An evaporating dish is a laboratory vessel designed to separate dissolved solid substances from a liquid solvent. Its primary purpose involves heating a solution to vaporize the liquid, leaving behind non-volatile solid components. This process is fundamental in various scientific disciplines for concentrating solutions or isolating solid residues. The dish allows liquids to be gently or vigorously heated to drive off the solvent.
Physical Characteristics and Materials
Evaporating dishes typically feature a shallow, wide-mouthed design with a flat bottom and sometimes a small pouring spout. This shape is engineered to maximize the surface area of the liquid exposed to the air, which enhances the rate of evaporation. Common materials for these dishes include porcelain, borosilicate glass, or ceramic. Porcelain and ceramic varieties are valued for their ability to withstand high temperatures and their resistance to thermal shock, making them suitable for direct heating over a flame.
Borosilicate glass dishes, while also heat-resistant, are often used for gentler evaporation processes, allowing for visual monitoring due to their transparency. Specialized dishes for high-temperature or corrosive substances may be made from materials like platinum or quartz, offering superior chemical inertness and heat tolerance. The inner surface of porcelain dishes is often glazed, which helps prevent sticking and makes cleaning easier.
How It Facilitates Evaporation
The design of an evaporating dish is optimized to accelerate evaporation. Its wide, shallow form provides a large surface area, allowing more solvent molecules to escape into the atmosphere simultaneously. This increased surface area speeds up the liquid’s transition to a gas. Heat is typically applied from an external source, such as a Bunsen burner or a hot plate.
Applying heat increases the kinetic energy of the liquid molecules, causing them to move faster and overcome the intermolecular forces holding them in the liquid phase. As the solvent evaporates, the concentration of the dissolved solid gradually increases until only the solid residue remains. The open nature of the dish prevents condensation of the evaporated liquid on the vessel walls, ensuring efficient solvent removal.
Common Uses and Applications
A common application for evaporating dishes involves isolating salts or other solid compounds by evaporating water from their solutions. Heating a sample containing a dissolved solid drives off the liquid, leaving the solid behind for collection or further analysis.
They are also used in crystallization processes, where a saturated solution is slowly evaporated to encourage the formation of pure crystals. Additionally, evaporating dishes assist in drying precipitates, which are solids formed from a chemical reaction in a liquid. The dish allows for gentle heating to remove residual moisture, ensuring the precipitate is completely dry before weighing or other analytical procedures. These dishes are used in quantitative analysis, where the precise weight of a solid residue is determined after solvent removal.