The evaporating dish is a common piece of laboratory equipment used primarily in chemistry to process liquid solutions. Its function is tied to the application of heat, facilitating the rapid escape of solvent. This action is required for concentrating solutions or isolating dissolved solids. This specialized vessel plays a distinct role that other glassware cannot efficiently replicate.
Defining the Evaporating Dish
The evaporating dish is characterized by its wide, shallow, and flat-bottomed shape, often resembling a small, lipped bowl. This geometry maximizes the liquid’s surface area exposed to the air, significantly accelerating the rate of evaporation. The dish frequently includes a small lip or spout, allowing for the precise pouring and transfer of concentrated solutions or residual solids.
Most evaporating dishes are constructed from glazed porcelain, a material chosen for its heat resistance and chemical inertness. Porcelain allows the dish to withstand direct, high-temperature heating from a Bunsen burner or hot plate without cracking or reacting. While some dishes are made from borosilicate glass for lower heat applications, porcelain is preferred for processes that require heating a substance to dryness. The capacity of these dishes is typically small, often ranging between 3 to 100 milliliters.
Primary Laboratory Applications
The primary utility of the evaporating dish is the evaporation of solvents from a solution to recover a solute or create a concentrated mixture. In this process, a liquid solution—such as saltwater—is heated, causing the solvent (e.g., water) to boil off and escape as a gas. The non-volatile dissolved substance, like salt, is left behind in a solid form, making the dish useful for separating compounds with different boiling points.
This technique is frequently used in quantitative analysis to determine the solid content of a sample, often by weighing the dish before and after solvent removal. For instance, a dish can be heated until only a stable precipitate remains, which is then weighed to calculate the original substance’s composition. Because the dish’s low walls encourage rapid solvent escape, the process must remain static; stirring could cause the concentrated liquid to splash and result in sample loss. The dish is also used for heating solid substances directly to a high temperature for processes like calcination or ignition.
Distinguishing It From Other Laboratory Equipment
The specialized function of the evaporating dish sets it apart from other common laboratory vessels, particularly beakers and watch glasses. A standard beaker is deeper and narrower than an evaporating dish, meaning the liquid inside has a smaller surface area, which slows the rate of evaporation. Beakers are primarily designed for mixing, holding, and measuring liquids, whereas the evaporating dish is engineered to maximize solvent removal.
The evaporating dish is also distinct from a watch glass, despite the latter sometimes being used for small-scale evaporation. An evaporating dish is made of heat-resistant materials like porcelain, allowing it to be placed directly over a flame or heat source. A watch glass, conversely, is generally made of thin glass and is used as a temporary cover for a beaker or as a surface for weighing solid materials.