Evaporating Dishes

Evaporating dishes are shallow open containers used for the evaporation of solutions and supernatant liquids to produce a concentrated solution or a solid precipitate of the dissolved substance. Evaporating dishes have a relatively flat shell and a large liquid surface that promotes evaporation, as well as a pouring spout that permits easy sample transfer. Most evaporating dishes are made of porcelain or borosilicate glass, which are non-reactive and can withstand heat from a Bunsen burner, but those made from other materials such as glassy carbon and PTFE are also available.

Evaporating dishes are pieces of heat-tolerant laboratory glassware that are used to hold substances or liquids that are being melted or evaporated. They are typically made of borosilicate glass (as in Pyrex evaporating dishes) or porcelain (also called “china dishes”). Porcelain evaporating dishes can withstand higher temperatures than glass but can also be delicate. Evaporating dishes made of platinum or another refractory metal may be used for very high-temperature work.

These shallow dishes are sometimes called "watch glasses" because they resemble the clear covers of watch faces. The shallow shape offers a larger evaporation surface and eliminates the possibility of condensation that can occur with other lab vessels. Stirring or swirling could cause spillage, so evaporation requires a static liquid.

Evaporating dishes may be weighed before use. The weight can then be subtracted from the final weight to provide the actual weight of the evaporated substance.

Use caution when evaporating substances that can produce noxious gases. These procedures should be performed under a hood or with other appropriate ventilation. Evaporation dish capacities are usually less than10 mL, but larger, more hemispherical dishes can be used to accommodate larger liquid volumes.

When evaporating dishes are used, gas burner heating is preferred; evaporating dishes should never be placed on a hot plate. Always handle heated dishes carefully to avoid getting burned, because they may not appear to be hot. Allow them to cool first, and use tongs, gripping devices (silicone rubber heat protectors), or heat-resistant gloves for safety.