An evaporating dish is a fundamental piece of laboratory equipment designed to hold a liquid sample while a solvent is removed. The primary goal of using this apparatus is to separate a non-volatile dissolved substance (the solute) from the volatile liquid (the solvent). This process leaves behind a more concentrated solution or a solid residue for further analysis or use. The dish facilitates the efficient removal of the liquid component from a mixture under controlled laboratory conditions.
Primary Function and Design
The main function of the evaporating dish is to accelerate the natural process of evaporation, achieved through its unique physical design. The dish is shallow and features a wide mouth, creating a large surface area for the liquid sample. This increased surface area maximizes the rate at which solvent molecules can escape into the atmosphere as a vapor.
To speed up the process, the dish is typically placed over a gentle heat source, such as a hot plate or a Bunsen burner flame. The heat supplies the thermal energy necessary for the solvent molecules to transition into the gaseous state. Unlike beakers or flasks, the dish’s low walls prevent the vaporized solvent from condensing and dripping back into the sample, ensuring a faster separation.
Common Applications in the Lab
Evaporating dishes are widely used in chemistry and analytical laboratories for various separation and preparation techniques. One common application involves separating dissolved solids from a liquid, such as isolating mineral salts from a water sample. By heating the solution, the water evaporates, leaving the solid residue behind for recovery or examination.
The apparatus is also employed to concentrate solutions before a reaction or analysis can proceed. This involves evaporating only a portion of the solvent to increase the concentration of the dissolved substance. Furthermore, they are used in quantitative analysis to prepare small samples by drying them completely, allowing scientists to weigh the remaining dry solid to accurately determine the total amount of non-volatile material present.
Materials and Safe Handling
Evaporating dishes must be constructed from materials that can withstand high temperatures and resist chemical attack. The most common materials are porcelain and borosilicate glass, both selected for their high thermal resistance and inert properties. Porcelain tolerates higher direct heat, while borosilicate glass offers transparency for visual monitoring during heating.
When handling a heated dish, safety procedures dictate the use of specialized tools, such as tongs or heat-resistant gloves, to prevent burns. To avoid thermal shock, apply heat gradually and avoid placing a hot dish directly onto a cold countertop. If the liquid produces harmful vapors, the procedure should always be conducted under a fume hood with proper ventilation.