An evaporating dish is a specialized laboratory vessel designed for evaporation. This shallow, often lipped container facilitates the removal of solvents from solutions, leaving behind a concentrated solute or solid residue. Scientists use them to separate dissolved substances, such as salts, from liquids, playing a role in various chemical procedures.
Design and Material
Evaporating dishes typically feature a shallow, wide-mouthed design, often with a pouring spout. This shape maximizes the liquid’s surface area exposed to air, accelerating evaporation. The broad, open surface allows more solvent molecules to escape, making the process efficient compared to deeper containers. The spout aids in transferring concentrated solutions or solid residues once evaporation is complete.
These dishes are predominantly manufactured from materials that can withstand high temperatures and resist chemical reactions. Porcelain is a common choice, known for its ability to endure significant heat, often up to 1150°C (2102°F), making it suitable for heating over a Bunsen burner or hot plate. Porcelain evaporating dishes are also resistant to a wide range of chemicals, including acids and bases, and their glazed interior surfaces prevent sample sticking and facilitate easy cleaning.
Borosilicate glass, such as Pyrex, is another material used for evaporating dishes. While generally less durable under extreme heat compared to porcelain, borosilicate glass offers good heat resistance and excellent thermal shock resistance, meaning it can tolerate rapid temperature changes without cracking. Its transparent nature allows for clear visibility of the contents during evaporation. Both materials are selected for their durability and inertness, ensuring they do not react with the substances being evaporated.
Purpose and Safe Handling
The primary purpose of an evaporating dish is to efficiently remove a liquid solvent from a solution, obtaining the dissolved solid or a more concentrated solution. This process uses heat, which provides the energy for solvent molecules to transition into a gaseous state and escape. As the solvent evaporates, the non-volatile solute remains behind. This technique is widely used in laboratories for tasks such as crystallizing substances, separating mixtures, or preparing samples for further analysis.
When using an evaporating dish, careful attention to safety protocols is important. Heating is typically performed using a Bunsen burner or a hot plate, but direct flame heating should be done cautiously to prevent uneven heating and potential cracking of the dish. Rapid heating should be avoided to prevent thermal shock, which can cause the dish to break. It is also important to ensure proper ventilation, especially when evaporating volatile or potentially noxious substances, by conducting the procedure in a fume hood.
Always handle heated evaporating dishes with appropriate tools, such as tongs or heat-resistant gloves, as they retain heat for an extended period and may not appear hot. Placing a hot dish directly onto a cold surface can cause it to crack due to sudden temperature changes, so it should be allowed to cool on a heat-resistant mat or wire gauze. Avoiding stirring or swirling the liquid during evaporation helps prevent spillage, as the process typically requires a static liquid for optimal results.