An evaporating dish is a piece of

laboratory glassware Laboratory glassware is a variety of equipment used in science, scientific work, traditionally made of glass. Glass may be blown, bent, cut, molded, or formed into many sizes and shapes. It is commonly used in chemistry, biology, and analytical ...

used for the

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

of solutions and

supernatant In an aqueous solution, precipitation is the "sedimentation of a solid material (a precipitate) from a liquid solution". The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemic ...

liquids, and sometimes to their

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

. Evaporating dishes are used to evaporate excess solvents – most commonly water – to produce a concentrated solution or a solid precipitate of the dissolved substance.

Description

Most evaporating dishes are made of

porcelain Porcelain (), also called china, is a ceramic material made by heating Industrial mineral, raw materials, generally including kaolinite, in a kiln to temperatures between . The greater strength and translucence of porcelain, relative to oth ...

borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...

. Shallow glass evaporating dishes are commonly termed "watch glasses", since they resemble the front window of a

pocket watch A pocket watch is a watch that is made to be carried in a pocket, as opposed to a wristwatch, which is strapped to the wrist. They were the most common type of watch from their development in the 16th century until wristwatches became popula ...

. Some used for high-temperature work are of

refractory metal Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definitions of which elements belong to this group di ...

s, usually of

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

, owing to its non-reactive behaviour and low risk of contamination. The capacity of evaporators is usually small – in the range 3–10 ml. Larger dishes, up to 100 ml, are different in shape, and are more hemispherical. The evaporator is used most often in quantitative analysis.

Analysis

In the determination of

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

content in an organic sample, a small and accurately-measured quantity of a substance is added to the large amount of

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

, then heated in an evaporating dish. The dish is heated with a

Bunsen burner A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is main ...

, until only stable precipitate remains, which contains the silica content. The dish is then closed and heated at high temperature until completely clean, fused silica is produced. Comparison of the initial weight of the substance and that of the fused silica allows the content of silicon in the sample to be determined. Parowniczka3D

The shape of the evaporating dish encourages evaporation in two ways: * The shell is relatively flat. A relatively large liquid surface promotes evaporation. * If heated in a flask or beaker, a part of the evaporated liquid condenses on the vessel walls and flows back into the solution. This does not happen in a dish. When heating liquid in an evaporating dish, the low walls encourage splashes and so stirring or swirling of evaporating liquids is considered bad practice, owing to the risk of spillage. Evaporation, especially in production quantities rather than merely for analysis, is now mostly performed in a

rotary evaporator A rotary evaporator (rotovap) is a device used in chemical laboratories for the efficient and gentle removal of solvents from samples by evaporation. When referenced in the chemistry research literature, description of the use of this technique an ...

. This is preferred because it works much faster and may be used under vacuum, avoiding unwanted reactions with the atmosphere and allowing control of noxious fumes. Evaporation under vacuum also reduces the severity of bumping and violent ebullition.

Notes

References

{{Laboratory equipment Laboratory glassware Laboratory porcelainware

Description

Analysis

See also

Notes

References