape distributes heat effectively.
Since they are thermally stable evaporating dishes may be heated to dryness over a Bunsen burner flame, in a laboratory oven, or on a hot plate with minimal concern of shattering. Quenching of hot porcelain should NOT be attempted. The evaporating dish should be allowed to air cool completely before handling.
Porcelain evaporating dishes are slightly porous. After washing they should be gently heated to dryness (10-20 seconds over a Bunsen burner) if their weight needs to be recorded as data.
A watch glass is a simple concavo-convex glass circle that is used to cover reaction vessels to prevent spattering during chemical reactions or to cover larger storage containers (e.g., beakers, evaporating dishes) temporarily to reduce loss of chemicals from evaporation.
The transparency of the watch glass allows for the observation of substances in an opaque reaction vessel during a chemical reaction or physical change.
When using the watch glass to cover reaction vessels the glass should always be placed with the concave side facing up so the glass nests into the reaction vessel.